chore: address feedback

Motivations:
- We can simplify the big mutual recursion and the implementation.
- We can implement the support for `match`-expressions in the `pre` method.
- It is easier to define and simplify `Simprocs`.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -33,7 +33,7 @@ assignees: ''
 
 ### Versions
 
-[Output of `lean --version` in the folder that the issue occured in]
+[Output of `#eval Lean.versionString` or of `lean --version` in the folder that the issue occured in]
 [OS version]
 
 ### Additional Information

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,14 +1,14 @@
-* [ ] Put an X in this bracket to confirm you have read the
-  [External Contribution Guidelines](https://github.com/leanprover/lean4/blob/master/doc/contributions.md).
+# Read this section before submitting
 
-* Please put the link to your `RFC` or `bug` issue here.
-  PRs missing this link will be marked as `missing RFC`.
+* Ensure your PR follows the [External Contribution Guidelines](https://github.com/leanprover/lean4/blob/master/CONTRIBUTING.md).
+* Please make sure the PR has excellent documentation and tests. If we label it `missing documentation` or `missing tests` then it needs fixing!
+* Include the link to your `RFC` or `bug` issue in the description.
+* If the issue does not already have approval from a developer, submit the PR as draft.
+* The PR title/description will become the commit message. Keep it up-to-date as the PR evolves.
+* If you rebase your PR onto `nightly-with-mathlib` then CI will test Mathlib against your PR.
+* You can manage the `awaiting-review`, `awaiting-author`, and `WIP` labels yourself, by writing a comment containing one of these labels on its own line.
+* Remove this section, up to and including the `---` before submitting.
 
-* If that issue does not already have approval from a developer,
-  please be sure to open this PR in "Draft" mode.
+---
 
-* Please make sure the PR has excellent documentation and tests.
-  If we label it `missing documentation` or `missing tests` then it needs fixing!
-
-* You can manage the `awaiting-review`, `awaiting-author`, and `WIP` labels
-  yourself, by writing a comment containing one of these labels on its own line.
+Closes #0000 (`RFC` or `bug` issue number fixed by this PR, if any)

.github/workflows/changelog.yml

@@ -1,33 +0,0 @@
-name: add PR to changelog
-
-on:
-  # needs read/write GH token, do *not* execute arbitrary code from PR
-  pull_request_target:
-    types: [closed]
-
-jobs:
-  update-changelog:
-    if: |
-      github.event.pull_request.merged == true &&
-      contains(github.event.pull_request.labels.*.name, 'changelog') &&
-      github.base_ref == 'master'
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v3
-        with:
-          # needs sufficiently elevated token to override branch protection rules
-          token: ${{ secrets.PUSH_NIGHTLY_TOKEN }}
-      - name: Update changelog
-        run: |
-          set -euxo pipefail
-          escaped_link=$(sed -e 's/[\/&]/\\&/g' <<'EOF'
-          [${{ github.event.pull_request.title}}](${{ github.event.pull_request.html_url }})
-          EOF
-          )
-          # insert link below first dashes line (https://stackoverflow.com/a/9453461/161659)
-          sed -i "0,/^---*/s/^---*/\0\n\n* $escaped_link./" RELEASES.md
-          # commit as github-actions bot (https://github.com/orgs/community/discussions/26560#discussioncomment-3252339)
-          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
-          git config user.name "github-actions[bot]"
-          git commit -i RELEASES.md -m "doc: update changelog"
-          git push

.github/workflows/ci.yml

@@ -6,8 +6,8 @@ on:
     tags:
       - '*'
   pull_request:
-    branches:
-      - master
+    types: [opened, synchronize, reopened, labeled]
+  merge_group:
   schedule:
     - cron: '0 7 * * *'  # 8AM CET/11PM PT
 
@@ -16,18 +16,185 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  set-nightly:
+
+  # This job determines various settings for the following CI runs; see the `outputs` for details
+  configure:
     runs-on: ubuntu-latest
     outputs:
-      nightly: ${{ steps.set.outputs.nightly }}
+      # Should we run only a quick CI? Yes on a pull request without the full-ci label
+      quick: ${{ steps.set-quick.outputs.quick }}
+      # The build matrix, dynamically generated here
+      matrix: ${{ steps.set-matrix.outputs.result }}
+      # Should we make a nightly release? If so, this output contains the lean version string, else it is empty
+      nightly: ${{ steps.set-nightly.outputs.nightly }}
+      # Should this be the CI for a tagged release?
+      # Yes only if a tag is pushed to the `leanprover` repository, and the tag is "v" followed by a valid semver.
+      # It sets `set-release.outputs.RELEASE_TAG` to the tag
+      # and sets `set-release.outputs.{LEAN_VERSION_MAJOR,LEAN_VERSION_MINOR,LEAN_VERSION_PATCH,LEAN_SPECIAL_VERSION_DESC}`
+      # to the semver components parsed via regex.
+      LEAN_VERSION_MAJOR: ${{ steps.set-release.outputs.LEAN_VERSION_MAJOR }}
+      LEAN_VERSION_MINOR: ${{ steps.set-release.outputs.LEAN_VERSION_MINOR }}
+      LEAN_VERSION_PATCH: ${{ steps.set-release.outputs.LEAN_VERSION_PATCH }}
+      LEAN_SPECIAL_VERSION_DESC: ${{ steps.set-release.outputs.LEAN_SPECIAL_VERSION_DESC }}
+      RELEASE_TAG: ${{ steps.set-release.outputs.RELEASE_TAG }}
+
     steps:
+      - name: Run quick CI?
+        id: set-quick
+        env:
+          quick: ${{
+            github.event_name == 'pull_request' && !contains( github.event.pull_request.labels.*.name, 'full-ci')
+           }}
+        run: |
+          echo "quick=${{env.quick}}" >> $GITHUB_OUTPUT
+
+      - name: Configure build matrix
+        id: set-matrix
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const quick = ${{ steps.set-quick.outputs.quick }};
+            console.log(`quick: ${quick}`)
+            let matrix = [
+              {
+                // portable release build: use channel with older glibc (2.27)
+                "name": "Linux LLVM",
+                "os": "ubuntu-latest",
+                "release": false,
+                "quick": false,
+                "shell": "nix-shell --arg pkgsDist \"import (fetchTarball \\\"channel:nixos-19.03\\\") {{}}\" --run \"bash -euxo pipefail {0}\"",
+                "llvm-url": "https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-linux-gnu.tar.zst",
+                "prepare-llvm": "../script/prepare-llvm-linux.sh lean-llvm*",
+                "binary-check": "ldd -v",
+                // foreign code may be linked against more recent glibc
+                // reverse-ffi needs to be updated to link to LLVM libraries
+                "CTEST_OPTIONS": "-E 'foreign|leanlaketest_reverse-ffi'",
+                "CMAKE_OPTIONS": "-DLLVM=ON -DLLVM_CONFIG=${GITHUB_WORKSPACE}/build/llvm-host/bin/llvm-config"
+              },
+              {
+                "name": "Linux release",
+                "os": "ubuntu-latest",
+                "release": true,
+                "quick": true,
+                "shell": "nix-shell --arg pkgsDist \"import (fetchTarball \\\"channel:nixos-19.03\\\") {{}}\" --run \"bash -euxo pipefail {0}\"",
+                "llvm-url": "https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-linux-gnu.tar.zst",
+                "prepare-llvm": "../script/prepare-llvm-linux.sh lean-llvm*",
+                "binary-check": "ldd -v",
+                // foreign code may be linked against more recent glibc
+                "CTEST_OPTIONS": "-E 'foreign'"
+              },
+              {
+                "name": "Linux",
+                "os": "ubuntu-latest",
+                "check-stage3": true,
+                "test-speedcenter": true,
+                "quick": false,
+              },
+              {
+                "name": "Linux Debug",
+                "os": "ubuntu-latest",
+                "quick": false,
+                "CMAKE_OPTIONS": "-DCMAKE_BUILD_TYPE=Debug",
+                // exclude seriously slow tests
+                "CTEST_OPTIONS": "-E 'interactivetest|leanpkgtest|laketest|benchtest'"
+              },
+              {
+                "name": "Linux fsanitize",
+                "os": "ubuntu-latest",
+                "quick": false,
+                // turn off custom allocator & symbolic functions to make LSAN do its magic
+                "CMAKE_OPTIONS": "-DLEAN_EXTRA_CXX_FLAGS=-fsanitize=address,undefined -DLEANC_EXTRA_FLAGS='-fsanitize=address,undefined -fsanitize-link-c++-runtime' -DSMALL_ALLOCATOR=OFF -DBSYMBOLIC=OFF",
+                // exclude seriously slow/problematic tests (laketests crash)
+                "CTEST_OPTIONS": "-E 'interactivetest|leanpkgtest|laketest|benchtest'"
+              },
+              {
+                "name": "macOS",
+                "os": "macos-latest",
+                "release": true,
+                "quick": false,
+                "shell": "bash -euxo pipefail {0}",
+                "llvm-url": "https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-apple-darwin.tar.zst",
+                "prepare-llvm": "../script/prepare-llvm-macos.sh lean-llvm*",
+                "binary-check": "otool -L",
+                "tar": "gtar" // https://github.com/actions/runner-images/issues/2619
+              },
+              {
+                "name": "macOS aarch64",
+                "os": "macos-latest",
+                "release": true,
+                "quick": false,
+                "cross": true,
+                "shell": "bash -euxo pipefail {0}",
+                "CMAKE_OPTIONS": "-DUSE_GMP=OFF -DLEAN_INSTALL_SUFFIX=-darwin_aarch64",
+                "llvm-url": "https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-aarch64-apple-darwin.tar.zst https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-apple-darwin.tar.zst",
+                "prepare-llvm": "EXTRA_FLAGS=--target=aarch64-apple-darwin ../script/prepare-llvm-macos.sh lean-llvm-aarch64-* lean-llvm-x86_64-*",
+                "binary-check": "otool -L",
+                "tar": "gtar" // https://github.com/actions/runner-images/issues/2619
+              },
+              {
+                "name": "Windows",
+                "os": "windows-2022",
+                "release": true,
+                "quick": false,
+                "shell": "msys2 {0}",
+                "CMAKE_OPTIONS": "-G \"Unix Makefiles\" -DUSE_GMP=OFF",
+                // for reasons unknown, interactivetests are flaky on Windows
+                "CTEST_OPTIONS": "--repeat until-pass:2",
+                "llvm-url": "https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-w64-windows-gnu.tar.zst",
+                "prepare-llvm": "../script/prepare-llvm-mingw.sh lean-llvm*",
+                "binary-check": "ldd"
+              },
+              {
+                "name": "Linux aarch64",
+                "os": "ubuntu-latest",
+                "CMAKE_OPTIONS": "-DUSE_GMP=OFF -DLEAN_INSTALL_SUFFIX=-linux_aarch64",
+                "release": true,
+                "quick": false,
+                "cross": true,
+                "shell": "nix-shell --arg pkgsDist \"import (fetchTarball \\\"channel:nixos-19.03\\\") {{ localSystem.config = \\\"aarch64-unknown-linux-gnu\\\"; }}\" --run \"bash -euxo pipefail {0}\"",
+                "llvm-url": "https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-linux-gnu.tar.zst https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-aarch64-linux-gnu.tar.zst",
+                "prepare-llvm": "EXTRA_FLAGS=--target=aarch64-unknown-linux-gnu ../script/prepare-llvm-linux.sh lean-llvm-aarch64-* lean-llvm-x86_64-*"
+              },
+              {
+                "name": "Linux 32bit",
+                "os": "ubuntu-latest",
+                // Use 32bit on stage0 and stage1 to keep oleans compatible
+                "CMAKE_OPTIONS": "-DSTAGE0_USE_GMP=OFF -DSTAGE0_LEAN_EXTRA_CXX_FLAGS='-m32' -DSTAGE0_LEANC_OPTS='-m32' -DSTAGE0_MMAP=OFF -DUSE_GMP=OFF -DLEAN_EXTRA_CXX_FLAGS='-m32' -DLEANC_OPTS='-m32' -DMMAP=OFF -DLEAN_INSTALL_SUFFIX=-linux_x86",
+                "cmultilib": true,
+                "release": true,
+                "quick": false,
+                "cross": true,
+                "shell": "bash -euxo pipefail {0}"
+              },
+              {
+                "name": "Web Assembly",
+                "os": "ubuntu-latest",
+                // Build a native 32bit binary in stage0 and use it to compile the oleans and the wasm build
+                "CMAKE_OPTIONS": "-DCMAKE_C_COMPILER_WORKS=1 -DSTAGE0_USE_GMP=OFF -DSTAGE0_LEAN_EXTRA_CXX_FLAGS='-m32' -DSTAGE0_LEANC_OPTS='-m32' -DSTAGE0_CMAKE_CXX_COMPILER=clang++ -DSTAGE0_CMAKE_C_COMPILER=clang -DSTAGE0_CMAKE_EXECUTABLE_SUFFIX=\"\" -DUSE_GMP=OFF -DMMAP=OFF -DSTAGE0_MMAP=OFF -DCMAKE_AR=../emsdk/emsdk-main/upstream/emscripten/emar -DCMAKE_TOOLCHAIN_FILE=../emsdk/emsdk-main/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -DLEAN_INSTALL_SUFFIX=-linux_wasm32",
+                "wasm": true,
+                "cmultilib": true,
+                "release": true,
+                "quick": false,
+                "cross": true,
+                "shell": "bash -euxo pipefail {0}",
+                // Just a few selected tests because wasm is slow
+                "CTEST_OPTIONS": "-R \"leantest_1007\\.lean|leantest_Format\\.lean|leanruntest\\_1037.lean|leanruntest_ac_rfl\\.lean\""
+              }
+            ];
+            console.log(`matrix:\n${JSON.stringify(matrix, null, 2)}`)
+            if (quick) {
+              return matrix.filter((job) => job.quick)
+            } else {
+              return matrix
+            }
+
       - name: Checkout
         uses: actions/checkout@v3
         # don't schedule nightlies on forks
         if: github.event_name == 'schedule' && github.repository == 'leanprover/lean4'
       - name: Set Nightly
         if: github.event_name == 'schedule' && github.repository == 'leanprover/lean4'
-        id: set
+        id: set-nightly
         run: |
           if [[ -n '${{ secrets.PUSH_NIGHTLY_TOKEN }}' ]]; then
             git remote add nightly https://foo:'${{ secrets.PUSH_NIGHTLY_TOKEN }}'@github.com/${{ github.repository_owner }}/lean4-nightly.git
@@ -39,26 +206,9 @@ jobs:
             fi
           fi
 
-  # This job determines if this CI build is for a tagged release.
-  # It only runs when a tag is pushed to the `leanprover` repository.
-  # It sets `set-release.outputs.RELEASE_TAG` to the tag, if the tag is "v" followed by a valid semver,
-  # and sets `set-release.outputs.{LEAN_VERSION_MAJOR,LEAN_VERSION_MINOR,LEAN_VERSION_PATCH,LEAN_SPECIAL_VERSION_DESC}`
-  # to the semver components parsed via regex.
-  set-release:
-    runs-on: ubuntu-latest
-    outputs:
-      LEAN_VERSION_MAJOR: ${{ steps.set.outputs.LEAN_VERSION_MAJOR }}
-      LEAN_VERSION_MINOR: ${{ steps.set.outputs.LEAN_VERSION_MINOR }}
-      LEAN_VERSION_PATCH: ${{ steps.set.outputs.LEAN_VERSION_PATCH }}
-      LEAN_SPECIAL_VERSION_DESC: ${{ steps.set.outputs.LEAN_SPECIAL_VERSION_DESC }}
-      RELEASE_TAG: ${{ steps.set.outputs.RELEASE_TAG }}
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v3
-        if: startsWith(github.ref, 'refs/tags/') && github.repository == 'leanprover/lean4'
       - name: Check for official release
         if: startsWith(github.ref, 'refs/tags/') && github.repository == 'leanprover/lean4'
-        id: set
+        id: set-release
         run: |
           TAG_NAME=${GITHUB_REF##*/}
 
@@ -87,98 +237,17 @@ jobs:
           fi
 
   build:
-    needs: [set-nightly, set-release]
+    needs: [configure]
     if: github.event_name != 'schedule' || github.repository == 'leanprover/lean4'
+    strategy:
+      matrix:
+        include: ${{fromJson(needs.configure.outputs.matrix)}}
+      # complete all jobs
+      fail-fast: false
     runs-on: ${{ matrix.os }}
     defaults:
       run:
         shell: ${{ matrix.shell || 'nix-shell --run "bash -euxo pipefail {0}"' }}
-    strategy:
-      matrix:
-        include:
-          # portable release build: use channel with older glibc (2.27)
-          - name: Linux LLVM
-            os: ubuntu-latest
-            release: false
-            shell: nix-shell --arg pkgsDist "import (fetchTarball \"channel:nixos-19.03\") {{}}" --run "bash -euxo pipefail {0}"
-            llvm-url: https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-linux-gnu.tar.zst
-            prepare-llvm: ../script/prepare-llvm-linux.sh lean-llvm*
-            binary-check: ldd -v
-            # foreign code may be linked against more recent glibc
-            # reverse-ffi needs to be updated to link to LLVM libraries
-            CTEST_OPTIONS: -E 'foreign|leanlaketest_reverse-ffi'
-            CMAKE_OPTIONS: -DLLVM=ON -DLLVM_CONFIG=${GITHUB_WORKSPACE}/build/llvm-host/bin/llvm-config
-          - name: Linux release
-            os: ubuntu-latest
-            release: true
-            shell: nix-shell --arg pkgsDist "import (fetchTarball \"channel:nixos-19.03\") {{}}" --run "bash -euxo pipefail {0}"
-            llvm-url: https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-linux-gnu.tar.zst
-            prepare-llvm: ../script/prepare-llvm-linux.sh lean-llvm*
-            binary-check: ldd -v
-            # foreign code may be linked against more recent glibc
-            CTEST_OPTIONS: -E 'foreign'
-          - name: Linux
-            os: ubuntu-latest
-            check-stage3: true
-            test-speedcenter: true
-          - name: Linux Debug
-            os: ubuntu-latest
-            CMAKE_OPTIONS: -DCMAKE_BUILD_TYPE=Debug
-            # exclude seriously slow tests
-            CTEST_OPTIONS: -E 'interactivetest|leanpkgtest|laketest|benchtest'
-          - name: Linux fsanitize
-            os: ubuntu-latest
-            # turn off custom allocator & symbolic functions to make LSAN do its magic
-            CMAKE_OPTIONS: -DLEAN_EXTRA_CXX_FLAGS=-fsanitize=address,undefined -DLEANC_EXTRA_FLAGS='-fsanitize=address,undefined -fsanitize-link-c++-runtime' -DSMALL_ALLOCATOR=OFF -DBSYMBOLIC=OFF
-            # exclude seriously slow/problematic tests (laketests crash)
-            CTEST_OPTIONS: -E 'interactivetest|leanpkgtest|laketest|benchtest'
-          - name: macOS
-            os: macos-latest
-            release: true
-            shell: bash -euxo pipefail {0}
-            llvm-url: https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-apple-darwin.tar.zst
-            prepare-llvm: ../script/prepare-llvm-macos.sh lean-llvm*
-            binary-check: otool -L
-            tar: gtar  # https://github.com/actions/runner-images/issues/2619
-          - name: macOS aarch64
-            os: macos-latest
-            release: true
-            cross: true
-            shell: bash -euxo pipefail {0}
-            CMAKE_OPTIONS: -DUSE_GMP=OFF -DLEAN_INSTALL_SUFFIX=-darwin_aarch64
-            llvm-url: https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-aarch64-apple-darwin.tar.zst https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-apple-darwin.tar.zst
-            prepare-llvm: EXTRA_FLAGS=--target=aarch64-apple-darwin ../script/prepare-llvm-macos.sh lean-llvm-aarch64-* lean-llvm-x86_64-*
-            binary-check: otool -L
-            tar: gtar  # https://github.com/actions/runner-images/issues/2619
-          - name: Windows
-            os: windows-2022
-            release: true
-            shell: msys2 {0}
-            CMAKE_OPTIONS: -G "Unix Makefiles" -DUSE_GMP=OFF
-            # for reasons unknown, interactivetests are flaky on Windows
-            CTEST_OPTIONS: --repeat until-pass:2
-            llvm-url: https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-w64-windows-gnu.tar.zst
-            prepare-llvm: ../script/prepare-llvm-mingw.sh lean-llvm*
-            binary-check: ldd
-          - name: Linux aarch64
-            os: ubuntu-latest
-            CMAKE_OPTIONS: -DUSE_GMP=OFF -DLEAN_INSTALL_SUFFIX=-linux_aarch64
-            release: true
-            cross: true
-            shell: nix-shell --arg pkgsDist "import (fetchTarball \"channel:nixos-19.03\") {{ localSystem.config = \"aarch64-unknown-linux-gnu\"; }}" --run "bash -euxo pipefail {0}"
-            llvm-url: https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-x86_64-linux-gnu.tar.zst https://github.com/leanprover/lean-llvm/releases/download/15.0.1/lean-llvm-aarch64-linux-gnu.tar.zst
-            prepare-llvm: EXTRA_FLAGS=--target=aarch64-unknown-linux-gnu ../script/prepare-llvm-linux.sh lean-llvm-aarch64-* lean-llvm-x86_64-*
-          - name: Web Assembly
-            os: ubuntu-latest
-            # Build a native 32bit binary in stage0 and use it to compile the oleans and the wasm build
-            CMAKE_OPTIONS: -DCMAKE_C_COMPILER_WORKS=1 -DSTAGE0_USE_GMP=OFF -DSTAGE0_LEAN_EXTRA_CXX_FLAGS='-m32' -DSTAGE0_LEANC_OPTS='-m32' -DSTAGE0_CMAKE_CXX_COMPILER=clang++ -DSTAGE0_CMAKE_C_COMPILER=clang -DSTAGE0_CMAKE_EXECUTABLE_SUFFIX="" -DUSE_GMP=OFF -DMMAP=OFF -DSTAGE0_MMAP=OFF -DUSE_GMP=OFF -DCMAKE_AR=../emsdk/emsdk-main/upstream/emscripten/emar -DCMAKE_TOOLCHAIN_FILE=../emsdk/emsdk-main/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake
-            wasm: true
-            cross: true
-            shell: bash -euxo pipefail {0}
-            # Just a few selected test because wasm is slow
-            CTEST_OPTIONS: -R "leantest_1007\.lean|leantest_Format\.lean|leanruntest\_1037.lean|leanruntest_ac_rfl\.lean"
-      # complete all jobs
-      fail-fast: false
     name: ${{ matrix.name }}
     env:
       # must be inside workspace
@@ -197,11 +266,13 @@ jobs:
         uses: actions/checkout@v3
         with:
           submodules: true
+          # the default is to use a virtual merge commit between the PR and master: just use the PR
+          ref: ${{ github.event.pull_request.head.sha }}
       - name: Install Nix
         uses: cachix/install-nix-action@v18
         with:
           install_url: https://releases.nixos.org/nix/nix-2.12.0/install
-        if: matrix.os == 'ubuntu-latest' && !matrix.wasm
+        if: matrix.os == 'ubuntu-latest' && !matrix.cmultilib
       - name: Install MSYS2
         uses: msys2/setup-msys2@v2
         with:
@@ -214,7 +285,7 @@ jobs:
           brew install ccache tree zstd coreutils gmp
         if: matrix.os == 'macos-latest'
       - name: Setup emsdk
-        uses: mymindstorm/setup-emsdk@v11
+        uses: mymindstorm/setup-emsdk@v12
         with:
           version: 3.1.44
           actions-cache-folder: emsdk
@@ -223,7 +294,7 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install -y gcc-multilib g++-multilib ccache
-        if: matrix.wasm
+        if: matrix.cmultilib
       - name: Cache
         uses: actions/cache@v3
         with:
@@ -248,21 +319,23 @@ jobs:
           mkdir build
           cd build
           ulimit -c unlimited # coredumps
-          OPTIONS=()
+          # this also enables githash embedding into stage 1 library
+          OPTIONS=(-DCHECK_OLEAN_VERSION=ON)
+          OPTIONS+=(-DLEAN_EXTRA_MAKE_OPTS=-DwarningAsError=true)
           if [[ -n '${{ matrix.prepare-llvm }}' ]]; then
             wget -q ${{ matrix.llvm-url }}
             PREPARE="$(${{ matrix.prepare-llvm }})"
             eval "OPTIONS+=($PREPARE)"
           fi
-          if [[ -n '${{ matrix.release }}' && -n '${{ needs.set-nightly.outputs.nightly }}' ]]; then
-            OPTIONS+=(-DLEAN_SPECIAL_VERSION_DESC=${{ needs.set-nightly.outputs.nightly }})
+          if [[ -n '${{ matrix.release }}' && -n '${{ needs.configure.outputs.nightly }}' ]]; then
+            OPTIONS+=(-DLEAN_SPECIAL_VERSION_DESC=${{ needs.configure.outputs.nightly }})
           fi
-          if [[ -n '${{ matrix.release }}' && -n '${{ needs.set-release.outputs.RELEASE_TAG }}' ]]; then
-            OPTIONS+=(-DLEAN_VERSION_MAJOR=${{ needs.set-release.outputs.LEAN_VERSION_MAJOR }})
-            OPTIONS+=(-DLEAN_VERSION_MINOR=${{ needs.set-release.outputs.LEAN_VERSION_MINOR }})
-            OPTIONS+=(-DLEAN_VERSION_PATCH=${{ needs.set-release.outputs.LEAN_VERSION_PATCH }})
+          if [[ -n '${{ matrix.release }}' && -n '${{ needs.configure.outputs.RELEASE_TAG }}' ]]; then
+            OPTIONS+=(-DLEAN_VERSION_MAJOR=${{ needs.configure.outputs.LEAN_VERSION_MAJOR }})
+            OPTIONS+=(-DLEAN_VERSION_MINOR=${{ needs.configure.outputs.LEAN_VERSION_MINOR }})
+            OPTIONS+=(-DLEAN_VERSION_PATCH=${{ needs.configure.outputs.LEAN_VERSION_PATCH }})
             OPTIONS+=(-DLEAN_VERSION_IS_RELEASE=1)
-            OPTIONS+=(-DLEAN_SPECIAL_VERSION_DESC=${{ needs.set-release.outputs.LEAN_SPECIAL_VERSION_DESC }})
+            OPTIONS+=(-DLEAN_SPECIAL_VERSION_DESC=${{ needs.configure.outputs.LEAN_SPECIAL_VERSION_DESC }})
           fi
           # contortion to support empty OPTIONS with old macOS bash
           cmake .. ${{ matrix.CMAKE_OPTIONS }} ${OPTIONS[@]+"${OPTIONS[@]}"} -DLEAN_INSTALL_PREFIX=$PWD/..
@@ -273,13 +346,13 @@ jobs:
       - name: List Install Tree
         run: |
           # omit contents of Init/, ...
-          tree --du -h lean-* | grep -E ' (Init|Lean|Lake|LICENSE|[a-z])'
+          tree --du -h lean-*-* | grep -E ' (Init|Lean|Lake|LICENSE|[a-z])'
       - name: Pack
         run: |
-          dir=$(echo lean-*)
+          dir=$(echo lean-*-*)
           mkdir pack
           # high-compression tar.zst + zip for release, fast tar.zst otherwise
-          if [[ '${{ startsWith(github.ref, 'refs/tags/') && matrix.release }}' == true || -n '${{ needs.set-nightly.outputs.nightly }}' || -n '${{ needs.set-release.outputs.RELEASE_TAG }}' ]]; then
+          if [[ '${{ startsWith(github.ref, 'refs/tags/') && matrix.release }}' == true || -n '${{ needs.configure.outputs.nightly }}' || -n '${{ needs.configure.outputs.RELEASE_TAG }}' ]]; then
             ${{ matrix.tar || 'tar' }} cf - $dir | zstd -T0 --no-progress -19 -o pack/$dir.tar.zst
             zip -rq pack/$dir.zip $dir
           else
@@ -300,22 +373,22 @@ jobs:
           ulimit -c unlimited # coredumps
           # exclude nonreproducible test
           ctest -j4 --output-on-failure ${{ matrix.CTEST_OPTIONS }} < /dev/null
-        if: matrix.wasm || !matrix.cross
+        if: (matrix.wasm || !matrix.cross) && needs.configure.outputs.quick == 'false'
       - name: Check Test Binary
         run: ${{ matrix.binary-check }} tests/compiler/534.lean.out
-        if: ${{ !matrix.cross }}
+        if: ${{ !matrix.cross && needs.configure.outputs.quick == 'false' }}
       - name: Build Stage 2
         run: |
           cd build
           ulimit -c unlimited # coredumps
           make -j4 stage2
-        if: matrix.build-stage2 || matrix.check-stage3
+        if: matrix.test-speedcenter
       - name: Check Stage 3
         run: |
           cd build
           ulimit -c unlimited # coredumps
           make -j4 check-stage3
-        if: matrix.check-stage3
+        if: matrix.test-speedcenter
       - name: Test Speedcenter Benchmarks
         run: |
           echo -1 | sudo tee /proc/sys/kernel/perf_event_paranoid
@@ -328,7 +401,7 @@ jobs:
           cd build
           ulimit -c unlimited # coredumps
           make update-stage0 && make -j4
-        if: matrix.name == 'Linux'
+        if: matrix.name == 'Linux' && needs.configure.outputs.quick == 'false'
       - name: CCache stats
         run: ccache -s
       - name: Show stacktrace for coredumps
@@ -352,6 +425,21 @@ jobs:
             ./build/stage2/bin/lean
             ./build/stage2/lib/lean/libleanshared.so
 
+  # This job collects results from all the matrix jobs
+  # This can be made the “required” job, instead of listing each
+  # matrix job separately
+  all-done:
+    name: Build matrix complete
+    runs-on: ubuntu-latest
+    needs: build
+    if: ${{ always() }}
+    steps:
+    - if: contains(needs.*.result, 'failure') ||  contains(needs.*.result, 'cancelled')
+      uses: actions/github-script@v7
+      with:
+        script: |
+            core.setFailed('Some jobs failed')
+
   # This job creates releases from tags
   # (whether they are "unofficial" releases for experiments, or official releases when the tag is "v" followed by a semver string.)
   # We do not attempt to automatically construct a changelog here:
@@ -375,8 +463,8 @@ jobs:
   # This job creates nightly releases during the cron job.
   # It is responsible for creating the tag, and automatically generating a changelog.
   release-nightly:
-    needs: [set-nightly, build]
-    if: needs.set-nightly.outputs.nightly
+    needs: [configure, build]
+    if: needs.configure.outputs.nightly
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -392,8 +480,9 @@ jobs:
         run: |
           git remote add nightly https://foo:'${{ secrets.PUSH_NIGHTLY_TOKEN }}'@github.com/${{ github.repository_owner }}/lean4-nightly.git
           git fetch nightly --tags
-          git tag ${{ needs.set-nightly.outputs.nightly }}
-          git push nightly ${{ needs.set-nightly.outputs.nightly }}
+          git tag ${{ needs.configure.outputs.nightly }}
+          git push nightly ${{ needs.configure.outputs.nightly }}
+          git push -f origin refs/tags/${{ needs.configure.outputs.nightly }}:refs/heads/nightly
           last_tag=$(git log HEAD^ --simplify-by-decoration --pretty="format:%d" | grep -o "nightly-[-0-9]*" | head -n 1)
           echo -e "*Changes since ${last_tag}:*\n\n" > diff.md
           git show $last_tag:RELEASES.md > old.md
@@ -408,7 +497,7 @@ jobs:
           prerelease: true
           files: artifacts/*/*
           fail_on_unmatched_files: true
-          tag_name: ${{ needs.set-nightly.outputs.nightly }}
+          tag_name: ${{ needs.configure.outputs.nightly }}
           repository: ${{ github.repository_owner }}/lean4-nightly
         env:
           GITHUB_TOKEN: ${{ secrets.PUSH_NIGHTLY_TOKEN }}

.github/workflows/labels-from-comments.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
     - name: Add label based on comment
-      uses: actions/github-script@v6
+      uses: actions/github-script@v7
       with:
         github-token: ${{ secrets.GITHUB_TOKEN }}
         script: |

.github/workflows/nix-ci.yml

@@ -6,8 +6,7 @@ on:
     tags:
       - '*'
   pull_request:
-    branches:
-      - master
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -18,7 +17,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     defaults:
       run:
-        shell: nix -v --experimental-features "nix-command flakes" run .#ciShell -- bash -euxo pipefail {0}
+        shell: nix run .#ciShell -- bash -euxo pipefail {0}
     strategy:
       matrix:
         include:
@@ -30,18 +29,13 @@ jobs:
       fail-fast: false
     name: ${{ matrix.name }}
     env:
-      NIX_BUILD_ARGS: -v --print-build-logs --fallback
+      NIX_BUILD_ARGS: --print-build-logs --fallback
     steps:
       - name: Checkout
         uses: actions/checkout@v3
-      - name: Install Nix
-        uses: cachix/install-nix-action@v18
         with:
-          # https://github.com/NixOS/nix/issues/6572
-          install_url: https://releases.nixos.org/nix/nix-2.7.0/install
-          extra_nix_config: |
-            extra-sandbox-paths = /nix/var/cache/ccache
-            substituters = file://${{ github.workspace }}/nix-store-cache-copy?priority=10&trusted=true https://cache.nixos.org
+          # the default is to use a virtual merge commit between the PR and master: just use the PR
+          ref: ${{ github.event.pull_request.head.sha }}
       - name: Set Up Nix Cache
         uses: actions/cache@v3
         with:
@@ -55,8 +49,13 @@ jobs:
         run: |
           # Nix seems to mutate the cache, so make a copy
           cp -r nix-store-cache nix-store-cache-copy || true
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+        with:
+          extra-conf: |
+            extra-sandbox-paths = /nix/var/cache/ccache?
+            substituters = file://${{ github.workspace }}/nix-store-cache-copy?priority=10&trusted=true https://cache.nixos.org  
       - name: Prepare CCache Cache
-        shell: bash -euxo pipefail {0}
         run: |
           sudo mkdir -m0770 -p /nix/var/cache/ccache
           sudo chown -R $USER /nix/var/cache/ccache
@@ -69,7 +68,6 @@ jobs:
           restore-keys: |
             ${{ matrix.name }}-nix-ccache
       - name: Further Set Up CCache Cache
-        shell: bash -euxo pipefail {0}
         run: |
           sudo chown -R root:nixbld /nix/var/cache
           sudo chmod -R 770 /nix/var/cache

.github/workflows/pr-release.yml

@@ -16,25 +16,16 @@ on:
 jobs:
   on-success:
     runs-on: ubuntu-latest
-    if: github.event.workflow_run.conclusion == 'success' && github.repository == 'leanprover/lean4'
+    if: github.event.workflow_run.conclusion == 'success' && github.event.workflow_run.event == 'pull_request' && github.repository == 'leanprover/lean4'
     steps:
       - name: Retrieve information about the original workflow
         uses: potiuk/get-workflow-origin@v1_1 # https://github.com/marketplace/actions/get-workflow-origin
+        # This action is deprecated and archived, but it seems hard to find a better solution for getting the PR number
+        # see https://github.com/orgs/community/discussions/25220 for some discussion
         id: workflow-info
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
           sourceRunId: ${{ github.event.workflow_run.id }}
-      - name: Checkout
-        # Only proceed if the previous workflow had a pull request number.
-        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
-        uses: actions/checkout@v3
-        with:
-          token: ${{ secrets.PR_RELEASES_TOKEN }}
-          # Since `workflow_run` runs on master, we need to specify which commit to check out,
-          # so that we tag the PR.
-          ref: ${{ steps.workflow-info.outputs.targetCommitSha }}
-          # We need a full checkout, so that we can push the PR commits to the `lean4-pr-releases` repo.
-          fetch-depth: 0
 
       - name: Download artifact from the previous workflow.
         if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
@@ -45,14 +36,22 @@ jobs:
           path: artifacts
           name: build-.*
           name_is_regexp: true
-      - name: Prepare release
+
+      - name: Push branch and tag
+        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
+        run: |
+          git init --bare lean4.git
+          git -C lean4.git remote add origin https://github.com/${{ github.repository_owner }}/lean4.git
+          git -C lean4.git fetch -n origin master
+          git -C lean4.git fetch -n origin "${{ steps.workflow-info.outputs.sourceHeadSha }}"
+          git -C lean4.git tag -f pr-release-${{ steps.workflow-info.outputs.pullRequestNumber }} "${{ steps.workflow-info.outputs.sourceHeadSha }}"
+          git -C lean4.git remote add pr-releases https://foo:'${{ secrets.PR_RELEASES_TOKEN }}'@github.com/${{ github.repository_owner }}/lean4-pr-releases.git
+          git -C lean4.git push -f pr-releases pr-release-${{ steps.workflow-info.outputs.pullRequestNumber }}
+      - name: Delete existing release if present
         if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
         run: |
-          git remote add pr-releases https://foo:'${{ secrets.PR_RELEASES_TOKEN }}'@github.com/${{ github.repository_owner }}/lean4-pr-releases.git
           # Try to delete any existing release for the current PR.
           gh release delete --repo ${{ github.repository_owner }}/lean4-pr-releases pr-release-${{ steps.workflow-info.outputs.pullRequestNumber }} -y || true
-          git tag -f pr-release-${{ steps.workflow-info.outputs.pullRequestNumber }}
-          git push -f pr-releases pr-release-${{ steps.workflow-info.outputs.pullRequestNumber }}
         env:
           GH_TOKEN: ${{ secrets.PR_RELEASES_TOKEN }}
       - name: Release
@@ -72,35 +71,143 @@ jobs:
 
       - name: Add label
         if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
-        uses: actions-ecosystem/action-add-labels@v1
+        uses: actions/github-script@v7
         with:
-          number: ${{ steps.workflow-info.outputs.pullRequestNumber }}
-          labels: toolchain-available
+          script: |
+            await github.rest.issues.addLabels({
+              issue_number: ${{ steps.workflow-info.outputs.pullRequestNumber }},
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              labels: ['toolchain-available']
+            })
+
+      # Next, determine the most recent nightly release in this PR's history.
+      - name: Find most recent nightly in feature branch
+        id: most-recent-nightly-tag
+        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
+        run: |
+          git -C lean4.git remote add nightly https://github.com/leanprover/lean4-nightly.git
+          git -C lean4.git fetch nightly '+refs/tags/nightly-*:refs/tags/nightly-*'
+          git -C lean4.git tag --merged "${{ steps.workflow-info.outputs.sourceHeadSha }}" --list "nightly-*" \
+            | sort -rV | head -n 1 | sed "s/^nightly-*/MOST_RECENT_NIGHTLY=/" | tee -a $GITHUB_ENV
+
+      - name: 'Setup jq'
+        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
+        uses: dcarbone/install-jq-action@v1.0.1
+
+      # Check that the most recently nightly coincides with 'git merge-base HEAD master'
+      - name: Check merge-base and nightly-testing-YYYY-MM-DD
+        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
+        id: ready
+        run: |
+          echo "Most recent nightly in your branch: $MOST_RECENT_NIGHTLY"
+          NIGHTLY_SHA=$(git -C lean4.git rev-parse "nightly-$MOST_RECENT_NIGHTLY^{commit}")
+          echo "SHA of most recent nightly: $NIGHTLY_SHA"
+          MERGE_BASE_SHA=$(git -C lean4.git merge-base origin/master "${{ steps.workflow-info.outputs.sourceHeadSha }}")
+          echo "SHA of merge-base: $MERGE_BASE_SHA"
+          if [ "$NIGHTLY_SHA" = "$MERGE_BASE_SHA" ]; then
+            echo "Most recent nightly tag agrees with the merge base."
+
+            REMOTE_BRANCHES=$(git ls-remote -h https://github.com/leanprover-community/mathlib4.git nightly-testing-$MOST_RECENT_NIGHTLY)
+
+            if [[ -n "$REMOTE_BRANCHES" ]]; then
+              echo "... and Mathlib has a 'nightly-testing-$MOST_RECENT_NIGHTLY' branch."
+              MESSAGE=""
+            else
+              echo "... but Mathlib does not yet have a 'nightly-testing-$MOST_RECENT_NIGHTLY' branch."
+              MESSAGE="- ❗ Mathlib CI can not be attempted yet, as the 'nightly-testing-$MOST_RECENT_NIGHTLY' branch does not exist there yet. We will retry when you push more commits. It may be necessary to rebase onto 'nightly' tomorrow."
+            fi
+
+          else
+            echo "The most recently nightly tag on this branch has SHA: $NIGHTLY_SHA"
+            echo "but 'git merge-base origin/master HEAD' reported: $MERGE_BASE_SHA"
+            git -C lean4.git log -10 origin/master
+
+            MESSAGE="- ❗ Mathlib CI will not be attempted unless you rebase your PR onto the 'nightly' branch."
+          fi
+
+          if [[ -n "$MESSAGE" ]]; then
+
+            echo "Checking existing messages"
+
+            # Use GitHub API to check if a comment already exists
+            existing_comment=$(curl -L -s -H "Authorization: token ${{ secrets.MATHLIB4_BOT }}" \
+                                    -H "Accept: application/vnd.github.v3+json" \
+                                    "https://api.github.com/repos/leanprover/lean4/issues/${{ steps.workflow-info.outputs.pullRequestNumber }}/comments" \
+                                    | jq '.[] | select(.body | startswith("- ❗ Mathlib") or startswith("- ✅ Mathlib") or startswith("- ❌ Mathlib") or startswith("- 💥 Mathlib") or startswith("- 🟡 Mathlib"))')
+            existing_comment_id=$(echo "$existing_comment" | jq -r .id)
+            existing_comment_body=$(echo "$existing_comment" | jq -r .body)
+
+            if [[ "$existing_comment_body" != *"$MESSAGE"* ]]; then
+              MESSAGE="$MESSAGE ($(date "+%Y-%m-%d %H:%M:%S"))"
+
+              echo "Posting message to the comments: $MESSAGE"
+
+              # Append new result to the existing comment or post a new comment
+              # It's essential we use the MATHLIB4_BOT token here, so that Mathlib CI can subsequently edit the comment.
+              if [ -z "$existing_comment_id" ]; then
+                # Post new comment with a bullet point
+                echo "Posting as new comment at leanprover/lean4/issues/${{ steps.workflow-info.outputs.pullRequestNumber }}/comments"
+                curl -L -s \
+                  -X POST \
+                  -H "Authorization: token ${{ secrets.MATHLIB4_BOT }}" \
+                  -H "Accept: application/vnd.github.v3+json" \
+                  -d "$(jq --null-input --arg val "$MESSAGE" '{"body": $val}')" \
+                  "https://api.github.com/repos/leanprover/lean4/issues/${{ steps.workflow-info.outputs.pullRequestNumber }}/comments"
+              else
+                # Append new result to the existing comment
+                echo "Appending to existing comment at leanprover/lean4/issues/${{ steps.workflow-info.outputs.pullRequestNumber }}/comments"
+                curl -L -s \
+                  -X PATCH \
+                  -H "Authorization: token ${{ secrets.MATHLIB4_BOT }}" \
+                  -H "Accept: application/vnd.github.v3+json" \
+                  -d "$(jq --null-input --arg existing "$existing_comment_body" --arg message "$MESSAGE" '{"body":($existing + "\n" + $message)}')" \
+                  "https://api.github.com/repos/leanprover/lean4/issues/comments/$existing_comment_id"
+              fi
+            else
+              echo "The message already exists in the comment body."
+            fi
+            echo "mathlib_ready=false" >> $GITHUB_OUTPUT
+          else
+            echo "mathlib_ready=true" >> $GITHUB_OUTPUT
+          fi
 
       # We next automatically create a Mathlib branch using this toolchain.
       # Mathlib CI will be responsible for reporting back success or failure
       # to the PR comments asynchronously.
       - name: Cleanup workspace
-        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
+        if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.ready.outputs.mathlib_ready == 'true'
         run: |
           sudo rm -rf *
 
       # Checkout the mathlib4 repository with all branches
       - name: Checkout mathlib4 repository
-        uses: actions/checkout@v2
+        if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.ready.outputs.mathlib_ready == 'true'
+        uses: actions/checkout@v3
         with:
           repository: leanprover-community/mathlib4
           token: ${{ secrets.MATHLIB4_BOT }}
-          ref: nightly-testing # This is more likely than `master` to work with the base of this PR.
-          fetch-depth: 0
+          ref: nightly-testing
+          fetch-depth: 0 # This ensures we check out all tags and branches.
 
       - name: Check if branch exists
-        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
+        if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.ready.outputs.mathlib_ready == 'true'
         id: check_branch
         run: |
           git config user.name "leanprover-community-mathlib4-bot"
           git config user.email "leanprover-community-mathlib4-bot@users.noreply.github.com"
 
+          if git branch -r | grep -q "nightly-testing-${MOST_RECENT_NIGHTLY}"; then
+            BASE=nightly-testing-${MOST_RECENT_NIGHTLY}
+          else
+            echo "This shouldn't be possible: couldn't find a 'nightly-testing-${MOST_RECENT_NIGHTLY}' branch at Mathlib. Falling back to 'nightly-testing'."
+            BASE=nightly-testing
+          fi
+
+          echo "Using base branch: $BASE"
+
+          git checkout $BASE
+
           EXISTS=$(git ls-remote --heads origin lean-pr-testing-${{ steps.workflow-info.outputs.pullRequestNumber }} | wc -l)
           echo "Branch exists: $EXISTS"
           if [ "$EXISTS" = "0" ]; then
@@ -112,15 +219,12 @@ jobs:
           else
             echo "Branch already exists, pushing an empty commit."
             git checkout lean-pr-testing-${{ steps.workflow-info.outputs.pullRequestNumber }}
-            # The Mathlib `nightly-testing` branch may have moved since this branch was created, so merge their changes.
-            # If the base of this Lean4 PR becomes significantly older than the nightly being used by `nightly-testing`
-            # this will cause breakages rather than fixing them!
-            # Without cumbersome requirements that Lean4 PRs are based off nightlies, I'm not sure there is a perfect solution here.
-            git merge nightly-testing --strategy-option ours --no-commit --allow-unrelated-histories
+            # The Mathlib `nightly-testing` or `nightly-testing-YYYY-MM-DD` branch may have moved since this branch was created, so merge their changes.
+            git merge $BASE --strategy-option ours --no-commit --allow-unrelated-histories
             git commit --allow-empty -m "Trigger CI for https://github.com/leanprover/lean4/pull/${{ steps.workflow-info.outputs.pullRequestNumber }}"
           fi
 
       - name: Push changes
-        if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}
+        if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.ready.outputs.mathlib_ready == 'true'
         run: |
           git push origin lean-pr-testing-${{ steps.workflow-info.outputs.pullRequestNumber }}

.github/workflows/pr-title.yml

@@ -0,0 +1,20 @@
+name: Check PR title for commit convention
+
+on:
+  merge_group:
+  pull_request:
+    types: [opened, synchronize, reopened, edited]
+
+jobs:
+  check-pr-title:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check PR title
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const msg = context.payload.pull_request? context.payload.pull_request.title : context.payload.merge_group.head_commit.message;
+            console.log(`Message: ${msg}`)
+            if (!/^(feat|fix|doc|style|refactor|test|chore|perf): .*[^.]($|\n\n)/.test(msg)) {
+              core.setFailed('PR title does not follow the Commit Convention (https://leanprover.github.io/lean4/doc/dev/commit_convention.html).');
+            }

.github/workflows/pr.yml

@@ -1,31 +0,0 @@
-name: sanity-check opened PRs
-
-on:
-  # needs read/write GH token, do *not* execute arbitrary code from PR
-  pull_request_target:
-    types: [opened]
-
-jobs:
-  check-pr:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Check Commit Message
-        uses: actions/github-script@v6
-        with:
-          github-token: ${{ secrets.GITHUB_TOKEN }}
-          script: |
-            const { data: commits } = await github.rest.pulls.listCommits({
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              pull_number: context.issue.number,
-            });
-            console.log(commits[0].commit.message);
-            // check first commit only (and only once) since later commits might be intended to be squashed away
-            if (!/^(feat|fix|doc|style|refactor|test|chore|perf): .*[^.]($|\n\n)/.test(commits[0].commit.message)) {
-              await github.rest.issues.createComment({
-                owner: context.repo.owner,
-                repo: context.repo.repo,
-                issue_number: context.issue.number,
-                body: 'Thanks for your contribution! Please make sure to follow our [Commit Convention](https://leanprover.github.io/lean4/doc/dev/commit_convention.html).',
-              });
-            }

.github/workflows/update-stage0.yml

@@ -0,0 +1,64 @@
+name: Update stage0
+
+# This action will update stage0 on master as soon as
+# src/stdlib_flags.h and stage0/src/stdlib_flags.h
+# are out of sync there, or when manually triggered.
+# The update bypasses the merge queue to be quick.
+# Also see <doc/dev/bootstrap.md>.
+
+on:
+  push:
+    branches:
+      - 'master'
+  workflow_dispatch:
+
+concurrency:
+  group: stage0
+  cancel-in-progress: true
+
+jobs:
+  update-stage0:
+    runs-on: ubuntu-latest
+    steps:
+    # This action should push to an otherwise protected branch, so it
+    # uses a deploy key with write permissions, as suggested at
+    # https://stackoverflow.com/a/76135647/946226
+    - uses: actions/checkout@v3
+      with:
+        ssh-key: ${{secrets.STAGE0_SSH_KEY}}
+    - run: echo "should_update_stage0=yes" >> "$GITHUB_ENV"
+    - name: Check if automatic update is needed
+      if: github.event_name == 'push'
+      run: |
+        if diff -u src/stdlib_flags.h stage0/src/stdlib_flags.h
+        then
+          echo "src/stdlib_flags.h and stage0/src/stdlib_flags.h agree, nothing to do"
+          echo "should_update_stage0=no" >> "$GITHUB_ENV"
+        fi
+    - name: Setup git user
+      if: env.should_update_stage0 == 'yes'
+      run: |
+          git config --global user.name "Lean stage0 autoupdater"
+          git config --global user.email "<>"
+    - if: env.should_update_stage0 == 'yes'
+      uses: DeterminateSystems/nix-installer-action@main
+    # Would be nice, but does not work yet:
+    # https://github.com/DeterminateSystems/magic-nix-cache/issues/39
+    # This action does not run that often and building runs in a few minutes, so ok for now
+    #- if: env.should_update_stage0 == 'yes'
+    #  uses: DeterminateSystems/magic-nix-cache-action@v2
+    - if: env.should_update_stage0 == 'yes'
+      name: Install Cachix
+      uses: cachix/cachix-action@v12
+      with:
+        name: lean4
+    - if: env.should_update_stage0 == 'yes'
+      run: nix run .#update-stage0-commit
+    - if: env.should_update_stage0 == 'yes'
+      run: git show --stat
+    - if: env.should_update_stage0 == 'yes' && github.event_name == 'push'
+      name: Sanity check # to avoid loops
+      run: |
+        diff -u src/stdlib_flags.h stage0/src/stdlib_flags.h || exit 1
+    - if: env.should_update_stage0 == 'yes'
+      run: git push origin

.gitignore

@@ -2,6 +2,8 @@
 \#*
 .#*
 *.lock
+.lake
+lake-manifest.json
 build
 !/src/lake/Lake/Build
 GPATH

.vscode/settings.json

@@ -1,7 +0,0 @@
-{
-    "files.insertFinalNewline": true,
-    "files.trimTrailingWhitespace": true,
-    "[markdown]": {
-        "rewrap.wrappingColumn": 70
-    }
-}

CMakeLists.txt

@@ -11,7 +11,7 @@ foreach(var ${vars})
     list(APPEND STAGE0_ARGS "-D${CMAKE_MATCH_1}=${${var}}")
   elseif("${currentHelpString}" MATCHES "No help, variable specified on the command line." OR "${currentHelpString}" STREQUAL "")
     list(APPEND CL_ARGS "-D${var}=${${var}}")
-    if("${var}" STREQUAL "USE_GMP")
+    if("${var}" MATCHES "USE_GMP|CHECK_OLEAN_VERSION")
       # must forward options that generate incompatible .olean format
       list(APPEND STAGE0_ARGS "-D${var}=${${var}}")
     endif()
@@ -35,6 +35,8 @@ ExternalProject_add(stage0
   SOURCE_SUBDIR src
   BINARY_DIR stage0
   # do not rebuild stage0 when git hash changes; it's not from this commit anyway
+  # (however, `CHECK_OLEAN_VERSION=ON` in CI will override this as we need to
+  # embed the githash into the stage 1 library built by stage 0)
   CMAKE_ARGS -DSTAGE=0 -DUSE_GITHASH=OFF ${PLATFORM_ARGS} ${STAGE0_ARGS}
   BUILD_ALWAYS ON  # cmake doesn't auto-detect changes without a download method
   INSTALL_COMMAND ""  # skip install

CODEOWNERS

@@ -0,0 +1,22 @@
+# Code Owners
+#
+# Documents responsible people per component.
+# Listed persons will automatically be asked by GitHub to review a PR touching these paths.
+# If multiple names are listed, a review by any of them is considered sufficient by default.
+
+/.github/ @Kha @semorrison
+/RELEASES.md @semorrison
+/src/ @leodemoura @Kha
+/src/Init/IO.lean @joehendrix
+/src/kernel/ @leodemoura
+/src/lake/ @tydeu
+/src/Lean/Compiler/ @leodemoura
+/src/Lean/Data/Lsp/ @mhuisi
+/src/Lean/Elab/Deriving/ @semorrison
+/src/Lean/Elab/Tactic/ @semorrison
+/src/Lean/Meta/Tactic/ @leodemoura
+/src/Lean/Parser/ @Kha
+/src/Lean/PrettyPrinter/ @Kha
+/src/Lean/Server/ @mhuisi
+/src/Lean/Widget/ @Vtec234
+/src/runtime/io.cpp @joehendrix

CONTRIBUTING.md

@@ -1,61 +1,79 @@
-# Contribution Guidelines
+External Contribution Guidelines
+============
 
-Thank you for your interest in contributing to Lean! There are many ways to contribute and we appreciate all of them.
+In the past, we accepted most pull requests. This practice produced hard to maintain code, performance problems, and bugs. In order to improve the quality and maintainability of our codebase, we've established the following guidelines for external contributions.
 
-## Bug reports
+Helpful links
+-------
 
-Bug reports as new issues are always welcome. Please check the existing [issues](https://github.com/leanprover/lean4/issues) first.
-Reduce the issue to a self-contained, reproducible test case.
-If you have the chance, before reporting a bug, please search existing issues, as it's possible that
-someone else has already reported your error.
-If you're not sure if something is a bug or not, feel free to file a bug anyway. You may also want to discuss it with the Lean
-community using the [lean4 Zulip channel](https://leanprover.zulipchat.com/#narrow/stream/270676-lean4).
+* [Development Setup](./doc/dev/index.md)
+* [Testing](./doc/dev/testing.md)
+* [Commit convention](./doc/dev/commit_convention.md)
 
-## Simple fixes
+Before You Submit a Pull Request (PR):
+-------
 
-Simple fixes for **typos and clear bugs** are welcome.
+**Start with an Issue**: Before submitting a PR, always open an issue discussing the problem you wish to solve or the feature you'd like to add. Use the prefix `RFC:` (request for comments) if you are proposing a new feature. Ask for feedback from other users. Take the time to summarize all the feedback. This allows the maintainers to evaluate your proposal more efficiently. When creating a RFC, consider the following questions:
 
-# **IMPORTANT**
+  - **User Experience**: How does this feature improve the user experience?
 
-We are currently overwhelmed. We respectfully request that you hold off on submitting Pull Requests and creating Request for Comments (RFCs) at this time. Our team is actively seeking funding to expand the Lean development team and improve our capacity to review and integrate contributions. We appreciate your understanding and look forward to being able to accept contributions in the near future. In the meantime, the process described in the following sections is temporarily suspended.
+  - **Beneficiaries**: Which Lean users and projects do benefit most from this feature/change?
 
-## Documentation
+  - **Community Feedback**: Have you sought feedback or insights from other Lean users?
 
-Tutorial-like examples are very welcome.
-They are useful for finding rough edges and bugs in Lean 4, for highlighting new features, and for showing how to use Lean.
-If you want to store your tutorial in the Lean 4 repository to make sure future changes will not break it, we suggest the following workflow:
-* Contact one of the Lean developers on Zulip, and check whether your tutorial is a good match for the Lean 4 repository.
-* Send bug reports and report rough edges. We will work with you until the tutorial looks great.
-* Add plenty of comments and make sure others will be able to follow it.
-* Create a pull request in the Lean 4 repository. After merging, we will link it to the official documentation and make sure it becomes part of our test suite.
+  - **Maintainability**: Will this change streamline code maintenance or simplify its structure?
 
-You can use `.lean` or `.md` files to create your tutorial. The `.md` files are ideal when you want to format your prose using markdown. For an example, see [this `.md` file](https://github.com/leanprover/lean4/blob/master/doc/lean3changes.md).
+**Understand the Project**: Familiarize yourself with the project, existing issues, and latest commits. Ensure your contribution aligns with the project's direction and priorities.
 
-Contributions to the reference manual are also welcome, but since Lean 4 is changing rapidly, please contact us first using Zulip
-to find out which parts are stable enough to document. We will work with you to get this kind of
-pull request merged. We are also happy to meet using Zoom, Skype or Google hangout to coordinate this kind of effort.
+**Stay Updated**: Regularly fetch and merge changes from the main branch to ensure your branch is up-to-date and can be smoothly integrated.
 
-As Lean 4 matures, other forms of documentation (e.g., doc-strings) will be welcome too.
+**Help wanted**: We have issues tagged with ["help wanted"](https://github.com/leanprover/lean4/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22), if you want to contribute to the project, please take a look at them. If you are interested in one of them, post comments, ask questions, and engage with the core developers there.
 
-## "Help wanted"
+Quality Over Quantity:
+-----
 
-For issues marked as [`help wanted`](https://github.com/leanprover/lean4/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22), pull requests (PR) are welcome and we will work with you to get a PR merged. Some of these issues are nontrivial. If you are interested, please consider adding comments to the issue and/or messaging the Lean developers in [Zulip](https://leanprover.zulipchat.com/#).
+**Focused Changes**: Each PR should address a single, clearly-defined issue or feature. Avoid making multiple unrelated changes in a single PR.
 
-## Unexpected Pull Requests
+**Write Tests**: Every new feature or bug fix should come with relevant tests. This ensures the robustness and reliability of the contribution.
 
-We have very few core developers, and we cannot review arbitrary pull requests (PRs). Moreover, many features involve subtle tradeoffs, and it may require significant time and energy to even assess a proposed design. We suggest the following workflow:
+**Documentation**: Update relevant documentation, including comments in the code, to explain the logic and reasoning behind your changes.
 
-* First, discuss your idea with the Lean community on Zulip. Ask the community to help collect examples, document the requirements, and detect complications.
-* If there is broad support, create a detailed issue for it on the Lean 4 repository at GitHub, and tag the issue with `RFC`.
-* Ask the community for help documenting the requirements, and for collecting examples and concerns.
-* Wait for one of the core developers to give you a "go ahead". At this point, the core developers will work with you to make sure your PR gets merged.
+Coding Standards:
+----
 
-We don't want to waste your time by you implementing a feature and then us not being able to merge it.
+**Follow the Code Style**: Ensure that your code follows the established coding style of the project.
 
-## How to Contribute
+**Lean on Lean**: Use Lean's built-in features and libraries effectively, avoiding reinventions.
 
-* Always follow the [commit convention](https://lean-lang.org/lean4/doc/dev/commit_convention.html).
-* Follow the style of the surrounding code. When in doubt, look at other files using the particular syntax as well.
-* Make sure your code is documented.
-* New features or bug fixes should come with appropriate tests.
-* Ensure all tests work before submitting a PR; see [Development Setup](https://lean-lang.org/lean4/doc/make/index.html#development-setup) and [Fixing Tests](https://lean-lang.org/lean4/doc/dev/fixing_tests.html).
+**Performance**: Make sure that your changes do not introduce performance regressions. If possible, optimize the solution for speed and resource usage.
+
+PR Submission:
+---
+
+**Descriptive Title and Summary**: The PR title should briefly explain the purpose of the PR. The summary should give more detailed information on what changes are made and why. Links to Zulip threads are not acceptable as a summary. You are responsible for summarizing the discussion, and getting support for it.
+
+**Follow the commit convention**: Pull requests are squash merged, and the
+commit message is taken from the pull request title and body, so make sure they adhere to the [commit convention](https://github.com/leanprover/lean4/blob/master/doc/dev/commit_convention.md). Put questions and extra information, which should not be part of the final commit message, into a first comment rather than the Pull Request description.
+Because the change will be squashed, there is no need to polish the commit messages and history on the branch.
+
+**Link to Relevant Issues**: Reference any issues that your PR addresses to provide context.
+
+**Stay Responsive**: Once the PR is submitted, stay responsive to feedback and be prepared to make necessary revisions. We will close any PR that has been inactive (no response or updates from the submitter) for more than a month.
+
+Reviews and Feedback:
+----
+
+**Be Patient**: Given the limited number of full-time maintainers and the volume of PRs, reviews may take some time.
+
+**Engage Constructively**: Always approach feedback positively and constructively. Remember, reviews are about ensuring the best quality for the project, not personal criticism.
+
+**Continuous Integration**: Ensure that all CI checks pass on your PR. Failed checks will delay the review process. The maintainers will not check PRs containing failures.
+
+What to Expect:
+----
+
+**Not All PRs Get Merged**: While we appreciate every contribution, not all PRs will be merged. Ensure your changes align with the project's goals and quality standards.
+
+**Feedback is a Gift**: It helps improve the project and can also help you grow as a developer or contributor.
+
+**Community Involvement**: Engage with the Lean community on our communication channels. This can lead to better collaboration and understanding of the project's direction.

README.md

@@ -14,7 +14,7 @@ and have just begun regular [stable point releases](https://github.com/leanprove
 - [Manual](https://lean-lang.org/lean4/doc/)
 - [Release notes](RELEASES.md) starting at v4.0.0-m3
 - [Examples](https://lean-lang.org/lean4/doc/examples.html)
-- [External Contribution Guidelines](https://github.com/leanprover/lean4/blob/master/doc/contributions.md)
+- [External Contribution Guidelines](CONTRIBUTING.md)
 - [FAQ](https://lean-lang.org/lean4/doc/faq.html)
 
 # Installation

RELEASES.md

@@ -5,18 +5,186 @@ There is not yet a strong guarantee of backwards compatibility between versions,
 only an expectation that breaking changes will be documented in this file.
 
 This file contains work-in-progress notes for the upcoming release, as well as previous stable releases.
-Please check the [releases](https://github.com/leanprover/lean4/releases) page for the current status of each version.
+Please check the [releases](https://github.com/leanprover/lean4/releases) page for the current status
+of each version.
 
-v4.3.0 (development in progress)
+v4.6.0 (development in progress)
 ---------
 
-* The derive handler for `DecidableEq` [now handles](https://github.com/leanprover/lean4/pull/2591) mutual inductive types.
-* [Show path of failed import in Lake](https://github.com/leanprover/lean4/pull/2616).
-* [Fix linker warnings on macOS](https://github.com/leanprover/lean4/pull/2598).
+v4.5.0
+---------
+
+* Modify the lexical syntax of string literals to have string gaps, which are escape sequences of the form `"\" newline whitespace*`.
+  These have the interpetation of an empty string and allow a string to flow across multiple lines without introducing additional whitespace.
+  The following is equivalent to `"this is a string"`.
+  ```lean
+  "this is \
+     a string"
+  ```
+  [PR #2821](https://github.com/leanprover/lean4/pull/2821) and [RFC #2838](https://github.com/leanprover/lean4/issues/2838).
+
+* Add raw string literal syntax. For example, `r"\n"` is equivalent to `"\\n"`, with no escape processing.
+  To include double quote characters in a raw string one can add sufficiently many `#` characters before and after
+  the bounding `"`s, as in `r#"the "the" is in quotes"#` for `"the \"the\" is in quotes"`.
+  [PR #2929](https://github.com/leanprover/lean4/pull/2929) and [issue #1422](https://github.com/leanprover/lean4/issues/1422).
+
+* The low-level `termination_by'` clause is no longer supported.
+
+  Migration guide: Use `termination_by` instead, e.g.:
+  ```diff
+  -termination_by' measure (fun ⟨i, _⟩ => as.size - i)
+  +termination_by go i _ => as.size - i
+  ```
+
+  If the well-founded relation you want to use is not the one that the
+  `WellFoundedRelation` type class would infer for your termination argument,
+  you can use `WellFounded.wrap` from the std libarary to explicitly give one:
+  ```diff
+  -termination_by' ⟨r, hwf⟩
+  +termination_by _ x => hwf.wrap x
+  ```
+
+* Support snippet edits in LSP `TextEdit`s. See `Lean.Lsp.SnippetString` for more details.
+
+* Deprecations and changes in the widget API.
+  - `Widget.UserWidgetDefinition` is deprecated in favour of `Widget.Module`. The annotation `@[widget]` is deprecated in favour of `@[widget_module]`. To migrate a definition of type `UserWidgetDefinition`, remove the `name` field and replace the type with `Widget.Module`. Removing the `name` results in a title bar no longer being drawn above your panel widget. To add it back, draw it as part of the component using `<details open=true><summary class='mv2 pointer'>{name}</summary>{rest_of_widget}</details>`. See an example migration [here](https://github.com/leanprover/std4/pull/475/files#diff-857376079661a0c28a53b7ff84701afabbdf529836a6944d106c5294f0e68109R43-R83).
+  - The new command `show_panel_widgets` allows displaying always-on and locally-on panel widgets.
+  - `RpcEncodable` widget props can now be stored in the infotree.
+  - See [RFC 2963](https://github.com/leanprover/lean4/issues/2963) for more details and motivation. 
+
+* If no usable lexicographic order can be found automatically for a termination proof, explain why.
+  See [feat: GuessLex: if no measure is found, explain why](https://github.com/leanprover/lean4/pull/2960).
+
+* Option to print [inferred termination argument](https://github.com/leanprover/lean4/pull/3012).
+  With `set_option showInferredTerminationBy true` you will get messages like
+  ```
+  Inferred termination argument:
+  termination_by
+  ackermann n m => (sizeOf n, sizeOf m)
+  ```
+  for automatically generated `termination_by` clauses.
+
+* More detailed error messages for [invalid mutual blocks](https://github.com/leanprover/lean4/pull/2949).
+
+* [Multiple](https://github.com/leanprover/lean4/pull/2923) [improvements](https://github.com/leanprover/lean4/pull/2969) to the output of `simp?` and `simp_all?`.
+
+* Tactics with `withLocation *` [no longer fail](https://github.com/leanprover/lean4/pull/2917) if they close the main goal.
+
+* Implementation of a `test_extern` command for writing tests for `@[extern]` and `@[implemented_by]` functions.
+  Usage is 
+  ```
+  import Lean.Util.TestExtern
+
+  test_extern Nat.add 17 37
+  ```
+  The head symbol must be the constant with the `@[extern]` or `@[implemented_by]` attribute. The return type must have a `DecidableEq` instance.
+
+Bug fixes for 
+[#2853](https://github.com/leanprover/lean4/issues/2853), [#2953](https://github.com/leanprover/lean4/issues/2953), [#2966](https://github.com/leanprover/lean4/issues/2966), 
+[#2971](https://github.com/leanprover/lean4/issues/2971), [#2990](https://github.com/leanprover/lean4/issues/2990), [#3094](https://github.com/leanprover/lean4/issues/3094).
+
+Bug fix for [eager evaluation of default value](https://github.com/leanprover/lean4/pull/3043) in `Option.getD`.
+Avoid [panic in `leanPosToLspPos`](https://github.com/leanprover/lean4/pull/3071) when file source is unavailable.
+Improve [short-circuiting behavior](https://github.com/leanprover/lean4/pull/2972) for `List.all` and `List.any`.
+
+Several Lake bug fixes: [#3036](https://github.com/leanprover/lean4/issues/3036), [#3064](https://github.com/leanprover/lean4/issues/3064), [#3069](https://github.com/leanprover/lean4/issues/3069).
+
+v4.4.0
+---------
+
+* Lake and the language server now support per-package server options using the `moreServerOptions` config field, as well as options that apply to both the language server and `lean` using the `leanOptions` config field. Setting either of these fields instead of `moreServerArgs` ensures that viewing files from a dependency uses the options for that dependency. Additionally, `moreServerArgs` is being deprecated in favor of the `moreGlobalServerArgs` field. See PR [#2858](https://github.com/leanprover/lean4/pull/2858).
+  
+  A Lakefile with the following deprecated package declaration:
+  ```lean
+  def moreServerArgs := #[
+    "-Dpp.unicode.fun=true"
+  ]
+  def moreLeanArgs := moreServerArgs
+  
+  package SomePackage where
+    moreServerArgs := moreServerArgs
+    moreLeanArgs := moreLeanArgs
+  ```
+  
+  ... can be updated to the following package declaration to use per-package options:
+  ```lean
+  package SomePackage where
+    leanOptions := #[⟨`pp.unicode.fun, true⟩]
+  ```
+* [Rename request handler](https://github.com/leanprover/lean4/pull/2462).
+* [Import auto-completion](https://github.com/leanprover/lean4/pull/2904).
+* [`pp.beta`` to apply beta reduction when pretty printing](https://github.com/leanprover/lean4/pull/2864).
+* [Embed and check githash in .olean](https://github.com/leanprover/lean4/pull/2766).
+* [Guess lexicographic order for well-founded recursion](https://github.com/leanprover/lean4/pull/2874).
+* [Allow trailing comma in tuples, lists, and tactics](https://github.com/leanprover/lean4/pull/2643).
+
+Bug fixes for [#2628](https://github.com/leanprover/lean4/issues/2628), [#2883](https://github.com/leanprover/lean4/issues/2883),
+[#2810](https://github.com/leanprover/lean4/issues/2810), [#2925](https://github.com/leanprover/lean4/issues/2925), and [#2914](https://github.com/leanprover/lean4/issues/2914).
+
+**Lake:**
+
+* `lake init .` and a bare `lake init` and will now use the current directory as the package name. [#2890](https://github.com/leanprover/lean4/pull/2890)
+* `lake new` and `lake init` will now produce errors on invalid package names such as `..`, `foo/bar`, `Init`, `Lean`, `Lake`, and `Main`. See issue [#2637](https://github.com/leanprover/lean4/issues/2637) and PR [#2890](https://github.com/leanprover/lean4/pull/2890).
+* `lean_lib` no longer converts its name to upper camel case (e.g., `lean_lib bar` will include modules named `bar.*` rather than `Bar.*`). See issue [#2567](https://github.com/leanprover/lean4/issues/2567) and PR [#2889](https://github.com/leanprover/lean4/pull/2889).
+* Lean and Lake now properly support non-identifier library names (e.g., `lake new 123-hello` and `import «123Hello»` now work correctly). See issue [#2865](https://github.com/leanprover/lean4/issues/2865) and PR [#2889](https://github.com/leanprover/lean4/pull/2888).
+* Lake now filters the environment extensions loaded from a compiled configuration (`lakefile.olean`) to include only those relevant to Lake's workspace loading process. This resolves segmentation faults caused by environment extension type mismatches (e.g., when defining custom elaborators via `elab` in configurations). See issue [#2632](https://github.com/leanprover/lean4/issues/2632) and PR [#2896](https://github.com/leanprover/lean4/pull/2896).
+* Cloud releases will now properly be re-unpacked if the build directory is removed. See PR [#2928](https://github.com/leanprover/lean4/pull/2928).
+* Lake's `math` template has been simplified. See PR [#2930](https://github.com/leanprover/lean4/pull/2930).
+* `lake exe <target>` now parses `target` like a build target (as the help text states it should) rather than as a basic name. For example, `lake exe @mathlib/runLinter` should now work. See PR [#2932](https://github.com/leanprover/lean4/pull/2932).
+* `lake new foo.bar [std]` now generates executables named `foo-bar` and `lake new foo.bar exe` properly creates `foo/bar.lean`. See PR [#2932](https://github.com/leanprover/lean4/pull/2932).
+* Later packages and libraries in the dependency tree are now preferred over earlier ones. That is, the later ones "shadow" the earlier ones. Such an ordering is more consistent with how declarations generally work in programming languages. This will break any package that relied on the previous ordering. See issue [#2548](https://github.com/leanprover/lean4/issues/2548) and PR [#2937](https://github.com/leanprover/lean4/pull/2937).
+* Executable roots are no longer mistakenly treated as importable. They will no longer be picked up by `findModule?`. See PR [#2937](https://github.com/leanprover/lean4/pull/2937).
+
+v4.3.0
+---------
+
+* `simp [f]` does not unfold partial applications of `f` anymore. See issue [#2042](https://github.com/leanprover/lean4/issues/2042).
+  To fix proofs affected by this change, use `unfold f` or `simp (config := { unfoldPartialApp := true }) [f]`.
+* By default, `simp` will no longer try to use Decidable instances to rewrite terms. In particular, not all decidable goals will be closed by `simp`, and the `decide` tactic may be useful in such cases. The `decide` simp configuration option can be used to locally restore the old `simp` behavior, as in `simp (config := {decide := true})`; this includes using Decidable instances to verify side goals such as numeric inequalities.
+
+* Many bug fixes:
+  * [Add left/right actions to term tree coercion elaborator and make `^`` a right action](https://github.com/leanprover/lean4/pull/2778)
+  * [Fix for #2775, don't catch max recursion depth errors](https://github.com/leanprover/lean4/pull/2790)
+  * [Reduction of `Decidable` instances very slow when using `cases` tactic](https://github.com/leanprover/lean4/issues/2552)
+  * [`simp` not rewriting in binder](https://github.com/leanprover/lean4/issues/1926)
+  * [`simp` unfolding `let` even with `zeta := false` option](https://github.com/leanprover/lean4/issues/2669)
+  * [`simp` (with beta/zeta disabled) and discrimination trees](https://github.com/leanprover/lean4/issues/2281)
+  * [unknown free variable introduced by `rw ... at h`](https://github.com/leanprover/lean4/issues/2711)
+  * [`dsimp` doesn't use `rfl` theorems which consist of an unapplied constant](https://github.com/leanprover/lean4/issues/2685)
+  * [`dsimp` does not close reflexive equality goals if they are wrapped in metadata](https://github.com/leanprover/lean4/issues/2514)
+  * [`rw [h]` uses `h` from the environment in preference to `h` from the local context](https://github.com/leanprover/lean4/issues/2729)
+  * [missing `withAssignableSyntheticOpaque` for `assumption` tactic](https://github.com/leanprover/lean4/issues/2361)
+  * [ignoring default value for field warning](https://github.com/leanprover/lean4/issues/2178)
+* [Cancel outstanding tasks on document edit in the language server](https://github.com/leanprover/lean4/pull/2648).
+* [Remove unnecessary `%` operations in `Fin.mod` and `Fin.div`](https://github.com/leanprover/lean4/pull/2688)
+* [Avoid `DecidableEq` in `Array.mem`](https://github.com/leanprover/lean4/pull/2774)
+* [Ensure `USize.size` unifies with `?m + 1`](https://github.com/leanprover/lean4/issues/1926)
+* [Improve compatibility with emacs eglot client](https://github.com/leanprover/lean4/pull/2721)
+
+**Lake:**
+
+* [Sensible defaults for `lake new MyProject math`](https://github.com/leanprover/lean4/pull/2770)
+* Changed `postUpdate?` configuration option to a `post_update` declaration. See the `post_update` syntax docstring for more information on the new syntax.
+* [A manifest is automatically created on workspace load if one does not exists.](https://github.com/leanprover/lean4/pull/2680).
+* The `:=` syntax for configuration declarations (i.e., `package`, `lean_lib`, and `lean_exe`) has been deprecated. For example, `package foo := {...}` is deprecated.
+* [support for overriding package URLs via `LAKE_PKG_URL_MAP`](https://github.com/leanprover/lean4/pull/2709)
+* Moved the default build directory (e.g., `build`), default packages directory (e.g., `lake-packages`), and the compiled configuration (e.g., `lakefile.olean`) into a new dedicated directory for Lake outputs, `.lake`. The cloud release build archives are also stored here, fixing [#2713](https://github.com/leanprover/lean4/issues/2713).
+* Update manifest format to version 7 (see [lean4#2801](https://github.com/leanprover/lean4/pull/2801) for details on the changes).
+* Deprecate the `manifestFile` field of a package configuration.
+* There is now a more rigorous check on `lakefile.olean` compatibility (see [#2842](https://github.com/leanprover/lean4/pull/2842) for more details).
 
 v4.2.0
 ---------
 
+* [isDefEq cache for terms not containing metavariables.](https://github.com/leanprover/lean4/pull/2644).
+* Make [`Environment.mk`](https://github.com/leanprover/lean4/pull/2604) and [`Environment.add`](https://github.com/leanprover/lean4/pull/2642) private, and add [`replay`](https://github.com/leanprover/lean4/pull/2617) as a safer alternative.
+* `IO.Process.output` no longer inherits the standard input of the caller.
+* [Do not inhibit caching](https://github.com/leanprover/lean4/pull/2612) of default-level `match` reduction.
+* [List the valid case tags](https://github.com/leanprover/lean4/pull/2629) when the user writes an invalid one.
+* The derive handler for `DecidableEq` [now handles](https://github.com/leanprover/lean4/pull/2591) mutual inductive types.
+* [Show path of failed import in Lake](https://github.com/leanprover/lean4/pull/2616).
+* [Fix linker warnings on macOS](https://github.com/leanprover/lean4/pull/2598).
+* **Lake:** Add `postUpdate?` package configuration option. Used by a package to specify some code which should be run after a successful `lake update` of the package or one of its downstream dependencies. ([lake#185](https://github.com/leanprover/lake/issues/185))
 * Improvements to Lake startup time ([#2572](https://github.com/leanprover/lean4/pull/2572), [#2573](https://github.com/leanprover/lean4/pull/2573))
 * `refine e` now replaces the main goal with metavariables which were created during elaboration of `e` and no longer captures pre-existing metavariables that occur in `e` ([#2502](https://github.com/leanprover/lean4/pull/2502)).
   * This is accomplished via changes to `withCollectingNewGoalsFrom`, which also affects `elabTermWithHoles`, `refine'`, `calc` (tactic), and `specialize`. Likewise, all of these now only include newly-created metavariables in their output.

doc/SUMMARY.md

@@ -4,7 +4,6 @@
 - [Tour of Lean](./tour.md)
 - [Setting Up Lean](./quickstart.md)
   - [Extended Setup Notes](./setup.md)
-  - [Nix Setup](./setup/nix.md)
 - [Theorem Proving in Lean](./tpil.md)
 - [Functional Programming in Lean](fplean.md)
 - [Examples](./examples.md)
@@ -86,7 +85,6 @@
   - [macOS Setup](./make/osx-10.9.md)
   - [Windows MSYS2 Setup](./make/msys2.md)
   - [Windows with WSL](./make/wsl.md)
-  - [Nix Setup (*Experimental*)](./make/nix.md)
 - [Bootstrapping](./dev/bootstrap.md)
 - [Testing](./dev/testing.md)
 - [Debugging](./dev/debugging.md)

doc/contributions.md

@@ -1,68 +0,0 @@
-External Contribution Guidelines
-============
-
-**In the past, we accepted most pull requests. This practice produced hard to maintain code, performance problems, and bugs.** In order to improve the quality and maintainability of our codebase, we've established the following guidelines for external contributions.
-
-Before You Submit a Pull Request (PR):
--------
-
-**Start with an Issue**: Before submitting a PR, always open an issue discussing the problem you wish to solve or the feature you'd like to add. Use the prefix `RFC:` (request for comments) if you are proposing a new feature. Ask for feedback from other users. Take the time to summarize all the feedback. This allows the maintainers to evaluate your proposal more efficiently. When creating a RFC, consider the following questions:
-
-  - **User Experience**: How does this feature improve the user experience?
-
-  - **Beneficiaries**: Which Lean users and projects do benefit most from this feature/change?
-
-  - **Community Feedback**: Have you sought feedback or insights from other Lean users?
-
-  - **Maintainability**: Will this change streamline code maintenance or simplify its structure?
-
-**Understand the Project**: Familiarize yourself with the project, existing issues, and latest commits. Ensure your contribution aligns with the project's direction and priorities.
-
-**Stay Updated**: Regularly fetch and merge changes from the main branch to ensure your branch is up-to-date and can be smoothly integrated.
-
-**Help wanted**: We have issues tagged with ["help wanted"](https://github.com/leanprover/lean4/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22), if you want to contribute to the project, please take a look at them. If you are interested in one of them, post comments, ask questions, and engage with the core developers there.
-
-Quality Over Quantity:
------
-
-**Focused Changes**: Each PR should address a single, clearly-defined issue or feature. Avoid making multiple unrelated changes in a single PR.
-
-**Write Tests**: Every new feature or bug fix should come with relevant tests. This ensures the robustness and reliability of the contribution.
-
-**Documentation**: Update relevant documentation, including comments in the code, to explain the logic and reasoning behind your changes.
-
-Coding Standards:
-----
-
-**Follow the Code Style**: Ensure that your code follows the established coding style of the project.
-
-**Lean on Lean**: Use Lean's built-in features and libraries effectively, avoiding reinventions.
-
-**Performance**: Make sure that your changes do not introduce performance regressions. If possible, optimize the solution for speed and resource usage.
-
-PR Submission:
----
-
-**Descriptive Title and Summary**: The PR title should briefly explain the purpose of the PR. The summary should give more detailed information on what changes are made and why. Links to Zulip threads are not acceptable as a summary. You are responsible for summarizing the discussion, and getting support for it.
-
-**Link to Relevant Issues**: Reference any issues that your PR addresses to provide context.
-
-**Stay Responsive**: Once the PR is submitted, stay responsive to feedback and be prepared to make necessary revisions. We will close any PR that has been inactive (no response or updates from the submitter) for more than a month.
-
-Reviews and Feedback:
-----
-
-**Be Patient**: Given the limited number of full-time maintainers and the volume of PRs, reviews may take some time.
-
-**Engage Constructively**: Always approach feedback positively and constructively. Remember, reviews are about ensuring the best quality for the project, not personal criticism.
-
-**Continuous Integration**: Ensure that all CI checks pass on your PR. Failed checks will delay the review process. The maintainers will not check PRs containing failures.
-
-What to Expect:
-----
-
-**Not All PRs Get Merged**: While we appreciate every contribution, not all PRs will be merged. Ensure your changes align with the project's goals and quality standards.
-
-**Feedback is a Gift**: It helps improve the project and can also help you grow as a developer or contributor.
-
-**Community Involvement**: Engage with the Lean community on our communication channels. This can lead to better collaboration and understanding of the project's direction.

doc/dev/bootstrap.md

@@ -65,9 +65,36 @@ You now have a Lean binary and library that include your changes, though their
 own compilation was not influenced by them, that you can use to test your
 changes on test programs whose compilation *will* be influenced by the changes.
 
+## Updating stage0
+
 Finally, when we want to use new language features in the library, we need to
-update the stage 0 compiler, which can be done via `make -C stageN update-stage0`.
-`make update-stage0` without `-C` defaults to stage1.
+update the archived C source code of the stage 0 compiler in `stage0/src`.
+
+The github repository will automatically update stage0 on `master` once
+`src/stdlib_flags.h` and `stage0/src/stdlib_flags.h` are out of sync.
+
+If you have write access to the lean4 repository, you can also also manually
+trigger that process, for example to be able to use new features in the compiler itself.
+You can do that on <https://github.com/nomeata/lean4/actions/workflows/update-stage0.yml>
+or using Github CLI with
+```
+gh workflow run update-stage0.yml
+```
+
+Leaving stage0 updates to the CI automation is preferrable, but should you need
+to do it locally, you can use `make update-stage0` in `build/release`, to
+update `stage0` from `stage1`, `make -C stageN update-stage0` to update from
+another stage, or `nix run .#update-stage0-commit` to update using nix.
+
+Updates to `stage0` should be their own commits in the Git history. So should
+you have to include the stage0 update in your PR (rather than using above
+automation after merging changes), commit your work before running `make
+update-stage0`, commit the updated `stage0` compiler code with the commit
+message:
+```
+chore: update stage0
+```
+and coordinate with the admins to not squash your PR.
 
 ## Further Bootstrapping Complications
 

doc/dev/commit_convention.md

@@ -1,10 +1,15 @@
 Git Commit Convention
 =====================
 
-We are using the following convention for writing git-commit messages.
-It is based on the one from AngularJS project([doc][angularjs-doc],
+We are using the following convention for writing git commit messages. For pull
+requests, make sure the pull request title and description follow this
+convention, as the squash-merge commit will inherit title and body from the
+pull request.
+
+This convention is based on the one from the AngularJS project ([doc][angularjs-doc],
 [commits][angularjs-git]).
 
+
 [angularjs-git]: https://github.com/angular/angular.js/commits/master
 [angularjs-doc]: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#
 

doc/dev/index.md

@@ -1,6 +1,6 @@
 # Development Workflow
 
-If you want to make changes to Lean itself, start by [building Lean](../make/index.html) from a clean checkout to make sure that everything is set up correctly.
+If you want to make changes to Lean itself, start by [building Lean](../make/index.md) from a clean checkout to make sure that everything is set up correctly.
 After that, read on below to find out how to set up your editor for changing the Lean source code, followed by further sections of the development manual where applicable such as on the [test suite](testing.md) and [commit convention](commit_convention.md).
 
 If you are planning to make any changes that may affect the compilation of Lean itself, e.g. changes to the parser, elaborator, or compiler, you should first read about the [bootstrapping pipeline](bootstrap.md).
@@ -30,20 +30,14 @@ powershell -f elan-init.ps1 --default-toolchain none
 del elan-init.ps1
 ```
 
-You can use `elan toolchain link` to give a specific stage build
-directory a reference name, then use `elan override set` to associate
-such a name to the current directory. We usually want to use `stage0`
-for editing files in `src` and `stage1` for everything else (e.g.
-tests).
+The `lean-toolchain` files in the Lean 4 repository are set up to use the `lean4-stage0`
+toolchain for editing files in `src` and the `lean4` toolchain for editing files in `tests`.
+
+Run the following commands to make `lean4` point at `stage1` and `lean4-stage0` point at `stage0`:
 ```bash
 # in the Lean rootdir
 elan toolchain link lean4 build/release/stage1
 elan toolchain link lean4-stage0 build/release/stage0
-# make `lean` etc. point to stage1 in the rootdir and subdirs
-elan override set lean4
-cd src
-# make `lean` etc. point to stage0 anywhere inside `src`
-elan override set lean4-stage0
 ```
 
 You can also use the `+toolchain` shorthand (e.g. `lean +lean4-debug`) to switch
@@ -64,3 +58,19 @@ simply by pushing a tag to your fork of the Lean 4 github repository
 If you push `my-tag` to a fork in your github account `my_name`,
 you can then put `my_name/lean4:my-tag` in your `lean-toolchain` file in a project using `lake`.
 (You must use a tag name that does not start with a numeral, or contain `_`).
+
+### VS Code
+
+There is a `lean.code-workspace` file that correctly sets up VS Code with workspace roots for the stage0/stage1 setup described above as well as with other settings.
+You should always load it when working on Lean, such as by invoking
+```
+code lean.code-workspace
+```
+on the command line.
+
+### `ccache`
+
+Lean's build process uses [`ccache`](https://ccache.dev/) if it is
+installed to speed up recompilation of the generated C code. Without
+`ccache`, you'll likely spend more time than necessary waiting on
+rebuilds - it's a good idea to make sure it's installed.

doc/dev/testing.md

@@ -5,7 +5,6 @@ After [building Lean](../make/index.md) you can run all the tests using
 cd build/release
 make test ARGS=-j4
 ```
-
 Change the 4 to the maximum number of parallel tests you want to
 allow. The best choice is the number of CPU cores on your machine as
 the tests are mostly CPU bound.  You can find the number of processors
@@ -17,6 +16,12 @@ adding the `-C stageN` argument. The default when run as above is stage 1.  The
 Lean tests will automatically use that stage's corresponding Lean
 executables
 
+Running `make test` will not pick up new test files; run
+```bash
+cmake build/release/stage1
+```
+to update the list of tests.
+
 You can also use `ctest` directly if you are in the right folder.  So
 to run stage1 tests with a 300 second timeout run this:
 
@@ -24,6 +29,9 @@ to run stage1 tests with a 300 second timeout run this:
 cd build/release/stage1
 ctest -j 4 --output-on-failure --timeout 300
 ```
+Useful `ctest` flags are `-R <name of test>` to run a single test, and
+`--rerun-failed` to run all tests that failed during the last run.
+You can also pass `ctest` flags via `make test ARGS="--rerun-failed"`.
 
 To get verbose output from ctest pass the `--verbose` command line
 option. Test output is normally suppressed and only summary
@@ -124,8 +132,3 @@ outputs. `meld` can also be used to repair the problems.
 
 In Emacs, we can also execute `M-x lean4-diff-test-file` to check/diff the file of the current buffer.
 To mass-copy all `.produced.out` files to the respective `.expected.out` file, use `tests/lean/copy-produced`.
-When using the Nix setup, add `--keep-failed` to the `nix build` call and then call
-```sh
-tests/lean/copy-produced <build-dir>/source/tests/lean
-```
-instead where `<build-dir>` is the path printed out by `nix build`.

doc/examples/widgets.lean

@@ -15,9 +15,8 @@ sections of a Lean document. User widgets are rendered in the Lean infoview.
 To try it out, simply type in the following code and place your cursor over the `#widget` command.
 -/
 
-@[widget]
-def helloWidget : UserWidgetDefinition where
-  name := "Hello"
+@[widget_module]
+def helloWidget : Widget.Module where
   javascript := "
     import * as React from 'react';
     export default function(props) {
@@ -25,7 +24,7 @@ def helloWidget : UserWidgetDefinition where
       return React.createElement('p', {}, name + '!')
     }"
 
-#widget helloWidget .null
+#widget helloWidget
 
 /-!
 If you want to dive into a full sample right away, check out
@@ -56,7 +55,11 @@ to the React component. In our first invocation of `#widget`, we set it to `.nul
 happens when you type in:
 -/
 
-#widget helloWidget (Json.mkObj [("name", "<your name here>")])
+structure HelloWidgetProps where
+  name? : Option String := none
+  deriving Server.RpcEncodable
+
+#widget helloWidget with { name? := "<your name here>" : HelloWidgetProps }
 
 /-!
 💡 NOTE: The RPC system presented below does not depend on JavaScript. However the primary use case
@@ -132,9 +135,8 @@ on this we either display an `InteractiveCode` with the type, `mapRpcError` the
 to turn it into a readable message, or show a `Loading..` message, respectively.
 -/
 
-@[widget]
-def checkWidget : UserWidgetDefinition where
-  name := "#check as a service"
+@[widget_module]
+def checkWidget : Widget.Module where
   javascript := "
 import * as React from 'react';
 const e = React.createElement;
@@ -160,7 +162,7 @@ export default function(props) {
 Finally we can try out the widget.
 -/
 
-#widget checkWidget .null
+#widget checkWidget
 
 /-!
 ![`#check` as a service](../images/widgets_caas.png)
@@ -193,9 +195,8 @@ interact with the text editor.
 You can see the full API for this [here](https://github.com/leanprover/vscode-lean4/blob/master/lean4-infoview-api/src/infoviewApi.ts#L52)
 -/
 
-@[widget]
-def insertTextWidget : UserWidgetDefinition where
-  name := "textInserter"
+@[widget_module]
+def insertTextWidget : Widget.Module where
   javascript := "
 import * as React from 'react';
 const e = React.createElement;
@@ -213,4 +214,4 @@ export default function(props) {
 
 /-! Finally, we can try this out: -/
 
-#widget insertTextWidget .null
+#widget insertTextWidget

doc/lexical_structure.md

@@ -8,7 +8,7 @@ A Lean program consists of a stream of UTF-8 tokens where each token
 is one of the following:
 
 ```
-   token: symbol | command | ident | string | char | numeral |
+   token: symbol | command | ident | string | raw_string | char | numeral |
         : decimal | doc_comment | mod_doc_comment | field_notation
 ```
 
@@ -79,15 +79,35 @@ special characters:
 [Unicode table](https://unicode-table.com/en/) so "\xA9 Copyright 2021" is "© Copyright 2021".
 - `\uHHHH` puts the character represented by the 4 digit hexadecimal into the string, so the following
 string "\u65e5\u672c" will become "日本" which means "Japan".
+- `\` followed by a newline and then any amount of whitespace is a "gap" that is equivalent to the empty string,
+useful for letting a string literal span across multiple lines. Gaps spanning multiple lines can be confusing,
+so the parser raises an error if the trailing whitespace contains any newlines.
 
 So the complete syntax is:
 
 ```
    string       : '"' string_item '"'
-   string_item  : string_char | string_escape
-   string_char  : [^\\]
-   string_escape: "\" ("\" | '"' | "'" | "n" | "t" | "x" hex_char{2} | "u" hex_char{4} )
+   string_item  : string_char | char_escape | string_gap
+   string_char  : [^"\\]
+   char_escape  : "\" ("\" | '"' | "'" | "n" | "t" | "x" hex_char{2} | "u" hex_char{4})
    hex_char     : [0-9a-fA-F]
+   string_gap   : "\" newline whitespace*
+```
+
+Raw String Literals
+===================
+
+Raw string literals are string literals without any escape character processing.
+They begin with `r##...#"` (with zero or more `#` characters) and end with `"#...##` (with the same number of `#` characters).
+The contents of a raw string literal may contain `"##..#` so long as the number of `#` characters
+is less than the number of `#` characters used to begin the raw string literal.
+
+```
+   raw_string          : raw_string_aux(0) | raw_string_aux(1) | raw_string_aux(2) | ...
+   raw_string_aux(n)   : 'r' '#'{n} '"' raw_string_item '"' '#'{n}
+   raw_string_item(n)  : raw_string_char | raw_string_quote(n)
+   raw_string_char     : [^"]
+   raw_string_quote(n) : '"' '#'{0..n-1}
 ```
 
 Char Literals
@@ -96,7 +116,9 @@ Char Literals
 Char literals are enclosed by single quotes (``'``).
 
 ```
-   char: "'" string_item "'"
+   char      : "'" char_item "'"
+   char_item : char_char | char_escape
+   char_char : [^'\\]
 ```
 
 Numeric Literals

doc/make/index.md

@@ -14,8 +14,6 @@ Platform-Specific Setup
 - [Windows (WSL)](wsl.md)
 - [macOS (homebrew)](osx-10.9.md)
 - Linux/macOS/WSL via [Nix](https://nixos.org/nix/): Call `nix-shell` in the project root. That's it.
-- There is also an [**experimental** setup based purely on Nix](nix.md) that works fundamentally differently from the
-  make/CMake setup described on this page.
 
 Generic Build Instructions
 --------------------------

doc/make/nix.md

@@ -1,110 +0,0 @@
-# Building with Nix
-
-While [Nix](https://nixos.org/nix/) can be used to quickly open a shell with all dependencies for the [standard setup](index.md) installed, the user-facing [Nix Setup](../setup.md#nix-setup) can also be used to work *on* Lean.
-
-## Setup
-
-Follow the setup in the link above; to open the Lean shell inside a Lean checkout, you can also use
-```bash
-# in the Lean root directory
-$ nix-shell -A nix
-```
-
-On top of the local and remote Nix cache, we do still rely on CCache as well to make C/C++ build steps incremental, which are atomic steps from Nix's point of view.
-To enable CCache, add the following line to the config file mentioned in the setup:
-```bash
-extra-sandbox-paths = /nix/var/cache/ccache
-```
-Then set up that directory as follows:
-```bash
-sudo mkdir -m0770 -p /nix/var/cache/ccache
-# macOS standard chown doesn't support --reference
-nix shell .#nixpkgs.coreutils -c sudo chown --reference=/nix/store /nix/var/cache/ccache
-```
-
-## Basic Build Commands
-
-From the Lean root directory inside the Lean shell:
-```bash
-nix build .#stage1  # build this stage's stdlib & executable
-nix build .#stage1.test  # run all tests
-nix run .#stage1.update-stage0  # update ./stage0 from this stage
-nix run .#stage1.update-stage0-commit  # ...and commit the results
-```
-The `stage1.` part in each command is optional:
-```bash
-nix build .#test  # run tests for stage 1
-nix build .  # build stage 1
-nix build  # ditto
-```
-
-## Build Process Description
-
-The Nix build process conceptually works the same as described in [Lean Build Pipeline](index.md#lean-build-pipeline).
-However, there are two important differences in practice apart from the standard Nix properties (hermeneutic, reproducible builds stored in a global hash-indexed store etc.):
-* Only files tracked by git (using `git add` or at least `git add --intent-to-add`) are compiled.
-This is actually a general property of Nix flakes, and has the benefit of making it basically impossible to forget to commit a file (at least in `src/`).
-* Only files reachable from `src/Lean.lean` are compiled.
-This is because modules are discovered not from a directory listing anymore but by recursively compiling all dependencies of that top module.
-
-## Editor Integration
-
-As in the standard Nix setup.
-After adding `src/` as an LSP workspace, it should automatically fall back to using stage 0 in there.
-
-Note that the UX of `{emacs,vscode}-dev` is quite different from the Make-based setup regarding the compilation of dependencies:
-there is no mutable directory incrementally filled by the build that we could point the editor at for .olean files.
-Instead, `emacs-dev` will gather the individual dependency outputs from the Nix store when checking a file -- and build them on the fly when necessary.
-However, it will only ever load changes saved to disk, not ones opened in other buffers.
-
-The absence of a mutable output directory also means that the Lean server will not automatically pick up `.ilean` metadata from newly compiled files.
-Instead, you can run `nix run .#link-ilean` to symlink the `.ilean` tree of the stdlib state at that point in time to `src/build/lib`, where the server should automatically find them.
-
-## Other Fun Stuff to Do with Nix
-
-Open Emacs with Lean set up from an arbitrary commit (without even cloning Lean beforehand... if your Nix is new enough):
-```bash
-nix run github:leanprover/lean4/7e4edeb#emacs-package
-```
-
-Open a shell with `lean` and `LEAN_PATH` set up for compiling a specific module (this is exactly what `emacs-dev` is doing internally):
-```bash
-nix develop .#mods.\"Lean.Parser.Basic\"
-# alternatively, directly pass a command to execute:
-nix develop .#stage2.mods.\"Init.Control.Basic\" -c bash -c 'lean $src -Dtrace.Elab.command=true'
-```
-
-Not sure what you just broke? Run Lean from (e.g.) the previous commit on a file:
-```bash
-nix run .\?rev=$(git rev-parse @^) scratch.lean
-```
-
-Work on two adjacent stages at the same time without the need for repeatedly updating and reverting `stage0/`:
-```bash
-# open an editor that will use only committed changes (so first commit them when changing files)
-nix run .#HEAD-as-stage1.emacs-dev&
-# open a second editor that will use those committed changes as stage 0
-# (so don't commit changes done here until you are done and ran a final `update-stage0-commit`)
-nix run .#HEAD-as-stage0.emacs-dev&
-```
-To run `nix build` on the second stage outside of the second editor, use
-```bash
-nix build .#stage0-from-input --override-input lean-stage0 .\?rev=$(git rev-parse HEAD)
-```
-This setup will inadvertently change your `flake.lock` file, which you can revert when you are done.
-
-...more surely to come...
-
-## Debugging
-
-Since Nix copies all source files before compilation, you will need to map debug symbols back to the original path using `set substitute-path` in GDB.
-For example, for a build on Linux with the Nix sandbox activated:
-```bash
-(gdb) f
-#1  0x0000000000d23a4f in lean_inc (o=0x1) at /build/source/build/include/lean/lean.h:562
-562	/build/source/build/include/lean/lean.h: No such file or directory.
-(gdb) set substitute-path /build/source/build src
-(gdb) f
-#1  0x0000000000d23a4f in lean_inc (o=0x1) at /build/source/build/include/lean/lean.h:562
-562	static inline void lean_inc(lean_object * o) { if (!lean_is_scalar(o)) lean_inc_ref(o); }
-```

doc/quickstart.md

@@ -1,55 +1,18 @@
 # Quickstart
 
-These instructions will walk you through setting up Lean using the "basic" setup and VS Code as the editor.
-See [Setup](./setup.md) for other ways, supported platforms, and more details on setting up Lean.
-
-See quick [walkthrough demo video](https://www.youtube.com/watch?v=yZo6k48L0VY).
+These instructions will walk you through setting up Lean 4 together with VS Code as an editor for Lean 4.
+See [Setup](./setup.md) for supported platforms and other ways to set up Lean 4.
 
 1. Install [VS Code](https://code.visualstudio.com/).
 
-1. Launch VS Code and install the `lean4` extension.
+1. Launch VS Code and install the `lean4` extension by clicking on the "Extensions" sidebar entry and searching for "lean4".
 
     ![installing the vscode-lean4 extension](images/code-ext.png)
 
-1. Create a new file using "File > New Text File" (`Ctrl+N`). Click the `Select a language` prompt, type in `lean4`, and hit ENTER.  You should see the following popup:
-    ![elan](images/install_elan.png)
+1. Open the Lean 4 setup guide by creating a new text file using "File > New Text File" (`Ctrl+N`), clicking on the ∀-symbol in the top right and selecting "Documentation… > Setup: Show Setup Guide".
 
-    Click the "Install Lean using Elan" button. You should see some progress output like this:
+    ![show setup guide](images/show-setup-guide.png)
 
-    ```
-    info: syncing channel updates for 'stable'
-    info: latest update on stable, lean version v4.0.0
-    info: downloading component 'lean'
-    ```
-    If there is no popup, you probably have Elan installed already.
-    You may want to make sure that your default toolchain is Lean 4 in this case by running `elan default leanprover/lean4:stable` and reopen the file, as the next step will fail otherwise.
+1. Follow the Lean 4 setup guide. It will walk you through learning resources for Lean 4, teach you how to set up Lean's dependencies on your platform, install Lean 4 for you at the click of a button and help you set up your first project.
 
-1. While it is installing, you can paste the following Lean program into the new file:
-
-    ```lean
-    #eval Lean.versionString
-    ```
-
-    When the installation has finished, the Lean Language Server should start automatically and you should get syntax-highlighting and a "Lean Infoview" popping up on the right.  You will see the output of the `#eval` statement when
-    you place your cursor at the end of the statement.
-
-    ![successful setup](images/code-success.png)
-
-You are set up!
-
-## Create a Lean Project
-
-*If your goal is to contribute to [mathlib4](https://github.com/leanprover-community/mathlib4) or use it as a dependency, please see its readme for specific instructions on how to do that.*
-
-You can now create a Lean project in a new folder. Run `lake init foo` from "View > Terminal" to create a package, followed by `lake build` to get an executable version of your Lean program.
-On Linux/macOS, you first have to follow the instructions printed by the Lean installation or log out and in again for the Lean executables to be available in you terminal.
-
-Note: Packages **have** to be opened using "File > Open Folder..." for imports to work.
-Saved changes are visible in other files after running "Lean 4: Refresh File Dependencies" (`Ctrl+Shift+X`).
-
-## Troubleshooting
-
-**The InfoView says "Waiting for Lean server to start..." forever.**
-
-Check that the VS Code Terminal is not showing some installation errors from `elan`.
-If that doesn't work, try also running the VS Code command `Developer: Reload Window`.
+    ![setup guide](images/setup_guide.png)

doc/setup.md

@@ -17,6 +17,8 @@ Issue reports and fixes are welcome.
 
 * aarch64 Linux with glibc 2.27+
 * aarch64 (Apple Silicon) macOS
+* x86 (32-bit) Linux
+* Emscripten Web Assembly
 
 <!--
 ### Tier 3
@@ -48,10 +50,10 @@ Foo.lean       # main file, import via `import Foo`
 Foo/
   A.lean       # further files, import via e.g. `import Foo.A`
   A/...        # further nesting
-build/         # `lake` build output directory
+.lake/         # `lake` build output directory
 ```
 
-After running `lake build` you will see a binary named `./build/bin/foo` and when you run it you should see the output:
+After running `lake build` you will see a binary named `./.lake/build/bin/foo` and when you run it you should see the output:
 ```
 Hello, world!
 ```

doc/setup/nix.md

@@ -1,71 +0,0 @@
-# Nix Setup
-
-An alternative setup based on Nix provides a perfectly reproducible development environment for your project from the Lean version down to the editor and Lean extension.
-However, it is still experimental and subject to change; in particular, it is heavily based on an unreleased version of Nix enabling [Nix Flakes](https://www.tweag.io/blog/2020-05-25-flakes/). The setup has been tested on NixOS, other Linux distributions, and macOS.
-
-After installing (any version of) Nix (<https://nixos.org/download.html>), you can easily open a shell with the particular pre-release version of Nix needed by and tested with our setup (called the "Lean shell" from here on):
-```bash
-$ nix-shell https://github.com/leanprover/lean4/archive/master.tar.gz -A nix
-```
-While this shell is sufficient for executing the steps below, it is recommended to also set the following options in `/etc/nix/nix.conf` (`nix.extraOptions` in NixOS):
-```
-max-jobs = auto  # Allow building multiple derivations in parallel
-keep-outputs = true  # Do not garbage-collect build time-only dependencies (e.g. clang)
-# Allow fetching build results from the Lean Cachix cache
-trusted-substituters = https://lean4.cachix.org/
-trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= lean4.cachix.org-1:mawtxSxcaiWE24xCXXgh3qnvlTkyU7evRRnGeAhD4Wk=
-```
-On a multi-user installation of Nix (the default), you need to restart the Nix daemon afterwards:
-```bash
-sudo pkill nix-daemon
-```
-
-The [Cachix](https://cachix.org/) integration will magically beam any build steps already executed by the CI right onto your machine when calling Nix commands in the shell opened above.
-It can be set up analogously as a cache for your own project.
-
-Note: Your system Nix might print warnings about not knowing some of the settings used by the Lean shell Nix, which can be ignored.
-
-## Basic Commands
-
-From a Lean shell, run
-```bash
-$ nix flake new mypkg -t github:leanprover/lean4
-```
-to create a new Lean package in directory `mypkg` using the latest commit of Lean 4.
-Such packages follow the same directory layout as described in the standard setup, except for a `lakefile.lean` replaced by a `flake.nix` file set up so you can run Nix commands on it, for example:
-```bash
-$ nix build  # build package and all dependencies
-$ nix build .#executable  # compile `main` definition into executable (after you've added one)
-$ nix run .#emacs-dev  # open a pinned version of Emacs with lean4-mode fully set up
-$ nix run .#emacs-dev MyPackage.lean  # arguments can be passed as well, e.g. the file to open
-$ nix run .#vscode-dev MyPackage.lean  # ditto, using VS Code
-```
-Note that if you rename `MyPackage.lean`, you also have to adjust the `name` attribute in `flake.nix` accordingly.
-Also note that if you turn the package into a Git repository, only tracked files will be visible to Nix.
-
-As in the standard setup, changes need to be saved to be visible in other files, which have then to be invalidated via an editor command.
-
-If you don't want to or cannot start the pinned editor from Nix, e.g. because you're running Lean inside WSL/a container/on a different machine, you can manually point your editor at the `lean` wrapper script the commands above use internally:
-```bash
-$ nix build .#lean-dev -o result-lean-dev
-```
-The resulting `./result-lean-dev/bin/lean` script essentially runs `nix run .#lean` in the current project's root directory when you open a Lean file or use the "refresh dependencies" command such that the correct Lean version for that project is executed.
-This includes selecting the correct stage of Lean (which it will compile on the fly, though without progress output) if you are [working on Lean itself](./make/nix.md#editor-integration).
-
-Package dependencies can be added as further input flakes and passed to the `deps` list of `buildLeanPackage`. Example: <https://github.com/Kha/testpkg2/blob/master/flake.nix#L5>
-
-For hacking, it can be useful to temporarily override an input with a local checkout/different version of a dependency:
-```bash
-$ nix build --override-input somedep path/to/somedep
-```
-
-On a build error, Nix will show the last 10 lines of the output by default. You can pass `-L` to `nix build` to show all lines, or pass the shown `*.drv` path to `nix log` to show the full log after the fact.
-
-Keeping all outputs ever built on a machine alive can accumulate to quite impressive amounts of disk space, so you might want to trigger the Nix GC when `/nix/store/` has grown too large:
-```bash
-nix-collect-garbage
-```
-This will remove everything not reachable from "GC roots" such as the `./result` symlink created by `nix build`.
-
-Note that the package information in `flake.nix` is currently completely independent from `lakefile.lean` used in the standard setup.
-Unifying the two formats is TBD.

doc/syntax_highlight_in_latex.md

@@ -67,6 +67,9 @@ theorem funext {f₁ f₂ : ∀ (x : α), β x} (h : ∀ x, f₁ x = f₂ x) : f
 \end{document}
 ```
 
+If your version of `minted` is v2.7 or newer, but before v3.0,
+you will additionally need to follow the workaround described in https://github.com/gpoore/minted/issues/360.
+
 You can then compile `test.tex` by executing the following command:
 
 ```bash

doc/typeclass.md

@@ -99,11 +99,11 @@ Let us start with the first step of the program above, declaring an appropriate
 
 ```lean
 # namespace Ex
-class Inhabited (a : Type u) where
+class Inhabited (a : Sort u) where
   default : a
 
 #check @Inhabited.default
--- Inhabited.default : {a : Type u} → [self : Inhabited a] → a
+-- Inhabited.default : {a : Sort u} → [self : Inhabited a] → a
 # end Ex
 ```
 Note `Inhabited.default` doesn't have any explicit argument.
@@ -114,7 +114,7 @@ Now we populate the class with some instances:
 
 ```lean
 # namespace Ex
-# class Inhabited (a : Type _) where
+# class Inhabited (a : Sort _) where
 #  default : a
 instance : Inhabited Bool where
   default := true
@@ -138,7 +138,7 @@ instance : Inhabited Prop where
 You can use the command `export` to create the alias `default` for `Inhabited.default`
 ```lean
 # namespace Ex
-# class Inhabited (a : Type _) where
+# class Inhabited (a : Sort _) where
 #  default : a
 # instance : Inhabited Bool where
 #  default := true
@@ -174,7 +174,7 @@ instance [Inhabited a] [Inhabited b] : Inhabited (a × b) where
 With this added to the earlier instance declarations, type class instance can infer, for example, a default element of ``Nat × Bool``:
 ```lean
 # namespace Ex
-# class Inhabited (a : Type u) where
+# class Inhabited (a : Sort u) where
 #  default : a
 # instance : Inhabited Bool where
 #  default := true
@@ -191,8 +191,14 @@ instance [Inhabited a] [Inhabited b] : Inhabited (a × b) where
 ```
 Similarly, we can inhabit type function with suitable constant functions:
 ```lean
+# namespace Ex
+# class Inhabited (a : Sort u) where
+#  default : a
+# opaque default [Inhabited a] : a :=
+#  Inhabited.default
 instance [Inhabited b] : Inhabited (a -> b) where
   default := fun _ => default
+# end Ex
 ```
 As an exercise, try defining default instances for other types, such as `List` and `Sum` types.
 

lean-toolchain

@@ -0,0 +1 @@
+lean4

lean.code-workspace

@@ -0,0 +1,57 @@
+{
+	"folders": [
+		{
+			"path": "."
+		},
+		{
+			"path": "src"
+		},
+		{
+			"path": "tests"
+		}
+	],
+	"settings": {
+		"files.insertFinalNewline": true,
+		"files.trimTrailingWhitespace": true,
+		"cmake.buildDirectory": "${workspaceFolder}/build/release",
+		"cmake.generator": "Unix Makefiles",
+		"[markdown]": {
+			"rewrap.wrappingColumn": 70
+		},
+		"[lean4]": {
+			"editor.rulers": [
+				100
+			]
+		}
+	},
+	"tasks": {
+		"version": "2.0.0",
+		"tasks": [
+			{
+				"label": "build",
+				"type": "shell",
+				"command": "make -C build/release -j$(nproc 2>/dev/null || sysctl -n hw.logicalcpu 2>/dev/null || echo 4)",
+				"problemMatcher": [],
+				"group": {
+					"kind": "build",
+					"isDefault": true
+				}
+			},
+			{
+				"label": "test",
+				"type": "shell",
+				"command": "NPROC=$(nproc 2>/dev/null || sysctl -n hw.logicalcpu 2>/dev/null || echo 4); CTEST_OUTPUT_ON_FAILURE=1 make -C build/release test -j$NPROC ARGS=\"-j$NPROC\"",
+				"problemMatcher": [],
+				"group": {
+					"kind": "test",
+					"isDefault": true
+				}
+			}
+		]
+	},
+	"extensions": {
+		"recommendations": [
+			"leanprover.lean4"
+		]
+	}
+}

nix/bootstrap.nix

@@ -83,13 +83,13 @@ rec {
         # use same stage for retrieving dependencies
         lean-leanDeps = stage0;
         lean-final = self;
-        leanFlags = [ "-DwarningAsError=true" ];
       } ({
         src = src + "/src";
         roots = [ { mod = args.name; glob = "andSubmodules"; } ];
         fullSrc = src;
         srcPath = "$PWD/src:$PWD/src/lake";
         inherit debug;
+        leanFlags = [ "-DwarningAsError=true" ];
       } // args);
       Init' = build { name = "Init"; deps = []; };
       Lean' = build { name = "Lean"; deps = [ Init' ]; };

nix/lake-dev.in

@@ -10,7 +10,7 @@ function pebkac() {
 [[ $# -gt 0 ]] || pebkac
 case $1 in
     --version)
-        # minimum version for `lake server` with fallback
+        # minimum version for `lake serve` with fallback
         echo 3.1.0
         ;;
     print-paths)

src/CMakeLists.txt

@@ -9,7 +9,7 @@ endif()
 include(ExternalProject)
 project(LEAN CXX C)
 set(LEAN_VERSION_MAJOR 4)
-set(LEAN_VERSION_MINOR 3)
+set(LEAN_VERSION_MINOR 6)
 set(LEAN_VERSION_PATCH 0)
 set(LEAN_VERSION_IS_RELEASE 0)  # This number is 1 in the release revision, and 0 otherwise.
 set(LEAN_SPECIAL_VERSION_DESC "" CACHE STRING "Additional version description like 'nightly-2018-03-11'")
@@ -64,10 +64,10 @@ option(BSYMBOLIC "Link with -Bsymbolic to reduce call overhead in shared librari
 option(USE_GMP "USE_GMP" ON)
 
 # development-specific options
-option(CHECK_OLEAN_VERSION "Only load .olean files compiled with the current version of Lean" ON)
+option(CHECK_OLEAN_VERSION "Only load .olean files compiled with the current version of Lean" OFF)
 
 set(LEAN_EXTRA_MAKE_OPTS  ""                           CACHE STRING "extra options to lean --make")
-set(LEANC_CC              "cc"                         CACHE STRING "C compiler to use in `leanc`")
+set(LEANC_CC              ${CMAKE_C_COMPILER}          CACHE STRING "C compiler to use in `leanc`")
 
 if ("${LAZY_RC}" MATCHES "ON")
   set(LEAN_LAZY_RC "#define LEAN_LAZY_RC")
@@ -93,8 +93,9 @@ if ("${RUNTIME_STATS}" MATCHES "ON")
   string(APPEND LEAN_EXTRA_CXX_FLAGS " -D LEAN_RUNTIME_STATS")
 endif()
 
-if (NOT("${CHECK_OLEAN_VERSION}" MATCHES "ON"))
-  string(APPEND LEAN_EXTRA_CXX_FLAGS " -D LEAN_IGNORE_OLEAN_VERSION")
+if ("${CHECK_OLEAN_VERSION}" MATCHES "ON")
+  set(USE_GITHASH ON)
+  string(APPEND LEAN_EXTRA_CXX_FLAGS " -D LEAN_CHECK_OLEAN_VERSION")
 endif()
 
 if(${CMAKE_SYSTEM_NAME} MATCHES "Emscripten")
@@ -401,26 +402,17 @@ if(MULTI_THREAD AND NOT MSVC AND (NOT ("${CMAKE_SYSTEM_NAME}" MATCHES "Darwin"))
 endif()
 
 # Git HASH
-set(LEAN_PACKAGE_VERSION "NOT-FOUND")
 if(USE_GITHASH)
   include(GetGitRevisionDescription)
   get_git_head_revision(GIT_REFSPEC GIT_SHA1)
   if(${GIT_SHA1} MATCHES "GITDIR-NOTFOUND")
     message(STATUS "Failed to read git_sha1")
     set(GIT_SHA1 "")
-    if(EXISTS "${LEAN_SOURCE_DIR}/bin/package_version")
-      file(STRINGS "${LEAN_SOURCE_DIR}/bin/package_version" LEAN_PACKAGE_VERSION)
-      message(STATUS "Package version detected: ${LEAN_PACKAGE_VERSION}")
-    endif()
   else()
     message(STATUS "git commit sha1: ${GIT_SHA1}")
   endif()
 else()
   set(GIT_SHA1 "")
-  if(EXISTS "${LEAN_SOURCE_DIR}/bin/package_version")
-    file(STRINGS "${LEAN_SOURCE_DIR}/bin/package_version" LEAN_PACKAGE_VERSION)
-    message(STATUS "Package version detected: ${LEAN_PACKAGE_VERSION}")
-  endif()
 endif()
 configure_file("${LEAN_SOURCE_DIR}/githash.h.in" "${LEAN_BINARY_DIR}/githash.h")
 
@@ -447,12 +439,13 @@ include_directories(${LEAN_SOURCE_DIR})
 include_directories(${CMAKE_BINARY_DIR})  # version.h etc., "private" headers
 include_directories(${CMAKE_BINARY_DIR}/include)  # config.h etc., "public" headers
 
+# Use CMake profile C++ flags for building Lean libraries, but do not embed in `leanc`
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
-# These are used in lean.mk (and libleanrt) and passed through by stdlib.make
-# They are not embedded into `leanc` since they are build profile/machine specific
 string(APPEND LEANC_OPTS " ${CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE}}")
+
+# Do embed flag for finding system libraries in dev builds
 if(CMAKE_OSX_SYSROOT AND NOT LEAN_STANDALONE)
-  string(APPEND LEANC_OPTS " ${CMAKE_CXX_SYSROOT_FLAG}${CMAKE_OSX_SYSROOT}")
+  string(APPEND LEANC_EXTRA_FLAGS " ${CMAKE_CXX_SYSROOT_FLAG}${CMAKE_OSX_SYSROOT}")
 endif()
 
 if(${STAGE} GREATER 1)

src/Init.lean

@@ -17,6 +17,7 @@ import Init.System
 import Init.Util
 import Init.Dynamic
 import Init.ShareCommon
+import Init.MetaTypes
 import Init.Meta
 import Init.NotationExtra
 import Init.SimpLemmas

src/Init/Classical.lean

@@ -22,7 +22,7 @@ noncomputable def choose {α : Sort u} {p : α → Prop} (h : ∃ x, p x) : α :
 theorem choose_spec {α : Sort u} {p : α → Prop} (h : ∃ x, p x) : p (choose h) :=
   (indefiniteDescription p h).property
 
-/-- Diaconescu's theorem: excluded middle from choice, Function extensionality and propositional extensionality. -/
+/-- **Diaconescu's theorem**: excluded middle from choice, Function extensionality and propositional extensionality. -/
 theorem em (p : Prop) : p ∨ ¬p :=
   let U (x : Prop) : Prop := x = True ∨ p
   let V (x : Prop) : Prop := x = False ∨ p

src/Init/Data/Array/Basic.lean

@@ -328,8 +328,9 @@ unsafe def anyMUnsafe {α : Type u} {m : Type → Type w} [Monad m] (p : α →
       else
         any (i+1) stop
   if start < stop then
-    if stop ≤ as.size then
-      any (USize.ofNat start) (USize.ofNat stop)
+    let stop' := min stop as.size
+    if start < stop' then
+      any (USize.ofNat start) (USize.ofNat stop')
     else
       pure false
   else
@@ -467,6 +468,9 @@ def elem [BEq α] (a : α) (as : Array α) : Bool :=
     else
       (true, r)
 
+/-- Convert a `Array α` into an `List α`. This is O(n) in the size of the array.  -/
+-- This function is exported to C, where it is called by `Array.data`
+-- (the projection) to implement this functionality.
 @[export lean_array_to_list]
 def toList (as : Array α) : List α :=
   as.foldr List.cons []

src/Init/Data/Array/Mem.lean

@@ -1,7 +1,7 @@
 /-
 Copyright (c) 2022 Microsoft Corporation. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
-Authors: Leonardo de Moura
+Authors: Leonardo de Moura, Joachim Breitner
 -/
 prelude
 import Init.Data.Array.Basic
@@ -20,32 +20,26 @@ theorem List.sizeOf_get_lt [SizeOf α] (as : List α) (i : Fin as.length) : size
 
 namespace Array
 
-instance [DecidableEq α] : Membership α (Array α) where
-  mem a as := as.contains a
+/-- `a ∈ as` is a predicate which asserts that `a` is in the array `as`. -/
+-- NB: This is defined as a structure rather than a plain def so that a lemma
+-- like `sizeOf_lt_of_mem` will not apply with no actual arrays around.
+structure Mem (a : α) (as : Array α) : Prop where
+  val : a ∈ as.data
+
+instance : Membership α (Array α) where
+  mem a as := Mem a as
 
 theorem sizeOf_get_lt [SizeOf α] (as : Array α) (i : Fin as.size) : sizeOf (as.get i) < sizeOf as := by
-  cases as; rename_i as
-  simp [get]
-  have ih := List.sizeOf_get_lt as i
-  exact Nat.lt_trans ih (by simp_arith)
+  cases as with | _ as =>
+  exact Nat.lt_trans (List.sizeOf_get_lt as i) (by simp_arith)
 
-theorem sizeOf_lt_of_mem [DecidableEq α] [SizeOf α] {as : Array α} (h : a ∈ as) : sizeOf a < sizeOf as := by
-  simp [Membership.mem, contains, any, Id.run, BEq.beq, anyM] at h
-  let rec aux (j : Nat) (h : anyM.loop (m := Id) (fun b => decide (a = b)) as as.size (Nat.le_refl ..) j = true) : sizeOf a < sizeOf as := by
-    unfold anyM.loop at h
-    split at h
-    · simp [Bind.bind, pure] at h; split at h
-      next he => subst a; apply sizeOf_get_lt
-      next => have ih := aux (j+1) h; assumption
-    · contradiction
-  apply aux 0 h
-termination_by aux j _ => as.size - j
+theorem sizeOf_lt_of_mem [SizeOf α] {as : Array α} (h : a ∈ as) : sizeOf a < sizeOf as := by
+  cases as with | _ as =>
+  exact Nat.lt_trans (List.sizeOf_lt_of_mem h.val) (by simp_arith)
 
 @[simp] theorem sizeOf_get [SizeOf α] (as : Array α) (i : Fin as.size) : sizeOf (as.get i) < sizeOf as := by
-  cases as
-  simp [get]
-  apply Nat.lt_trans (List.sizeOf_get ..)
-  simp_arith
+  cases as with | _ as =>
+  exact Nat.lt_trans (List.sizeOf_get ..) (by simp_arith)
 
 /-- This tactic, added to the `decreasing_trivial` toolbox, proves that
 `sizeOf arr[i] < sizeOf arr`, which is useful for well founded recursions
@@ -57,4 +51,17 @@ macro "array_get_dec" : tactic =>
 
 macro_rules | `(tactic| decreasing_trivial) => `(tactic| array_get_dec)
 
+/-- This tactic, added to the `decreasing_trivial` toolbox, proves that `sizeOf a < sizeOf arr`
+provided that `a ∈ arr` which is useful for well founded recursions over a nested inductive like
+`inductive T | mk : Array T → T`. -/
+-- NB: This is analogue to tactic `sizeOf_list_dec`
+macro "array_mem_dec" : tactic =>
+  `(tactic| first
+    | apply Array.sizeOf_lt_of_mem; assumption; done
+    | apply Nat.lt_trans (Array.sizeOf_lt_of_mem ?h)
+      case' h => assumption
+      simp_arith)
+
+macro_rules | `(tactic| decreasing_trivial) => `(tactic| array_mem_dec)
+
 end Array

src/Init/Data/ByteArray/Basic.lean

@@ -81,7 +81,7 @@ def isEmpty (s : ByteArray) : Bool :=
   If `exact` is `false`, the capacity will be doubled when grown. -/
 @[extern "lean_byte_array_copy_slice"]
 def copySlice (src : @& ByteArray) (srcOff : Nat) (dest : ByteArray) (destOff len : Nat) (exact : Bool := true) : ByteArray :=
-  ⟨dest.data.extract 0 destOff ++ src.data.extract srcOff (srcOff + len) ++ dest.data.extract (destOff + len) dest.data.size⟩
+  ⟨dest.data.extract 0 destOff ++ src.data.extract srcOff (srcOff + len) ++ dest.data.extract (destOff + min len (src.data.size - srcOff)) dest.data.size⟩
 
 def extract (a : ByteArray) (b e : Nat) : ByteArray :=
   a.copySlice b empty 0 (e - b)

src/Init/Data/Fin/Basic.lean

@@ -45,19 +45,19 @@ protected def sub : Fin n → Fin n → Fin n
   | ⟨a, h⟩, ⟨b, _⟩ => ⟨(a + (n - b)) % n, mlt h⟩
 
 /-!
-Remark: mod/div/modn/land/lor can be defined without using (% n), but
+Remark: land/lor can be defined without using (% n), but
 we are trying to minimize the number of Nat theorems
 needed to bootstrap Lean.
 -/
 
 protected def mod : Fin n → Fin n → Fin n
-  | ⟨a, h⟩, ⟨b, _⟩ => ⟨(a % b) % n, mlt h⟩
+  | ⟨a, h⟩, ⟨b, _⟩ => ⟨a % b,  Nat.lt_of_le_of_lt (Nat.mod_le _ _) h⟩
 
 protected def div : Fin n → Fin n → Fin n
-  | ⟨a, h⟩, ⟨b, _⟩ => ⟨(a / b) % n, mlt h⟩
+  | ⟨a, h⟩, ⟨b, _⟩ => ⟨a / b, Nat.lt_of_le_of_lt (Nat.div_le_self _ _) h⟩
 
 def modn : Fin n → Nat → Fin n
-  | ⟨a, h⟩, m => ⟨(a % m) % n, mlt h⟩
+  | ⟨a, h⟩, m => ⟨a % m, Nat.lt_of_le_of_lt (Nat.mod_le _ _) h⟩
 
 def land : Fin n → Fin n → Fin n
   | ⟨a, h⟩, ⟨b, _⟩ => ⟨(Nat.land a b) % n, mlt h⟩
@@ -110,7 +110,7 @@ theorem val_ne_of_ne {i j : Fin n} (h : i ≠ j) : val i ≠ val j :=
   fun h' => absurd (eq_of_val_eq h') h
 
 theorem modn_lt : ∀ {m : Nat} (i : Fin n), m > 0 → (modn i m).val < m
-  | _, ⟨_, _⟩, hp =>  Nat.lt_of_le_of_lt (mod_le _ _) (mod_lt _ hp)
+  | _, ⟨_, _⟩, hp =>  by simp [modn]; apply Nat.mod_lt; assumption
 
 theorem val_lt_of_le (i : Fin b) (h : b ≤ n) : i.val < n :=
   Nat.lt_of_lt_of_le i.isLt h

src/Init/Data/Float.lean

@@ -132,7 +132,7 @@ instance : ReprAtom Float  := ⟨⟩
 @[extern "round"] opaque Float.round : Float → Float
 @[extern "fabs"] opaque Float.abs : Float → Float
 
-instance : Pow Float Float := ⟨Float.pow⟩
+instance : HomogeneousPow Float := ⟨Float.pow⟩
 
 instance : Min Float := minOfLe
 

src/Init/Data/List/Basic.lean

@@ -6,9 +6,48 @@ Author: Leonardo de Moura
 prelude
 import Init.SimpLemmas
 import Init.Data.Nat.Basic
+import Init.Data.Nat.Div
 set_option linter.missingDocs true -- keep it documented
 open Decidable List
 
+/--
+The syntax `[a, b, c]` is shorthand for `a :: b :: c :: []`, or
+`List.cons a (List.cons b (List.cons c List.nil))`. It allows conveniently constructing
+list literals.
+
+For lists of length at least 64, an alternative desugaring strategy is used
+which uses let bindings as intermediates as in
+`let left := [d, e, f]; a :: b :: c :: left` to avoid creating very deep expressions.
+Note that this changes the order of evaluation, although it should not be observable
+unless you use side effecting operations like `dbg_trace`.
+-/
+syntax "[" withoutPosition(term,*,?) "]"  : term
+
+/--
+Auxiliary syntax for implementing `[$elem,*]` list literal syntax.
+The syntax `%[a,b,c|tail]` constructs a value equivalent to `a::b::c::tail`.
+It uses binary partitioning to construct a tree of intermediate let bindings as in
+`let left := [d, e, f]; a :: b :: c :: left` to avoid creating very deep expressions.
+-/
+syntax "%[" withoutPosition(term,*,? " | " term) "]" : term
+
+namespace Lean
+
+macro_rules
+  | `([ $elems,* ]) => do
+    -- NOTE: we do not have `TSepArray.getElems` yet at this point
+    let rec expandListLit (i : Nat) (skip : Bool) (result : TSyntax `term) : MacroM Syntax := do
+      match i, skip with
+      | 0,   _     => pure result
+      | i+1, true  => expandListLit i false result
+      | i+1, false => expandListLit i true  (← ``(List.cons $(⟨elems.elemsAndSeps.get! i⟩) $result))
+    let size := elems.elemsAndSeps.size
+    if size < 64 then
+      expandListLit size (size % 2 == 0) (← ``(List.nil))
+    else
+      `(%[ $elems,* | List.nil ])
+end Lean
+
 universe u v w
 
 variable {α : Type u} {β : Type v} {γ : Type w}
@@ -85,7 +124,8 @@ def appendTR (as bs : List α) : List α :=
   induction as with
   | nil  => rfl
   | cons a as ih =>
-    simp [reverseAux, List.append, ih, reverseAux_reverseAux]
+    rw [reverseAux, reverseAux_reverseAux]
+    simp [List.append, ih, reverseAux]
 
 instance : Append (List α) := ⟨List.append⟩
 
@@ -356,7 +396,7 @@ inductive Mem (a : α) : List α → Prop
 instance : Membership α (List α) where
   mem := Mem
 
-theorem mem_of_elem_eq_true [DecidableEq α] {a : α} {as : List α} : elem a as = true → a ∈ as := by
+theorem mem_of_elem_eq_true [BEq α] [LawfulBEq α] {a : α} {as : List α} : elem a as = true → a ∈ as := by
   match as with
   | [] => simp [elem]
   | a'::as =>
@@ -365,12 +405,12 @@ theorem mem_of_elem_eq_true [DecidableEq α] {a : α} {as : List α} : elem a as
     next h => intros; simp [BEq.beq] at h; subst h; apply Mem.head
     next _ => intro h; exact Mem.tail _ (mem_of_elem_eq_true h)
 
-theorem elem_eq_true_of_mem [DecidableEq α] {a : α} {as : List α} (h : a ∈ as) : elem a as = true := by
+theorem elem_eq_true_of_mem [BEq α] [LawfulBEq α] {a : α} {as : List α} (h : a ∈ as) : elem a as = true := by
   induction h with
   | head _ => simp [elem]
   | tail _ _ ih => simp [elem]; split; rfl; assumption
 
-instance [DecidableEq α] (a : α) (as : List α) : Decidable (a ∈ as) :=
+instance [BEq α] [LawfulBEq α] (a : α) (as : List α) : Decidable (a ∈ as) :=
   decidable_of_decidable_of_iff (Iff.intro mem_of_elem_eq_true elem_eq_true_of_mem)
 
 theorem mem_append_of_mem_left {a : α} {as : List α} (bs : List α) : a ∈ as → a ∈ as ++ bs := by
@@ -518,16 +558,22 @@ def takeWhile (p : α → Bool) : (xs : List α) → List α
 /--
 `O(|l|)`. Returns true if `p` is true for any element of `l`.
 * `any p [a, b, c] = p a || p b || p c`
+
+Short-circuits upon encountering the first `true`.
 -/
-@[inline] def any (l : List α) (p : α → Bool) : Bool :=
-  foldr (fun a r => p a || r) false l
+def any : List α -> (α → Bool) -> Bool
+  | [], _ => false
+  | h :: t, p => p h || any t p
 
 /--
 `O(|l|)`. Returns true if `p` is true for every element of `l`.
 * `all p [a, b, c] = p a && p b && p c`
+
+Short-circuits upon encountering the first `false`.
 -/
-@[inline] def all (l : List α) (p : α → Bool) : Bool :=
-  foldr (fun a r => p a && r) true l
+def all : List α -> (α → Bool) -> Bool
+  | [], _ => true
+  | h :: t, p => p h && all t p
 
 /--
 `O(|l|)`. Returns true if `true` is an element of the list of booleans `l`.

src/Init/Data/Nat/Basic.lean

@@ -629,7 +629,7 @@ protected theorem sub_lt_sub_left : ∀ {k m n : Nat}, k < m → k < n → m - n
 @[simp] protected theorem zero_sub (n : Nat) : 0 - n = 0 := by
   induction n with
   | zero => rfl
-  | succ n ih => simp [ih, Nat.sub_succ]
+  | succ n ih => simp only [ih, Nat.sub_succ]; decide
 
 protected theorem sub_self_add (n m : Nat) : n - (n + m) = 0 := by
   show (n + 0) - (n + m) = 0

src/Init/Data/UInt/Basic.lean

@@ -285,7 +285,7 @@ instance : Max UInt64 := maxOfLe
 instance : Min UInt64 := minOfLe
 
 theorem usize_size_gt_zero : USize.size > 0 :=
-  Nat.pos_pow_of_pos System.Platform.numBits (Nat.zero_lt_succ _)
+  Nat.zero_lt_succ ..
 
 @[extern "lean_usize_of_nat"]
 def USize.ofNat (n : @& Nat) : USize := ⟨Fin.ofNat' n usize_size_gt_zero⟩

src/Init/Meta.lean

@@ -6,6 +6,7 @@ Authors: Leonardo de Moura and Sebastian Ullrich
 Additional goodies for writing macros
 -/
 prelude
+import Init.MetaTypes
 import Init.Data.Array.Basic
 import Init.Data.Option.BasicAux
 
@@ -64,6 +65,15 @@ def toolchain :=
 @[extern "lean_internal_is_stage0"]
 opaque Internal.isStage0 (u : Unit) : Bool
 
+/--
+This function can be used to detect whether the compiler has support for
+generating LLVM instead of C. It is used by lake instead of the --features
+flag in order to avoid having to run a compiler for this every time on startup.
+See #2572.
+-/
+@[extern "lean_internal_has_llvm_backend"]
+opaque Internal.hasLLVMBackend (u : Unit) : Bool
+
 /-- Valid identifier names -/
 def isGreek (c : Char) : Bool :=
   0x391 ≤ c.val && c.val ≤ 0x3dd
@@ -217,11 +227,6 @@ instance : DecidableEq Name :=
 
 end Name
 
-structure NameGenerator where
-  namePrefix : Name := `_uniq
-  idx        : Nat  := 1
-  deriving Inhabited
-
 namespace NameGenerator
 
 @[inline] def curr (g : NameGenerator) : Name :=
@@ -452,11 +457,6 @@ end Syntax
   | none => x
   | some ref => withRef ref x
 
-/-- Syntax objects for a Lean module. -/
-structure Module where
-  header   : Syntax
-  commands : Array Syntax
-
 /--
   Expand macros in the given syntax.
   A node with kind `k` is visited only if `p k` is true.
@@ -773,6 +773,16 @@ def decodeQuotedChar (s : String) (i : String.Pos) : Option (Char × String.Pos)
   else
     none
 
+/--
+Decodes a valid string gap after the `\`.
+Note that this function matches `"\" whitespace+` rather than
+the more restrictive `"\" newline whitespace*` since this simplifies the implementation.
+Justification: this does not overlap with any other sequences beginning with `\`.
+-/
+def decodeStringGap (s : String) (i : String.Pos) : Option String.Pos := do
+  guard <| (s.get i).isWhitespace
+  s.nextWhile Char.isWhitespace (s.next i)
+
 partial def decodeStrLitAux (s : String) (i : String.Pos) (acc : String) : Option String := do
   let c := s.get i
   let i := s.next i
@@ -781,14 +791,49 @@ partial def decodeStrLitAux (s : String) (i : String.Pos) (acc : String) : Optio
   else if s.atEnd i then
     none
   else if c == '\\' then do
-    let (c, i) ← decodeQuotedChar s i
-    decodeStrLitAux s i (acc.push c)
+    if let some (c, i) := decodeQuotedChar s i then
+      decodeStrLitAux s i (acc.push c)
+    else if let some i := decodeStringGap s i then
+      decodeStrLitAux s i acc
+    else
+      none
   else
     decodeStrLitAux s i (acc.push c)
 
-def decodeStrLit (s : String) : Option String :=
-  decodeStrLitAux s ⟨1⟩ ""
+/--
+Takes a raw string literal, counts the number of `#`'s after the `r`, and interprets it as a string.
+The position `i` should start at `1`, which is the character after the leading `r`.
+The algorithm is simple: we are given `r##...#"...string..."##...#` with zero or more `#`s.
+By counting the number of leading `#`'s, we can extract the `...string...`.
+-/
+partial def decodeRawStrLitAux (s : String) (i : String.Pos) (num : Nat) : String :=
+  let c := s.get i
+  let i := s.next i
+  if c == '#' then
+    decodeRawStrLitAux s i (num + 1)
+  else
+    s.extract i ⟨s.utf8ByteSize - (num + 1)⟩
 
+/--
+Takes the string literal lexical syntax parsed by the parser and interprets it as a string.
+This is where escape sequences are processed for example.
+The string `s` is is either a plain string literal or a raw string literal.
+
+If it returns `none` then the string literal is ill-formed, which indicates a bug in the parser.
+The function is not required to return `none` if the string literal is ill-formed.
+-/
+def decodeStrLit (s : String) : Option String :=
+  if s.get 0 == 'r' then
+    decodeRawStrLitAux s ⟨1⟩ 0
+  else
+    decodeStrLitAux s ⟨1⟩ ""
+
+/--
+If the provided `Syntax` is a string literal, returns the string it represents.
+
+Even if the `Syntax` is a `str` node, the function may return `none` if its internally ill-formed.
+The parser should always create well-formed `str` nodes.
+-/
 def isStrLit? (stx : Syntax) : Option String :=
   match isLit? strLitKind stx with
   | some val => decodeStrLit val
@@ -1162,8 +1207,12 @@ private partial def decodeInterpStrLit (s : String) : Option String :=
     else if s.atEnd i then
       none
     else if c == '\\' then do
-      let (c, i) ← decodeInterpStrQuotedChar s i
-      loop i (acc.push c)
+      if let some (c, i) := decodeInterpStrQuotedChar s i then
+        loop i (acc.push c)
+      else if let some i := decodeStringGap s i then
+        loop i acc
+      else
+        none
     else
       loop i (acc.push c)
   loop ⟨1⟩ ""
@@ -1203,88 +1252,7 @@ end TSyntax
 
 namespace Meta
 
-inductive TransparencyMode where
-  | all | default | reducible | instances
-  deriving Inhabited, BEq, Repr
-
-inductive EtaStructMode where
-  /-- Enable eta for structure and classes. -/
-  | all
-  /-- Enable eta only for structures that are not classes. -/
-  | notClasses
-  /-- Disable eta for structures and classes. -/
-  | none
-  deriving Inhabited, BEq, Repr
-
-namespace DSimp
-
-structure Config where
-  zeta              : Bool := true
-  beta              : Bool := true
-  eta               : Bool := true
-  etaStruct         : EtaStructMode := .all
-  iota              : Bool := true
-  proj              : Bool := true
-  decide            : Bool := false
-  autoUnfold        : Bool := false
-  /-- If `failIfUnchanged := true`, then calls to `simp`, `dsimp`, or `simp_all`
-  will fail if they do not make progress. -/
-  failIfUnchanged   : Bool := true
-  deriving Inhabited, BEq, Repr
-
-end DSimp
-
-namespace Simp
-
-def defaultMaxSteps := 100000
-
-structure Config where
-  maxSteps          : Nat  := defaultMaxSteps
-  maxDischargeDepth : Nat  := 2
-  contextual        : Bool := false
-  memoize           : Bool := true
-  singlePass        : Bool := false
-  zeta              : Bool := true
-  beta              : Bool := true
-  eta               : Bool := true
-  etaStruct         : EtaStructMode := .all
-  iota              : Bool := true
-  proj              : Bool := true
-  decide            : Bool := true
-  arith             : Bool := false
-  autoUnfold        : Bool := false
-  /--
-    If `dsimp := true`, then switches to `dsimp` on dependent arguments where there is no congruence theorem that allows
-    `simp` to visit them. If `dsimp := false`, then argument is not visited.
-  -/
-  dsimp             : Bool := true
-  /-- If `failIfUnchanged := true`, then calls to `simp`, `dsimp`, or `simp_all`
-  will fail if they do not make progress. -/
-  failIfUnchanged   : Bool := true
-  deriving Inhabited, BEq, Repr
-
--- Configuration object for `simp_all`
-structure ConfigCtx extends Config where
-  contextual := true
-
-def neutralConfig : Simp.Config := {
-  zeta              := false
-  beta              := false
-  eta               := false
-  iota              := false
-  proj              := false
-  decide            := false
-  arith             := false
-  autoUnfold        := false
-}
-
-end Simp
-
-inductive Occurrences where
-  | all
-  | pos (idxs : List Nat)
-  | neg (idxs : List Nat)
-  deriving Inhabited, BEq
+deriving instance Repr for TransparencyMode, EtaStructMode, DSimp.Config, Simp.Config
 
 def Occurrences.contains : Occurrences → Nat → Bool
   | all,      _   => true
@@ -1342,14 +1310,14 @@ This will rewrite with all equation lemmas, which can be used to
 partially evaluate many definitions. -/
 declare_simp_like_tactic simpAutoUnfold "simp! " fun (c : Lean.Meta.Simp.Config) => { c with autoUnfold := true }
 
-/-- `simp_arith` is shorthand for `simp` with `arith := true`.
+/-- `simp_arith` is shorthand for `simp` with `arith := true` and `decide := true`.
 This enables the use of normalization by linear arithmetic. -/
-declare_simp_like_tactic simpArith "simp_arith " fun (c : Lean.Meta.Simp.Config) => { c with arith := true }
+declare_simp_like_tactic simpArith "simp_arith " fun (c : Lean.Meta.Simp.Config) => { c with arith := true, decide := true }
 
 /-- `simp_arith!` is shorthand for `simp_arith` with `autoUnfold := true`.
 This will rewrite with all equation lemmas, which can be used to
 partially evaluate many definitions. -/
-declare_simp_like_tactic simpArithAutoUnfold "simp_arith! " fun (c : Lean.Meta.Simp.Config) => { c with arith := true, autoUnfold := true }
+declare_simp_like_tactic simpArithAutoUnfold "simp_arith! " fun (c : Lean.Meta.Simp.Config) => { c with arith := true, autoUnfold := true, decide := true }
 
 /-- `simp_all!` is shorthand for `simp_all` with `autoUnfold := true`.
 This will rewrite with all equation lemmas, which can be used to
@@ -1357,10 +1325,10 @@ partially evaluate many definitions. -/
 declare_simp_like_tactic (all := true) simpAllAutoUnfold "simp_all! " fun (c : Lean.Meta.Simp.ConfigCtx) => { c with autoUnfold := true }
 
 /-- `simp_all_arith` combines the effects of `simp_all` and `simp_arith`. -/
-declare_simp_like_tactic (all := true) simpAllArith "simp_all_arith " fun (c : Lean.Meta.Simp.ConfigCtx) => { c with arith := true }
+declare_simp_like_tactic (all := true) simpAllArith "simp_all_arith " fun (c : Lean.Meta.Simp.ConfigCtx) => { c with arith := true, decide := true }
 
 /-- `simp_all_arith!` combines the effects of `simp_all`, `simp_arith` and `simp!`. -/
-declare_simp_like_tactic (all := true) simpAllArithAutoUnfold "simp_all_arith! " fun (c : Lean.Meta.Simp.ConfigCtx) => { c with arith := true, autoUnfold := true }
+declare_simp_like_tactic (all := true) simpAllArithAutoUnfold "simp_all_arith! " fun (c : Lean.Meta.Simp.ConfigCtx) => { c with arith := true, autoUnfold := true, decide := true }
 
 /-- `dsimp!` is shorthand for `dsimp` with `autoUnfold := true`.
 This will rewrite with all equation lemmas, which can be used to

src/Init/MetaTypes.lean

@@ -0,0 +1,124 @@
+/-
+Copyright (c) Leonardo de Moura. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Leonardo de Moura
+-/
+prelude
+import Init.Core
+
+namespace Lean
+
+structure NameGenerator where
+  namePrefix : Name := `_uniq
+  idx        : Nat  := 1
+  deriving Inhabited
+
+/-- Syntax objects for a Lean module. -/
+structure Module where
+  header   : Syntax
+  commands : Array Syntax
+
+namespace Meta
+
+inductive TransparencyMode where
+  /-- unfold all constants, even those tagged as `@[irreducible]`. -/
+  | all
+  /-- unfold all constants except those tagged as `@[irreducible]`. -/
+  | default
+  /-- unfold only constants tagged with the `@[reducible]` attribute. -/
+  | reducible
+  /-- unfold reducible constants and constants tagged with the `@[instance]` attribute. -/
+  | instances
+  deriving Inhabited, BEq
+
+inductive EtaStructMode where
+  /-- Enable eta for structure and classes. -/
+  | all
+  /-- Enable eta only for structures that are not classes. -/
+  | notClasses
+  /-- Disable eta for structures and classes. -/
+  | none
+  deriving Inhabited, BEq
+
+namespace DSimp
+
+structure Config where
+  zeta              : Bool := true
+  beta              : Bool := true
+  eta               : Bool := true
+  etaStruct         : EtaStructMode := .all
+  iota              : Bool := true
+  proj              : Bool := true
+  decide            : Bool := false
+  autoUnfold        : Bool := false
+  /-- If `failIfUnchanged := true`, then calls to `simp`, `dsimp`, or `simp_all`
+  will fail if they do not make progress. -/
+  failIfUnchanged   : Bool := true
+  /-- If `unfoldPartialApp := true`, then calls to `simp`, `dsimp`, or `simp_all`
+  will unfold even partial applications of `f` when we request `f` to be unfolded. -/
+  unfoldPartialApp  : Bool := false
+  deriving Inhabited, BEq
+
+end DSimp
+
+namespace Simp
+
+def defaultMaxSteps := 100000
+
+structure Config where
+  maxSteps          : Nat  := defaultMaxSteps
+  maxDischargeDepth : Nat  := 2
+  contextual        : Bool := false
+  memoize           : Bool := true
+  singlePass        : Bool := false
+  zeta              : Bool := true
+  beta              : Bool := true
+  eta               : Bool := true
+  etaStruct         : EtaStructMode := .all
+  iota              : Bool := true
+  proj              : Bool := true
+  decide            : Bool := false
+  arith             : Bool := false
+  autoUnfold        : Bool := false
+  /--
+    If `dsimp := true`, then switches to `dsimp` on dependent arguments where there is no congruence theorem that allows
+    `simp` to visit them. If `dsimp := false`, then argument is not visited.
+  -/
+  dsimp             : Bool := true
+  /-- If `failIfUnchanged := true`, then calls to `simp`, `dsimp`, or `simp_all`
+  will fail if they do not make progress. -/
+  failIfUnchanged   : Bool := true
+  /-- If `ground := true`, then ground terms are reduced. A term is ground when
+  it does not contain free or meta variables. Reduction is interrupted at a function application `f ...`
+  if `f` is marked to not be unfolded. -/
+  ground            : Bool := false
+  /-- If `unfoldPartialApp := true`, then calls to `simp`, `dsimp`, or `simp_all`
+  will unfold even partial applications of `f` when we request `f` to be unfolded. -/
+  unfoldPartialApp  : Bool := false
+  deriving Inhabited, BEq
+
+-- Configuration object for `simp_all`
+structure ConfigCtx extends Config where
+  contextual := true
+
+def neutralConfig : Simp.Config := {
+  zeta              := false
+  beta              := false
+  eta               := false
+  iota              := false
+  proj              := false
+  decide            := false
+  arith             := false
+  autoUnfold        := false
+  ground            := false
+}
+
+end Simp
+
+inductive Occurrences where
+  | all
+  | pos (idxs : List Nat)
+  | neg (idxs : List Nat)
+  deriving Inhabited, BEq
+
+end Lean.Meta

src/Init/Notation.lean

@@ -295,7 +295,8 @@ macro_rules | `($x - $y)   => `(binop% HSub.hSub $x $y)
 macro_rules | `($x * $y)   => `(binop% HMul.hMul $x $y)
 macro_rules | `($x / $y)   => `(binop% HDiv.hDiv $x $y)
 macro_rules | `($x % $y)   => `(binop% HMod.hMod $x $y)
-macro_rules | `($x ^ $y)   => `(binop% HPow.hPow $x $y)
+-- exponentiation should be considered a right action (#2220)
+macro_rules | `($x ^ $y)   => `(rightact% HPow.hPow $x $y)
 macro_rules | `($x ++ $y)  => `(binop% HAppend.hAppend $x $y)
 macro_rules | `(- $x)      => `(unop% Neg.neg $x)
 
@@ -460,42 +461,8 @@ expected type is known. So, `without_expected_type` is not effective in this cas
 -/
 macro "without_expected_type " x:term : term => `(let aux := $x; aux)
 
-/--
-The syntax `[a, b, c]` is shorthand for `a :: b :: c :: []`, or
-`List.cons a (List.cons b (List.cons c List.nil))`. It allows conveniently constructing
-list literals.
-
-For lists of length at least 64, an alternative desugaring strategy is used
-which uses let bindings as intermediates as in
-`let left := [d, e, f]; a :: b :: c :: left` to avoid creating very deep expressions.
-Note that this changes the order of evaluation, although it should not be observable
-unless you use side effecting operations like `dbg_trace`.
--/
-syntax "[" withoutPosition(term,*) "]"  : term
-
-/--
-Auxiliary syntax for implementing `[$elem,*]` list literal syntax.
-The syntax `%[a,b,c|tail]` constructs a value equivalent to `a::b::c::tail`.
-It uses binary partitioning to construct a tree of intermediate let bindings as in
-`let left := [d, e, f]; a :: b :: c :: left` to avoid creating very deep expressions.
--/
-syntax "%[" withoutPosition(term,* " | " term) "]" : term
-
 namespace Lean
 
-macro_rules
-  | `([ $elems,* ]) => do
-    -- NOTE: we do not have `TSepArray.getElems` yet at this point
-    let rec expandListLit (i : Nat) (skip : Bool) (result : TSyntax `term) : MacroM Syntax := do
-      match i, skip with
-      | 0,   _     => pure result
-      | i+1, true  => expandListLit i false result
-      | i+1, false => expandListLit i true  (← ``(List.cons $(⟨elems.elemsAndSeps.get! i⟩) $result))
-    if elems.elemsAndSeps.size < 64 then
-      expandListLit elems.elemsAndSeps.size false (← ``(List.nil))
-    else
-      `(%[ $elems,* | List.nil ])
-
 /--
 Category for carrying raw syntax trees between macros; any content is printed as is by the pretty printer.
 The only accepted parser for this category is an antiquotation.

src/Init/Prelude.lean

@@ -66,6 +66,19 @@ example (b : Bool) : Function.const Bool 10 b = 10 :=
 @[inline] def Function.const {α : Sort u} (β : Sort v) (a : α) : β → α :=
   fun _ => a
 
+/--
+The encoding of `let_fun x := v; b` is `letFun v (fun x => b)`.
+This is equal to `(fun x => b) v`, so the value of `x` is not accessible to `b`.
+This is in contrast to `let x := v; b`, where the value of `x` is accessible to `b`.
+
+There is special support for `letFun`.
+Both WHNF and `simp` are aware of `letFun` and can reduce it when zeta reduction is enabled,
+despite the fact it is marked `irreducible`.
+For metaprogramming, the function `Lean.Expr.letFun?` can be used to recognize a `let_fun` expression
+to extract its parts as if it were a `let` expression.
+-/
+@[irreducible] def letFun {α : Sort u} {β : α → Sort v} (v : α) (f : (x : α) → β x) : β v := f v
+
 set_option checkBinderAnnotations false in
 /--
 `inferInstance` synthesizes a value of any target type by typeclass
@@ -806,6 +819,12 @@ decidability instance instead of the proposition, which has no code).
 
 If a proposition `p` is `Decidable`, then `(by decide : p)` will prove it by
 evaluating the decidability instance to `isTrue h` and returning `h`.
+
+Because `Decidable` carries data,
+when writing `@[simp]` lemmas which include a `Decidable` instance on the LHS,
+it is best to use `{_ : Decidable p}` rather than `[Decidable p]`
+so that non-canonical instances can be found via unification rather than
+typeclass search.
 -/
 class inductive Decidable (p : Prop) where
   /-- Prove that `p` is decidable by supplying a proof of `¬p` -/
@@ -920,7 +939,9 @@ or derive `i < arr.size` from some other proposition that we are checking in the
 return `t` or `e` depending on whether `c` is true or false. The explicit argument
 `c : Prop` does not have any actual computational content, but there is an additional
 `[Decidable c]` argument synthesized by typeclass inference which actually
-determines how to evaluate `c` to true or false.
+determines how to evaluate `c` to true or false. Write `if h : c then t else e`
+instead for a "dependent if-then-else" `dite`, which allows `t`/`e` to use the fact
+that `c` is true/false.
 
 Because lean uses a strict (call-by-value) evaluation strategy, the signature of this
 function is problematic in that it would require `t` and `e` to be evaluated before
@@ -1297,11 +1318,38 @@ class Mod (α : Type u) where
 The homogeneous version of `HPow`: `a ^ b : α` where `a : α`, `b : β`.
 (The right argument is not the same as the left since we often want this even
 in the homogeneous case.)
+
+Types can choose to subscribe to particular defaulting behavior by providing
+an instance to either `NatPow` or `HomogeneousPow`:
+- `NatPow` is for types whose exponents is preferentially a `Nat`.
+- `HomogeneousPow` is for types whose base and exponent are preferentially the same.
 -/
 class Pow (α : Type u) (β : Type v) where
   /-- `a ^ b` computes `a` to the power of `b`. See `HPow`. -/
   pow : α → β → α
 
+/-- The homogenous version of `Pow` where the exponent is a `Nat`.
+The purpose of this class is that it provides a default `Pow` instance,
+which can be used to specialize the exponent to `Nat` during elaboration.
+
+For example, if `x ^ 2` should preferentially elaborate with `2 : Nat` then `x`'s type should
+provide an instance for this class. -/
+class NatPow (α : Type u) where
+  /-- `a ^ n` computes `a` to the power of `n` where `n : Nat`. See `Pow`. -/
+  protected pow : α → Nat → α
+
+/-- The completely homogeneous version of `Pow` where the exponent has the same type as the base.
+The purpose of this class is that it provides a default `Pow` instance,
+which can be used to specialize the exponent to have the same type as the base's type during elaboration.
+This is to say, a type should provide an instance for this class in case `x ^ y` should be elaborated
+with both `x` and `y` having the same type.
+
+For example, the `Float` type provides an instance of this class, which causes expressions
+such as `(2.2 ^ 2.2 : Float)` to elaborate. -/
+class HomogeneousPow (α : Type u) where
+  /-- `a ^ b` computes `a` to the power of `b` where `a` and `b` both have the same type. -/
+  protected pow : α → α → α
+
 /-- The homogeneous version of `HAppend`: `a ++ b : α` where `a b : α`. -/
 class Append (α : Type u) where
   /-- `a ++ b` is the result of concatenation of `a` and `b`. See `HAppend`. -/
@@ -1385,6 +1433,14 @@ instance [Mod α] : HMod α α α where
 instance [Pow α β] : HPow α β α where
   hPow a b := Pow.pow a b
 
+@[default_instance]
+instance [NatPow α] : Pow α Nat where
+  pow a n := NatPow.pow a n
+
+@[default_instance]
+instance [HomogeneousPow α] : Pow α α where
+  pow a b := HomogeneousPow.pow a b
+
 @[default_instance]
 instance [Append α] : HAppend α α α where
   hAppend a b := Append.append a b
@@ -1480,8 +1536,7 @@ protected def Nat.pow (m : @& Nat) : (@& Nat) → Nat
   | 0      => 1
   | succ n => Nat.mul (Nat.pow m n) m
 
-instance : Pow Nat Nat where
-  pow := Nat.pow
+instance : NatPow Nat := ⟨Nat.pow⟩
 
 set_option bootstrap.genMatcherCode false in
 /--
@@ -1774,7 +1829,7 @@ instance Fin.decLt {n} (a b : Fin n) : Decidable (LT.lt a b) := Nat.decLt ..
 instance Fin.decLe {n} (a b : Fin n) : Decidable (LE.le a b) := Nat.decLe ..
 
 /-- The size of type `UInt8`, that is, `2^8 = 256`. -/
-def UInt8.size : Nat := 256
+abbrev UInt8.size : Nat := 256
 
 /--
 The type of unsigned 8-bit integers. This type has special support in the
@@ -1813,7 +1868,7 @@ instance : Inhabited UInt8 where
   default := UInt8.ofNatCore 0 (by decide)
 
 /-- The size of type `UInt16`, that is, `2^16 = 65536`. -/
-def UInt16.size : Nat := 65536
+abbrev UInt16.size : Nat := 65536
 
 /--
 The type of unsigned 16-bit integers. This type has special support in the
@@ -1852,7 +1907,7 @@ instance : Inhabited UInt16 where
   default := UInt16.ofNatCore 0 (by decide)
 
 /-- The size of type `UInt32`, that is, `2^32 = 4294967296`. -/
-def UInt32.size : Nat := 4294967296
+abbrev UInt32.size : Nat := 4294967296
 
 /--
 The type of unsigned 32-bit integers. This type has special support in the
@@ -1929,7 +1984,7 @@ instance : Max UInt32 := maxOfLe
 instance : Min UInt32 := minOfLe
 
 /-- The size of type `UInt64`, that is, `2^64 = 18446744073709551616`. -/
-def UInt64.size : Nat := 18446744073709551616
+abbrev UInt64.size : Nat := 18446744073709551616
 /--
 The type of unsigned 64-bit integers. This type has special support in the
 compiler to make it actually 64 bits rather than wrapping a `Nat`.
@@ -1969,11 +2024,26 @@ instance : Inhabited UInt64 where
 /--
 The size of type `UInt16`, that is, `2^System.Platform.numBits`, which may
 be either `2^32` or `2^64` depending on the platform's architecture.
+
+Remark: we define `USize.size` using `(2^numBits - 1) + 1` to ensure the
+Lean unifier can solve contraints such as `?m + 1 = USize.size`. Recall that
+`numBits` does not reduce to a numeral in the Lean kernel since it is platform
+specific. Without this trick, the following definition would be rejected by the
+Lean type checker.
+```
+def one: Fin USize.size := 1
+```
+Because Lean would fail to synthesize instance `OfNat (Fin USize.size) 1`.
+Recall that the `OfNat` instance for `Fin` is
+```
+instance : OfNat (Fin (n+1)) i where
+  ofNat := Fin.ofNat i
+```
 -/
-def USize.size : Nat := hPow 2 System.Platform.numBits
+abbrev USize.size : Nat := Nat.succ (Nat.sub (hPow 2 System.Platform.numBits) 1)
 
 theorem usize_size_eq : Or (Eq USize.size 4294967296) (Eq USize.size 18446744073709551616) :=
-  show Or (Eq (hPow 2 System.Platform.numBits) 4294967296) (Eq (hPow 2 System.Platform.numBits) 18446744073709551616) from
+  show Or (Eq (Nat.succ (Nat.sub (hPow 2 System.Platform.numBits) 1)) 4294967296) (Eq (Nat.succ (Nat.sub (hPow 2 System.Platform.numBits) 1)) 18446744073709551616) from
   match System.Platform.numBits, System.Platform.numBits_eq with
   | _, Or.inl rfl => Or.inl (by decide)
   | _, Or.inr rfl => Or.inr (by decide)
@@ -2156,9 +2226,10 @@ returns `a` if `opt = some a` and `dflt` otherwise.
 This function is `@[macro_inline]`, so `dflt` will not be evaluated unless
 `opt` turns out to be `none`.
 -/
-@[macro_inline] def Option.getD : Option α → α → α
-  | some x, _ => x
-  | none,   e => e
+@[macro_inline] def Option.getD (opt : Option α) (dflt : α) : α :=
+  match opt with
+  | some x => x
+  | none => dflt
 
 /--
 Map a function over an `Option` by applying the function to the contained
@@ -2491,13 +2562,22 @@ is not observable from lean code. Arrays perform best when unshared; as long
 as they are used "linearly" all updates will be performed destructively on the
 array, so it has comparable performance to mutable arrays in imperative
 programming languages.
+
+From the point of view of proofs `Array α` is just a wrapper around `List α`.
 -/
 structure Array (α : Type u) where
-  /-- Convert a `List α` into an `Array α`. This function is overridden
-  to `List.toArray` and is O(n) in the length of the list. -/
+  /--
+  Converts a `List α` into an `Array α`.
+
+  At runtime, this constructor is implemented by `List.toArray` and is O(n) in the length of the
+  list.
+  -/
   mk ::
-  /-- Convert an `Array α` into a `List α`. This function is overridden
-  to `Array.toList` and is O(n) in the length of the list. -/
+  /--
+  Converts a `Array α` into an `List α`.
+
+  At runtime, this projection is implemented by `Array.toList` and is O(n) in the length of the
+  array. -/
   data : List α
 
 attribute [extern "lean_array_data"] Array.data
@@ -2645,12 +2725,9 @@ def List.redLength : List α → Nat
   | nil       => 0
   | cons _ as => as.redLength.succ
 
-/--
-Convert a `List α` into an `Array α`. This is O(n) in the length of the list.
-
-This function is exported to C, where it is called by `Array.mk`
-(the constructor) to implement this functionality.
--/
+/-- Convert a `List α` into an `Array α`. This is O(n) in the length of the list.  -/
+-- This function is exported to C, where it is called by `Array.mk`
+-- (the constructor) to implement this functionality.
 @[inline, match_pattern, export lean_list_to_array]
 def List.toArray (as : List α) : Array α :=
   as.toArrayAux (Array.mkEmpty as.redLength)

src/Init/SimpLemmas.lean

@@ -41,6 +41,12 @@ theorem implies_congr_ctx {p₁ p₂ q₁ q₂ : Prop} (h₁ : p₁ = p₂) (h
 theorem forall_congr {α : Sort u} {p q : α → Prop} (h : ∀ a, p a = q a) : (∀ a, p a) = (∀ a, q a) :=
   (funext h : p = q) ▸ rfl
 
+theorem forall_prop_domain_congr {p₁ p₂ : Prop} {q₁ : p₁ → Prop} {q₂ : p₂ → Prop}
+    (h₁ : p₁ = p₂)
+    (h₂ : ∀ a : p₂, q₁ (h₁.substr a) = q₂ a)
+    : (∀ a : p₁, q₁ a) = (∀ a : p₂, q₂ a) := by
+  subst h₁; simp [← h₂]
+
 theorem let_congr {α : Sort u} {β : Sort v} {a a' : α} {b b' : α → β}
     (h₁ : a = a') (h₂ : ∀ x, b x = b' x) : (let x := a; b x) = (let x := a'; b' x) :=
   h₁ ▸ (funext h₂ : b = b') ▸ rfl
@@ -78,6 +84,10 @@ theorem dite_congr {_ : Decidable b} [Decidable c]
 @[simp] theorem ite_false (a b : α) : (if False then a else b) = b := rfl
 @[simp] theorem dite_true {α : Sort u} {t : True → α} {e : ¬ True → α} : (dite True t e) = t True.intro := rfl
 @[simp] theorem dite_false {α : Sort u} {t : False → α} {e : ¬ False → α} : (dite False t e) = e not_false := rfl
+@[simp ↓] theorem ite_cond_true {_ : Decidable c} (a b : α) (h : c) : (if c then a else b) = a := by simp [h]
+@[simp ↓] theorem ite_cond_false {_ : Decidable c} (a b : α) (h : ¬ c) : (if c then a else b) = b := by simp [h]
+@[simp ↓] theorem dite_cond_true {α : Sort u} {_ : Decidable c} {t : c → α} {e : ¬ c → α} (h : c) : (dite c t e) = t h := by simp [h]
+@[simp ↓] theorem dite_cond_false {α : Sort u} {_ : Decidable c} {t : c → α} {e : ¬ c → α} (h : ¬ c) : (dite c t e) = e h := by simp [h]
 @[simp] theorem ite_self {α : Sort u} {c : Prop} {d : Decidable c} (a : α) : ite c a a = a := by cases d <;> rfl
 @[simp] theorem and_self (p : Prop) : (p ∧ p) = p := propext ⟨(·.1), fun h => ⟨h, h⟩⟩
 @[simp] theorem and_true (p : Prop) : (p ∧ True) = p := propext ⟨(·.1), (⟨·, trivial⟩)⟩
@@ -98,6 +108,7 @@ theorem dite_congr {_ : Decidable b} [Decidable c]
 @[simp] theorem implies_true (α : Sort u) : (α → True) = True := eq_true fun _ => trivial
 @[simp] theorem true_implies (p : Prop) : (True → p) = p := propext ⟨(· trivial), (fun _ => ·)⟩
 @[simp] theorem not_false_eq_true : (¬ False) = True := eq_true False.elim
+@[simp] theorem not_true_eq_false : (¬ True) = False := by decide
 
 @[simp] theorem Bool.or_false (b : Bool) : (b || false) = b  := by cases b <;> rfl
 @[simp] theorem Bool.or_true (b : Bool) : (b || true) = true := by cases b <;> rfl
@@ -136,9 +147,9 @@ theorem Bool.or_assoc (a b c : Bool) : (a || b || c) = (a || (b || c)) := by
 @[simp] theorem Bool.not_eq_true (b : Bool) : (¬(b = true)) = (b = false) := by cases b <;> decide
 @[simp] theorem Bool.not_eq_false (b : Bool) : (¬(b = false)) = (b = true) := by cases b <;> decide
 
-@[simp] theorem decide_eq_true_eq [Decidable p] : (decide p = true) = p := propext <| Iff.intro of_decide_eq_true decide_eq_true
-@[simp] theorem decide_not [h : Decidable p] : decide (¬ p) = !decide p := by cases h <;> rfl
-@[simp] theorem not_decide_eq_true [h : Decidable p] : ((!decide p) = true) = ¬ p := by cases h <;> simp [decide, *]
+@[simp] theorem decide_eq_true_eq {_ : Decidable p} : (decide p = true) = p := propext <| Iff.intro of_decide_eq_true decide_eq_true
+@[simp] theorem decide_not {h : Decidable p} : decide (¬ p) = !decide p := by cases h <;> rfl
+@[simp] theorem not_decide_eq_true {h : Decidable p} : ((!decide p) = true) = ¬ p := by cases h <;> simp [decide, *]
 
 @[simp] theorem heq_eq_eq {α : Sort u} (a b : α) : HEq a b = (a = b) := propext <| Iff.intro eq_of_heq heq_of_eq
 
@@ -152,7 +163,7 @@ theorem Bool.or_assoc (a b c : Bool) : (a || b || c) = (a || (b || c)) := by
 @[simp] theorem bne_self_eq_false' [DecidableEq α] (a : α) : (a != a) = false := by simp [bne]
 
 @[simp] theorem Nat.le_zero_eq (a : Nat) : (a ≤ 0) = (a = 0) :=
-  propext ⟨fun h => Nat.le_antisymm h (Nat.zero_le ..), fun h => by simp [h]⟩
+  propext ⟨fun h => Nat.le_antisymm h (Nat.zero_le ..), fun h => by rw [h]; decide⟩
 
 @[simp] theorem decide_False : decide False = false := rfl
 @[simp] theorem decide_True : decide True = true := rfl

src/Init/System/IO.lean

@@ -328,7 +328,41 @@ namespace FS
 namespace Handle
 
 @[extern "lean_io_prim_handle_mk"] opaque mk (fn : @& FilePath) (mode : FS.Mode) : IO Handle
+
+/--
+Acquires an exclusive or shared lock on the handle.
+Will block to wait for the lock if necessary.
+
+**NOTE:** Acquiring a exclusive lock while already possessing a shared lock
+will NOT reliably succeed (i.e., it works on Unix but not on Windows).
+-/
+@[extern "lean_io_prim_handle_lock"] opaque lock (h : @& Handle) (exclusive := true) : IO Unit
+/--
+Tries to acquire an exclusive or shared lock on the handle.
+Will NOT block for the lock, but instead return `false`.
+
+**NOTE:** Acquiring a exclusive lock while already possessing a shared lock
+will NOT reliably succeed (i.e., it works on Unix but not on Windows).
+-/
+@[extern "lean_io_prim_handle_try_lock"] opaque tryLock (h : @& Handle) (exclusive := true) : IO Bool
+/--
+Releases any previously acquired lock on the handle.
+Will succeed even if no lock has been acquired.
+-/
+@[extern "lean_io_prim_handle_unlock"] opaque unlock (h : @& Handle) : IO Unit
+
 @[extern "lean_io_prim_handle_flush"] opaque flush (h : @& Handle) : IO Unit
+/-- Rewinds the read/write cursor to the beginning of the handle. -/
+@[extern "lean_io_prim_handle_rewind"] opaque rewind (h : @& Handle) : IO Unit
+/--
+Truncates the handle to the read/write cursor.
+
+Does not automatically flush. Usually this is fine because the read/write
+cursor includes buffered writes. However, the combination of buffered writes,
+then `rewind`, then `truncate`, then close may lead to a file with content.
+If unsure, flush before truncating.
+-/
+@[extern "lean_io_prim_handle_truncate"] opaque truncate (h : @& Handle) : IO Unit
 /--
 Read up to the given number of bytes from the handle.
 If the returned array is empty, an end-of-file marker has been reached.
@@ -638,9 +672,12 @@ structure Output where
   stdout   : String
   stderr   : String
 
-/-- Run process to completion and capture output. -/
+/--
+Run process to completion and capture output.
+The process does not inherit the standard input of the caller.
+-/
 def output (args : SpawnArgs) : IO Output := do
-  let child ← spawn { args with stdout := Stdio.piped, stderr := Stdio.piped }
+  let child ← spawn { args with stdout := .piped, stderr := .piped, stdin := .null }
   let stdout ← IO.asTask child.stdout.readToEnd Task.Priority.dedicated
   let stderr ← child.stderr.readToEnd
   let exitCode ← child.wait

src/Init/Tactics.lean

@@ -250,7 +250,7 @@ macro:1 x:tactic tk:" <;> " y:tactic:2 : tactic => `(tactic|
     all_goals $y:tactic)
 
 /-- `eq_refl` is equivalent to `exact rfl`, but has a few optimizations. -/
-syntax (name := refl) "eq_refl" : tactic
+syntax (name := eqRefl) "eq_refl" : tactic
 
 /--
 `rfl` tries to close the current goal using reflexivity.
@@ -435,14 +435,14 @@ non-dependent hypotheses. It has many variants:
   other hypotheses.
 -/
 syntax (name := simp) "simp" (config)? (discharger)? (&" only")?
-  (" [" withoutPosition((simpStar <|> simpErase <|> simpLemma),*) "]")? (location)? : tactic
+  (" [" withoutPosition((simpStar <|> simpErase <|> simpLemma),*,?) "]")? (location)? : tactic
 /--
 `simp_all` is a stronger version of `simp [*] at *` where the hypotheses and target
 are simplified multiple times until no simplification is applicable.
 Only non-dependent propositional hypotheses are considered.
 -/
 syntax (name := simpAll) "simp_all" (config)? (discharger)? (&" only")?
-  (" [" withoutPosition((simpErase <|> simpLemma),*) "]")? : tactic
+  (" [" withoutPosition((simpErase <|> simpLemma),*,?) "]")? : tactic
 
 /--
 The `dsimp` tactic is the definitional simplifier. It is similar to `simp` but only
@@ -450,7 +450,7 @@ applies theorems that hold by reflexivity. Thus, the result is guaranteed to be
 definitionally equal to the input.
 -/
 syntax (name := dsimp) "dsimp" (config)? (discharger)? (&" only")?
-  (" [" withoutPosition((simpErase <|> simpLemma),*) "]")? (location)? : tactic
+  (" [" withoutPosition((simpErase <|> simpLemma),*,?) "]")? (location)? : tactic
 
 /--
 `delta id1 id2 ...` delta-expands the definitions `id1`, `id2`, ....

src/Init/Util.lean

@@ -85,3 +85,21 @@ private def outOfBounds [Inhabited α] : α :=
 
 macro:max x:term noWs "[" i:term "]" noWs "?" : term => `(getElem? $x $i)
 macro:max x:term noWs "[" i:term "]" noWs "!" : term => `(getElem! $x $i)
+
+/--
+  Marks given value and its object graph closure as multi-threaded if currently
+  marked single-threaded. This will make reference counter updates atomic and
+  thus more costly. It can still be useful to do eagerly when the value will be
+  shared between threads later anyway and there is available time budget to mark
+  it now. -/
+@[extern "lean_runtime_mark_multi_threaded"]
+def Runtime.markMultiThreaded (a : α) : α := a
+
+/--
+  Marks given value and its object graph closure as persistent. This will remove
+  reference counter updates but prevent the closure from being deallocated until
+  the end of the process! It can still be useful to do eagerly when the value
+  will be marked persistent later anyway and there is available time budget to
+  mark it now or it would be unnecessarily marked multi-threaded in between. -/
+@[extern "lean_runtime_mark_persistent"]
+def Runtime.markPersistent (a : α) : α := a

src/Init/WFTactics.lean

@@ -5,12 +5,13 @@ Author: Leonardo de Moura
 -/
 prelude
 import Init.SizeOf
+import Init.MetaTypes
 import Init.WF
 
 /-- Unfold definitions commonly used in well founded relation definitions.
 This is primarily intended for internal use in `decreasing_tactic`. -/
 macro "simp_wf" : tactic =>
-  `(tactic| try simp [invImage, InvImage, Prod.lex, sizeOfWFRel, measure, Nat.lt_wfRel, WellFoundedRelation.rel])
+  `(tactic| try simp (config := { unfoldPartialApp := true }) [invImage, InvImage, Prod.lex, sizeOfWFRel, measure, Nat.lt_wfRel, WellFoundedRelation.rel])
 
 /-- Extensible helper tactic for `decreasing_tactic`. This handles the "base case"
 reasoning after applying lexicographic order lemmas.

src/Lean/Attributes.lean

@@ -330,7 +330,7 @@ private def AttributeExtension.mkInitial : IO AttributeExtensionState := do
 
 unsafe def mkAttributeImplOfConstantUnsafe (env : Environment) (opts : Options) (declName : Name) : Except String AttributeImpl :=
   match env.find? declName with
-  | none      => throw ("unknow constant '" ++ toString declName ++ "'")
+  | none      => throw ("unknown constant '" ++ toString declName ++ "'")
   | some info =>
     match info.type with
     | Expr.const `Lean.AttributeImpl _ => env.evalConst AttributeImpl opts declName

src/Lean/Compiler/IR/EmitC.lean

@@ -472,6 +472,7 @@ def quoteString (s : String) : String :=
       else if c == '\t' then "\\t"
       else if c == '\\' then "\\\\"
       else if c == '\"' then "\\\""
+      else if c == '?' then "\\?" -- avoid trigraphs
       else if c.toNat <= 31 then
         "\\x" ++ toHexDigit (c.toNat / 16) ++ toHexDigit (c.toNat % 16)
       -- TODO(Leo): we should use `\unnnn` for escaping unicode characters.

src/Lean/Compiler/IR/EmitLLVM.lean

@@ -1541,15 +1541,6 @@ end EmitLLVM
 def getLeanHBcPath : IO System.FilePath := do
   return (← getLibDir (← getBuildDir)) / "lean.h.bc"
 
-def optimizeLLVMModule (mod : LLVM.Module ctx) : IO Unit := do
-  let pm  ← LLVM.createPassManager
-  let pmb ← LLVM.createPassManagerBuilder
-  pmb.setOptLevel 3
-  pmb.populateModulePassManager pm
-  LLVM.runPassManager pm mod
-  LLVM.disposePassManager pm
-  LLVM.disposePassManagerBuilder pmb
-
 /-- Get the names of all global symbols in the module -/
 partial def getModuleGlobals (mod : LLVM.Module llvmctx) : IO (Array (LLVM.Value llvmctx)) := do
   let rec go (v : LLVM.Value llvmctx) (acc : Array (LLVM.Value llvmctx)) : IO (Array (LLVM.Value llvmctx)) := do
@@ -1568,7 +1559,7 @@ partial def getModuleFunctions (mod : LLVM.Module llvmctx) : IO (Array (LLVM.Val
 `emitLLVM` is the entrypoint for the lean shell to code generate LLVM.
 -/
 @[export lean_ir_emit_llvm]
-def emitLLVM (env : Environment) (modName : Name) (filepath : String) (tripleStr? : Option String) : IO Unit := do
+def emitLLVM (env : Environment) (modName : Name) (filepath : String) : IO Unit := do
   LLVM.llvmInitializeTargetInfo
   let llvmctx ← LLVM.createContext
   let module ← LLVM.createModule llvmctx modName.toString
@@ -1601,17 +1592,7 @@ def emitLLVM (env : Environment) (modName : Name) (filepath : String) (tripleStr
            let some fn ← LLVM.getNamedFunction emitLLVMCtx.llvmmodule name
               | throw <| IO.Error.userError s!"ERROR: linked module must have function from runtime module: '{name}'"
            LLVM.setLinkage fn LLVM.Linkage.internal
-
-         optimizeLLVMModule emitLLVMCtx.llvmmodule
          LLVM.writeBitcodeToFile emitLLVMCtx.llvmmodule filepath
-         let tripleStr := tripleStr?.getD (← LLVM.getDefaultTargetTriple)
-         let target ← LLVM.getTargetFromTriple tripleStr
-         let cpu := "generic"
-         let features := ""
-         let targetMachine ← LLVM.createTargetMachine target tripleStr cpu features
-         let codegenType := LLVM.CodegenFileType.ObjectFile
-         LLVM.targetMachineEmitToFile targetMachine emitLLVMCtx.llvmmodule (filepath ++ ".o") codegenType
          LLVM.disposeModule emitLLVMCtx.llvmmodule
-         LLVM.disposeTargetMachine targetMachine
   | .error err => throw (IO.Error.userError err)
 end Lean.IR

src/Lean/Compiler/LCNF/PassManager.lean

@@ -46,7 +46,7 @@ structure Pass where
   Resulting phase.
   -/
   phaseOut : Phase := phase
-  phaseInv : phaseOut ≥ phase := by simp
+  phaseInv : phaseOut ≥ phase := by simp_arith
   /--
   The name of the `Pass`
   -/

src/Lean/Compiler/LCNF/ToLCNF.lean

@@ -658,7 +658,9 @@ where
       visit (f.beta e.getAppArgs)
 
   visitApp (e : Expr) : M Arg := do
-    if let .const declName _ := e.getAppFn then
+    if let some (args, n, t, v, b) := e.letFunAppArgs? then
+      visitCore <| mkAppN (.letE n t v b (nonDep := true)) args
+    else if let .const declName _ := e.getAppFn then
       if declName == ``Quot.lift then
         visitQuotLift e
       else if declName == ``Quot.mk then
@@ -725,11 +727,8 @@ where
       pushElement (.fun funDecl)
       return .fvar funDecl.fvarId
 
-  visitMData (mdata : MData) (e : Expr) : M Arg := do
-    if let some (.app (.lam n t b ..) v) := letFunAnnotation? (.mdata mdata e) then
-      visitLet (.letE n t v b (nonDep := true)) #[]
-    else
-      visit e
+  visitMData (_mdata : MData) (e : Expr) : M Arg := do
+    visit e
 
   visitProj (s : Name) (i : Nat) (e : Expr) : M Arg := do
     match (← visit e) with

src/Lean/CoreM.lean

@@ -65,6 +65,12 @@ structure Context where
   initHeartbeats : Nat := 0
   maxHeartbeats  : Nat := getMaxHeartbeats options
   currMacroScope : MacroScope := firstFrontendMacroScope
+  /--
+  If `catchRuntimeEx = false`, then given `try x catch ex => h ex`,
+  an runtime exception occurring in `x` is not handled by `h`.
+  Recall that runtime exceptions are `maxRecDepth` or `maxHeartbeats`.
+  -/
+  catchRuntimeEx : Bool := false
   deriving Nonempty
 
 /-- CoreM is a monad for manipulating the Lean environment.
@@ -199,6 +205,11 @@ instance [MetaEval α] : MetaEval (CoreM α) where
 protected def withIncRecDepth [Monad m] [MonadControlT CoreM m] (x : m α) : m α :=
   controlAt CoreM fun runInBase => withIncRecDepth (runInBase x)
 
+@[inline] def checkInterrupted : CoreM Unit := do
+  if (← IO.checkCanceled) then
+    -- should never be visible to users!
+    throw <| Exception.error .missing "elaboration interrupted"
+
 def throwMaxHeartbeat (moduleName : Name) (optionName : Name) (max : Nat) : CoreM Unit := do
   let msg := s!"(deterministic) timeout at '{moduleName}', maximum number of heartbeats ({max/1000}) has been reached (use 'set_option {optionName} <num>' to set the limit)"
   throw <| Exception.error (← getRef) (MessageData.ofFormat (Std.Format.text msg))
@@ -212,6 +223,11 @@ def checkMaxHeartbeatsCore (moduleName : String) (optionName : Name) (max : Nat)
 def checkMaxHeartbeats (moduleName : String) : CoreM Unit := do
   checkMaxHeartbeatsCore moduleName `maxHeartbeats (← read).maxHeartbeats
 
+def checkSystem (moduleName : String) : CoreM Unit := do
+  -- TODO: bring back more checks from the C++ implementation
+  checkInterrupted
+  checkMaxHeartbeats moduleName
+
 private def withCurrHeartbeatsImp (x : CoreM α) : CoreM α := do
   let heartbeats ← IO.getNumHeartbeats
   withReader (fun ctx => { ctx with initHeartbeats := heartbeats }) x
@@ -240,7 +256,7 @@ instance : MonadLog CoreM where
 
 end Core
 
-export Core (CoreM mkFreshUserName checkMaxHeartbeats withCurrHeartbeats)
+export Core (CoreM mkFreshUserName checkSystem withCurrHeartbeats)
 
 @[inline] def withAtLeastMaxRecDepth [MonadFunctorT CoreM m] (max : Nat) : m α → m α :=
   monadMap (m := CoreM) <| withReader (fun ctx => { ctx with maxRecDepth := Nat.max max ctx.maxRecDepth })
@@ -299,7 +315,7 @@ private def checkUnsupported [Monad m] [MonadEnv m] [MonadError m] (decl : Decla
     | _ => pure ()
 
 register_builtin_option compiler.enableNew : Bool := {
-  defValue := true
+  defValue := false
   group    := "compiler"
   descr    := "(compiler) enable the new code generator, this should have no significant effect on your code but it does help to test the new code generator; unset to only use the old code generator instead"
 }
@@ -340,4 +356,42 @@ def ImportM.runCoreM (x : CoreM α) : ImportM α := do
   let (a, _) ← x.toIO { options := ctx.opts, fileName := "<ImportM>", fileMap := default } { env := ctx.env }
   return a
 
+/-- Return `true` if the exception was generated by one our resource limits. -/
+def Exception.isRuntime (ex : Exception) : Bool :=
+  ex.isMaxHeartbeat || ex.isMaxRecDepth
+
+/--
+Custom `try-catch` for all monads based on `CoreM`. We don't want to catch "runtime exceptions"
+in these monads, but on `CommandElabM`. See issues #2775 and #2744
+-/
+@[inline] protected def Core.tryCatch (x : CoreM α) (h : Exception → CoreM α) : CoreM α := do
+  try
+    x
+  catch ex =>
+    if ex.isRuntime && !(← read).catchRuntimeEx then
+      throw ex
+    else
+      h ex
+
+instance : MonadExceptOf Exception CoreM where
+  throw    := throw
+  tryCatch := Core.tryCatch
+
+@[inline] def Core.withCatchingRuntimeEx (flag : Bool) (x : CoreM α) : CoreM α :=
+  withReader (fun ctx => { ctx with catchRuntimeEx := flag }) x
+
+@[inline] def mapCoreM [MonadControlT CoreM m] [Monad m] (f : forall {α}, CoreM α → CoreM α) {α} (x : m α) : m α :=
+  controlAt CoreM fun runInBase => f <| runInBase x
+
+/--
+Execute `x` with `catchRuntimeEx = flag`. That is, given `try x catch ex => h ex`,
+if `x` throws a runtime exception, the handler `h` will be invoked if `flag = true`
+Recall that
+-/
+@[inline] def withCatchingRuntimeEx [MonadControlT CoreM m] [Monad m] (x : m α) : m α :=
+  mapCoreM (Core.withCatchingRuntimeEx true) x
+
+@[inline] def withoutCatchingRuntimeEx [MonadControlT CoreM m] [Monad m] (x : m α) : m α :=
+  mapCoreM (Core.withCatchingRuntimeEx false) x
+
 end Lean

src/Lean/Data/Array.lean

@@ -0,0 +1,46 @@
+/-
+Copyright (c) 2023 Lean FRO, LLC. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Joachim Breitner
+-/
+
+import Init.Data.Array
+
+namespace Array
+
+/-!
+This module contains utility functions involving Arrays that are useful in a few places
+of the lean code base, but too specialized to live in `Init.Data.Array`, which arguably
+is part of the public, user-facing standard library.
+-/
+
+/--
+Given an array `a`, runs `f xᵢ xⱼ` for all `i < j`, removes those entries for which `f` returns
+`false` (and will subsequently skip pairs if one element is removed), and returns the array of
+remaining elements.
+
+This can be used to remove elements from an array where a “better” element, in some partial
+order, exists in the array.
+-/
+def filterPairsM {m} [Monad m] {α} (a : Array α) (f : α → α → m (Bool × Bool)) :
+    m (Array α) := do
+  let mut removed := Array.mkArray a.size false
+  let mut numRemoved := 0
+  for h1 : i in [:a.size] do for h2 : j in [i+1:a.size] do
+    unless removed[i]! || removed[j]! do
+      let xi := a[i]'h1.2
+      let xj := a[j]'h2.2
+      let (keepi, keepj) ← f xi xj
+      unless keepi do
+        numRemoved := numRemoved + 1
+        removed := removed.set! i true
+      unless keepj do
+        numRemoved := numRemoved + 1
+        removed := removed.set! j true
+  let mut a' := Array.mkEmpty numRemoved
+  for h : i in [:a.size] do
+    unless removed[i]! do
+      a' := a'.push (a[i]'h.2)
+  return a'
+
+end Array

src/Lean/Data/JsonRpc.lean

@@ -305,7 +305,7 @@ section
         throw $ userError s!"Expected method '{expectedMethod}', got method '{method}'"
     | _ => throw $ userError s!"Expected JSON-RPC notification, got: '{(toJson m).compress}'"
 
-  partial def readResponseAs (h : FS.Stream) (nBytes : Nat) (expectedID : RequestID) (α) [FromJson α] : IO (Response α) := do
+  def readResponseAs (h : FS.Stream) (nBytes : Nat) (expectedID : RequestID) (α) [FromJson α] : IO (Response α) := do
     let m ← h.readMessage nBytes
     match m with
     | Message.response id result =>
@@ -315,7 +315,6 @@ section
         | Except.error inner => throw $ userError s!"Unexpected result '{result.compress}'\n{inner}"
       else
         throw $ userError s!"Expected id {expectedID}, got id {id}"
-    | Message.notification .. => readResponseAs h nBytes expectedID α
     | _ => throw $ userError s!"Expected JSON-RPC response, got: '{(toJson m).compress}'"
 end
 

src/Lean/Data/KVMap.lean

@@ -97,6 +97,10 @@ def insert : KVMap → Name → DataValue → KVMap
 def contains (m : KVMap) (n : Name) : Bool :=
   (m.find n).isSome
 
+/-- Erase an entry from the map -/
+def erase : KVMap → Name → KVMap
+  | ⟨m⟩, k => ⟨m.filter fun a => a.1 ≠ k⟩
+
 def getString (m : KVMap) (k : Name) (defVal := "") : String :=
   match m.find k with
   | some (DataValue.ofString v) => v
@@ -145,6 +149,30 @@ def setName (m : KVMap) (k : Name) (v : Name) : KVMap :=
 def setSyntax (m : KVMap) (k : Name) (v : Syntax) : KVMap :=
   m.insert k (DataValue.ofSyntax v)
 
+/-- Update a `String` entry based on its current value. -/
+def updateString (m : KVMap) (k : Name) (f : String → String) : KVMap :=
+  m.insert k <| DataValue.ofString <| f <| m.getString k
+
+/-- Update a `Nat` entry based on its current value. -/
+def updateNat (m : KVMap) (k : Name) (f : Nat → Nat) : KVMap :=
+  m.insert k <| DataValue.ofNat <| f <| m.getNat k
+
+/-- Update an `Int` entry based on its current value. -/
+def updateInt (m : KVMap) (k : Name) (f : Int → Int) : KVMap :=
+  m.insert k <| DataValue.ofInt <| f <| m.getInt k
+
+/-- Update a `Bool` entry based on its current value. -/
+def updateBool (m : KVMap) (k : Name) (f : Bool → Bool) : KVMap :=
+  m.insert k <| DataValue.ofBool <| f <| m.getBool k
+
+/-- Update a `Name` entry based on its current value. -/
+def updateName (m : KVMap) (k : Name) (f : Name → Name) : KVMap :=
+  m.insert k <| DataValue.ofName <| f <| m.getName k
+
+/-- Update a `Syntax` entry based on its current value. -/
+def updateSyntax (m : KVMap) (k : Name) (f : Syntax → Syntax) : KVMap :=
+  m.insert k <| DataValue.ofSyntax <| f <| m.getSyntax k
+
 @[inline] protected def forIn.{w, w'} {δ : Type w} {m : Type w → Type w'} [Monad m]
   (kv : KVMap) (init : δ) (f : Name × DataValue → δ → m (ForInStep δ)) : m δ :=
   kv.entries.forIn init f
@@ -162,6 +190,16 @@ def subsetAux : List (Name × DataValue) → KVMap → Bool
 def subset : KVMap → KVMap → Bool
   | ⟨m₁⟩, m₂ => subsetAux m₁ m₂
 
+def mergeBy (mergeFn : Name → DataValue → DataValue → DataValue) (l r : KVMap)
+    : KVMap := Id.run do
+  let mut result := l
+  for ⟨k, vᵣ⟩ in r do
+    if let some vₗ := result.find k then
+      result := result.insert k (mergeFn k vₗ vᵣ)
+    else
+      result := result.insert k vᵣ
+  return result
+
 def eqv (m₁ m₂ : KVMap) : Bool :=
   subset m₁ m₂ && subset m₂ m₁
 
@@ -181,6 +219,11 @@ class Value (α : Type) where
 @[inline] def set {α : Type} [Value α] (m : KVMap) (k : Name) (v : α) : KVMap :=
   m.insert k (Value.toDataValue v)
 
+@[inline] def update {α : Type} [Value α] (m : KVMap) (k : Name) (f : Option α → Option α) : KVMap :=
+  match f (m.get? k) with
+  | some a => m.set k a
+  | none => m.erase k
+
 instance : Value DataValue where
   toDataValue  := id
   ofDataValue? := some

src/Lean/Data/Lsp/Basic.lean

@@ -78,6 +78,38 @@ structure Command where
   arguments? : Option (Array Json) := none
   deriving ToJson, FromJson
 
+/-- A snippet is a string that gets inserted into a document,
+and can afterwards be edited by the user in a structured way.
+
+Snippets contain instructions that
+specify how this structured editing should proceed.
+They are expressed in a domain-specific language
+based on one from TextMate,
+including the following constructs:
+- Designated positions for subsequent user input,
+  called "tab stops" after their most frequently-used keybinding.
+  They are denoted with `$1`, `$2`, and so forth.
+  `$0` denotes where the cursor should be positioned after all edits are completed,
+  defaulting to the end of the string.
+  Snippet tab stops are unrelated to tab stops used for indentation.
+- Tab stops with default values, called _placeholders_, written `${1:default}`.
+  The default may itself contain a tab stop or a further placeholder
+  and multiple options to select from may be provided
+  by surrounding them with `|`s and separating them with `,`,
+  as in `${1|if $2 then $3 else $4,if let $2 := $3 then $4 else $5|}`.
+- One of a set of predefined variables that are replaced with their values.
+  This includes the current line number (`$TM_LINE_NUMBER`)
+  or the text that was selected when the snippet was invoked (`$TM_SELECTED_TEXT`).
+- Formatting instructions to modify variables using regular expressions
+  or a set of predefined filters.
+
+The full syntax and semantics of snippets,
+including the available variables and the rules for escaping control characters,
+are described in the [LSP specification](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#snippet_syntax). -/
+structure SnippetString where
+  value : String
+  deriving ToJson, FromJson
+
 /-- A textual edit applicable to a text document.
 
 [reference](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textEdit) -/
@@ -87,6 +119,21 @@ structure TextEdit where
   range : Range
   /-- The string to be inserted. For delete operations use an empty string. -/
   newText : String
+  /-- If this field is present and the editor supports it,
+  `newText` is ignored
+  and an interactive snippet edit is performed instead.
+
+  The use of snippets in `TextEdit`s
+  is a Lean-specific extension to the LSP standard,
+  so `newText` should still be set to a correct value
+  as fallback in case the editor does not support this feature.
+  Even editors that support snippets may not always use them;
+  for instance, if the file is not already open,
+  VS Code will perform a normal text edit in the background instead. -/
+  /- NOTE: Similar functionality may be added to LSP in the future:
+  see [issue #592](https://github.com/microsoft/language-server-protocol/issues/592).
+  If such an addition occurs, this field should be deprecated. -/
+  leanExtSnippet? : Option SnippetString := none
   /-- Identifier for annotated edit.
 
     `WorkspaceEdit` has a `changeAnnotations` field that maps these identifiers to a `ChangeAnnotation`.
@@ -206,7 +253,7 @@ instance : FromJson DocumentChange where
 [reference](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspaceEdit) -/
 structure WorkspaceEdit where
   /-- Changes to existing resources. -/
-  changes : RBMap DocumentUri TextEditBatch compare := ∅
+  changes? : Option (RBMap DocumentUri TextEditBatch compare) := none
   /-- Depending on the client capability
     `workspace.workspaceEdit.resourceOperations` document changes are either
     an array of `TextDocumentEdit`s to express changes to n different text
@@ -220,14 +267,14 @@ structure WorkspaceEdit where
     If a client neither supports `documentChanges` nor
     `workspace.workspaceEdit.resourceOperations` then only plain `TextEdit`s
     using the `changes` property are supported. -/
-  documentChanges : Array DocumentChange := ∅
+  documentChanges? : Option (Array DocumentChange) := none
   /-- A map of change annotations that can be referenced in
       `AnnotatedTextEdit`s or create, rename and delete file / folder
       operations.
 
       Whether clients honor this property depends on the client capability
       `workspace.changeAnnotationSupport`. -/
-  changeAnnotations : RBMap String ChangeAnnotation compare := ∅
+  changeAnnotations? : Option (RBMap String ChangeAnnotation compare) := none
   deriving ToJson, FromJson
 
 namespace WorkspaceEdit
@@ -236,24 +283,24 @@ instance : EmptyCollection WorkspaceEdit := ⟨{}⟩
 
 instance : Append WorkspaceEdit where
   append x y := {
-    changes           := x.changes.mergeBy (fun _ v₁ v₂ => v₁ ++ v₂) y.changes
-    documentChanges   := x.documentChanges ++ y.documentChanges
-    changeAnnotations := x.changeAnnotations.mergeBy (fun _ _v₁ v₂ => v₂) y.changeAnnotations
+    changes?           :=
+      match x.changes?, y.changes? with
+      | v, none | none, v => v
+      | some x, some y => x.mergeBy (fun _ v₁ v₂ => v₁ ++ v₂) y
+    documentChanges?   :=
+      match x.documentChanges?, y.documentChanges? with
+      | v, none | none, v => v
+      | some x, some y => x ++ y
+    changeAnnotations? :=
+      match x.changeAnnotations?, y.changeAnnotations? with
+      | v, none | none, v => v
+      | some x, some y => x.mergeBy (fun _ _v₁ v₂ => v₂) y
   }
 
 def ofTextDocumentEdit (e : TextDocumentEdit) : WorkspaceEdit :=
-  { documentChanges := #[DocumentChange.edit e]}
+  { documentChanges? := #[DocumentChange.edit e]}
 
-def ofTextEdit (uri : DocumentUri) (te : TextEdit) : WorkspaceEdit :=
-  /- [note], there is a bug in vscode where not including the version will cause an error,
-  even though the version field is not used to validate the change.
-
-  References:
-  - [a fix in the wild](https://github.com/stylelint/vscode-stylelint/pull/330/files).
-    Note that the version field needs to be present, even if the value is `undefined`.
-  - [angry comment](https://github.com/tsqllint/tsqllint-vscode-extension/blob/727026fce9f8c6a33d113373666d0776f8f6c23c/server/src/server.ts#L70)
-  -/
-  let doc := {uri, version? := some 0}
+def ofTextEdit (doc : VersionedTextDocumentIdentifier) (te : TextEdit) : WorkspaceEdit :=
   ofTextDocumentEdit { textDocument := doc, edits := #[te]}
 
 end WorkspaceEdit

src/Lean/Data/Lsp/Capabilities.lean

@@ -74,6 +74,7 @@ structure ServerCapabilities where
   declarationProvider       : Bool                           := false
   typeDefinitionProvider    : Bool                           := false
   referencesProvider        : Bool                           := false
+  renameProvider?           : Option RenameOptions           := none
   workspaceSymbolProvider   : Bool                           := false
   foldingRangeProvider      : Bool                           := false
   semanticTokensProvider?   : Option SemanticTokensOptions   := none

src/Lean/Data/Lsp/Extra.lean

@@ -5,6 +5,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Marc Huisinga, Wojciech Nawrocki
 -/
 import Lean.Data.Lsp.Basic
+import Lean.Data.Lsp.TextSync
 import Lean.Server.Rpc.Basic
 
 /-! This file contains Lean-specific extensions to LSP. See the structures below for which
@@ -13,6 +14,22 @@ additional requests and notifications are supported. -/
 namespace Lean.Lsp
 open Json
 
+/--
+  Controls when dependencies are built on `textDocument/didOpen` notifications.
+-/
+inductive DependencyBuildMode where
+  /-- Always build dependencies. -/
+  | always
+  /-- Build dependencies once, but then do not build them again on import changes / crashes. Used for `didOpen` notifications that are explicitly intended for manually triggering the build. -/
+  | once
+  /--  Never build dependencies. -/
+  | never
+  deriving FromJson, ToJson, Inhabited
+
+structure LeanDidOpenTextDocumentParams extends DidOpenTextDocumentParams where
+  dependencyBuildMode? : Option DependencyBuildMode := none -- `none`: Compatibility with clients pre-build-mode
+  deriving FromJson, ToJson
+
 /-- `textDocument/waitForDiagnostics` client->server request.
 
 Yields a response when all the diagnostics for a version of the document greater or equal to the

src/Lean/Data/Lsp/Ipc.lean

@@ -62,8 +62,23 @@ def readMessage : IpcM JsonRpc.Message := do
 def readRequestAs (expectedMethod : String) (α) [FromJson α] : IpcM (Request α) := do
   (←stdout).readLspRequestAs expectedMethod α
 
-def readResponseAs (expectedID : RequestID) (α) [FromJson α] : IpcM (Response α) := do
-  (←stdout).readLspResponseAs expectedID α
+/--
+  Reads response, discarding notifications in between. This function is meant
+  purely for testing where we use `collectDiagnostics` explicitly if we do care
+  about such notifications. -/
+partial def readResponseAs (expectedID : RequestID) (α) [FromJson α] :
+    IpcM (Response α) := do
+  let m ← (←stdout).readLspMessage
+  match m with
+  | Message.response id result =>
+    if id == expectedID then
+      match fromJson? result with
+      | Except.ok v => pure ⟨expectedID, v⟩
+      | Except.error inner => throw $ userError s!"Unexpected result '{result.compress}'\n{inner}"
+    else
+      throw $ userError s!"Expected id {expectedID}, got id {id}"
+  | .notification .. => readResponseAs expectedID α
+  | _ => throw $ userError s!"Expected JSON-RPC response, got: '{(toJson m).compress}'"
 
 def waitForExit : IpcM UInt32 := do
   (←read).wait

src/Lean/Data/Lsp/LanguageFeatures.lean

@@ -351,5 +351,16 @@ structure FoldingRange where
   kind? : Option FoldingRangeKind := none
   deriving ToJson
 
+structure RenameOptions where
+  prepareProvider : Bool := false
+  deriving FromJson, ToJson
+
+structure RenameParams extends TextDocumentPositionParams where
+  newName : String
+  deriving FromJson, ToJson
+
+structure PrepareRenameParams extends TextDocumentPositionParams
+  deriving FromJson, ToJson
+
 end Lsp
 end Lean

src/Lean/Data/Lsp/Utf16.lean

@@ -62,21 +62,24 @@ end String
 namespace Lean
 namespace FileMap
 
+private def lineStartPos (text : FileMap) (line : Nat) : String.Pos :=
+  if h : line < text.positions.size then
+    text.positions.get ⟨line, h⟩
+  else if text.positions.isEmpty then
+    0
+  else
+    text.positions.back
+
 /-- Computes an UTF-8 offset into `text.source`
 from an LSP-style 0-indexed (ln, col) position. -/
 def lspPosToUtf8Pos (text : FileMap) (pos : Lsp.Position) : String.Pos :=
-  let colPos :=
-    if h : pos.line < text.positions.size then
-      text.positions.get ⟨pos.line, h⟩
-    else if text.positions.isEmpty then
-      0
-    else
-      text.positions.back
-  let chr := text.source.utf16PosToCodepointPosFrom pos.character colPos
-  text.source.codepointPosToUtf8PosFrom colPos chr
+  let lineStartPos := lineStartPos text pos.line
+  let chr := text.source.utf16PosToCodepointPosFrom pos.character lineStartPos
+  text.source.codepointPosToUtf8PosFrom lineStartPos chr
 
 def leanPosToLspPos (text : FileMap) : Lean.Position → Lsp.Position
-  | ⟨ln, col⟩ => ⟨ln-1, text.source.codepointPosToUtf16PosFrom col (text.positions.get! $ ln - 1)⟩
+  | ⟨line, col⟩ =>
+    ⟨line - 1, text.source.codepointPosToUtf16PosFrom col (lineStartPos text (line - 1))⟩
 
 def utf8PosToLspPos (text : FileMap) (pos : String.Pos) : Lsp.Position :=
   text.leanPosToLspPos (text.toPosition pos)

src/Lean/Data/NameTrie.lean

@@ -69,4 +69,10 @@ def NameTrie.forMatchingM [Monad m] (t : NameTrie β) (k : Name) (f : β → m U
 def NameTrie.forM [Monad m] (t : NameTrie β) (f : β → m Unit) : m Unit :=
   t.forMatchingM Name.anonymous f
 
+def NameTrie.matchingToArray (t : NameTrie β) (k : Name) : Array β :=
+  Id.run <| t.foldMatchingM k #[] fun v acc => acc.push v
+
+def NameTrie.toArray (t : NameTrie β) : Array β :=
+  Id.run <| t.foldM #[] fun v acc => acc.push v
+
 end Lean

src/Lean/Data/SMap.lean

@@ -97,7 +97,7 @@ def toList (m : SMap α β) : List (α × β) :=
 
 end SMap
 
-def List.toSMap [BEq α] [Hashable α] (es : List (α × β)) : SMap α β :=
+def _root_.List.toSMap [BEq α] [Hashable α] (es : List (α × β)) : SMap α β :=
   es.foldl (init := {}) fun s (a, b) => s.insert a b
 
 instance {_ : BEq α} {_ : Hashable α} [Repr α] [Repr β] : Repr (SMap α β) where

src/Lean/Elab/App.lean

@@ -1261,7 +1261,7 @@ def elabExplicitUnivs (lvls : Array Syntax) : TermElabM (List Level) := do
 - When we elaborate choice nodes (and overloaded identifiers), we track multiple results using the `observing x` combinator.
   The `observing x` executes `x` and returns a `TermElabResult`.
 
-`observing `x does not check for synthetic sorry's, just an exception. Thus, it may think `x` worked when it didn't
+`observing x` does not check for synthetic sorry's, just an exception. Thus, it may think `x` worked when it didn't
 if a synthetic sorry was introduced. We decided that checking for synthetic sorrys at `observing` is not a good solution
 because it would not be clear to decide what the "main" error message for the alternative is. When the result contains
 a synthetic `sorry`, it is not clear which error message corresponds to the `sorry`. Moreover, while executing `x`, many

src/Lean/Elab/Binders.lean

@@ -668,12 +668,11 @@ def elabLetDeclAux (id : Syntax) (binders : Array Syntax) (typeStx : Syntax) (va
       let body ← instantiateMVars body
       mkLetFVars #[x] body (usedLetOnly := usedLetOnly)
   else
-    let f ← withLocalDecl id.getId (kind := kind) .default type fun x => do
+    withLocalDecl id.getId (kind := kind) .default type fun x => do
       addLocalVarInfo id x
       let body ← elabTermEnsuringType body expectedType?
       let body ← instantiateMVars body
-      mkLambdaFVars #[x] body (usedLetOnly := false)
-    pure <| mkLetFunAnnotation (mkApp f val)
+      mkLetFun x val body
   if elabBodyFirst then
     forallBoundedTelescope type binders.size fun xs type => do
       -- the original `fvars` from above are gone, so add back info manually

src/Lean/Elab/BuiltinCommand.lean

@@ -118,6 +118,64 @@ private partial def elabChoiceAux (cmds : Array Syntax) (i : Nat) : CommandElabM
 @[builtin_command_elab choice] def elabChoice : CommandElab := fun stx =>
   elabChoiceAux stx.getArgs 0
 
+/-- Declares one or more universe variables.
+
+`universe u v`
+
+`Prop`, `Type`, `Type u` and `Sort u` are types that classify other types, also known as
+*universes*. In `Type u` and `Sort u`, the variable `u` stands for the universe's *level*, and a
+universe at level `u` can only classify universes that are at levels lower than `u`. For more
+details on type universes, please refer to [the relevant chapter of Theorem Proving in Lean][tpil
+universes].
+
+Just as type arguments allow polymorphic definitions to be used at many different types, universe
+parameters, represented by universe variables, allow a definition to be used at any required level.
+While Lean mostly handles universe levels automatically, declaring them explicitly can provide more
+control when writing signatures. The `universe` keyword allows the declared universe variables to be
+used in a collection of definitions, and Lean will ensure that these definitions use them
+consistently.
+
+[tpil universes]: https://lean-lang.org/theorem_proving_in_lean4/dependent_type_theory.html#types-as-objects
+(Type universes on Theorem Proving in Lean)
+
+```lean
+/- Explicit type-universe parameter. -/
+def id₁.{u} (α : Type u) (a : α) := a
+
+/- Implicit type-universe parameter, equivalent to `id₁`.
+  Requires option `autoImplicit true`, which is the default. -/
+def id₂ (α : Type u) (a : α) := a
+
+/- Explicit standalone universe variable declaration, equivalent to `id₁` and `id₂`. -/
+universe u
+def id₃ (α : Type u) (a : α) := a
+```
+
+On a more technical note, using a universe variable only in the right-hand side of a definition
+causes an error if the universe has not been declared previously.
+
+```lean
+def L₁.{u} := List (Type u)
+
+-- def L₂ := List (Type u) -- error: `unknown universe level 'u'`
+
+universe u
+def L₃ := List (Type u)
+```
+
+## Examples
+
+```lean
+universe u v w
+
+structure Pair (α : Type u) (β : Type v) : Type (max u v) where
+  a : α
+  b : β
+
+#check Pair.{v, w}
+-- Pair : Type v → Type w → Type (max v w)
+```
+-/
 @[builtin_command_elab «universe»] def elabUniverse : CommandElab := fun n => do
   n[1].forArgsM addUnivLevel
 
@@ -126,6 +184,30 @@ private partial def elabChoiceAux (cmds : Array Syntax) (i : Nat) : CommandElabM
   | Except.ok env   => setEnv env
   | Except.error ex => throwError (ex.toMessageData (← getOptions))
 
+/-- Adds names from other namespaces to the current namespace.
+
+The command `export Some.Namespace (name₁ name₂)` makes `name₁` and `name₂`:
+
+- visible in the current namespace without prefix `Some.Namespace`, like `open`, and
+- visible from outside the current namespace `N` as `N.name₁` and `N.name₂`.
+
+## Examples
+
+```lean
+namespace Morning.Sky
+  def star := "venus"
+end Morning.Sky
+
+namespace Evening.Sky
+  export Morning.Sky (star)
+  -- `star` is now in scope
+  #check star
+end Evening.Sky
+
+-- `star` is visible in `Evening.Sky`
+#check Evening.Sky.star
+```
+-/
 @[builtin_command_elab «export»] def elabExport : CommandElab := fun stx => do
   let `(export $ns ($ids*)) := stx | throwUnsupportedSyntax
   let nss ← resolveNamespace ns
@@ -135,9 +217,123 @@ private partial def elabChoiceAux (cmds : Array Syntax) (i : Nat) : CommandElabM
   for idStx in ids do
     let id := idStx.getId
     let declName ← resolveNameUsingNamespaces nss idStx
+    if (← getInfoState).enabled then
+      addConstInfo idStx declName
     aliases := aliases.push (currNamespace ++ id, declName)
   modify fun s => { s with env := aliases.foldl (init := s.env) fun env p => addAlias env p.1 p.2 }
 
+/-- Makes names from other namespaces visible without writing the namespace prefix.
+
+Names that are made available with `open` are visible within the current `section` or `namespace`
+block. This makes referring to (type) definitions and theorems easier, but note that it can also
+make [scoped instances], notations, and attributes from a different namespace available.
+
+The `open` command can be used in a few different ways:
+
+* `open Some.Namespace.Path1 Some.Namespace.Path2` makes all non-protected names in
+  `Some.Namespace.Path1` and `Some.Namespace.Path2` available without the prefix, so that
+  `Some.Namespace.Path1.x` and `Some.Namespace.Path2.y` can be referred to by writing only `x` and
+  `y`.
+
+* `open Some.Namespace.Path hiding def1 def2` opens all non-protected names in `Some.Namespace.Path`
+  except `def1` and `def2`.
+
+* `open Some.Namespace.Path (def1 def2)` only makes `Some.Namespace.Path.def1` and
+  `Some.Namespace.Path.def2` available without the full prefix, so `Some.Namespace.Path.def3` would
+  be unaffected.
+
+  This works even if `def1` and `def2` are `protected`.
+
+* `open Some.Namespace.Path renaming def1 → def1', def2 → def2'` same as `open Some.Namespace.Path
+  (def1 def2)` but `def1`/`def2`'s names are changed to `def1'`/`def2'`.
+
+  This works even if `def1` and `def2` are `protected`.
+
+* `open scoped Some.Namespace.Path1 Some.Namespace.Path2` **only** opens [scoped instances],
+  notations, and attributes from `Namespace1` and `Namespace2`; it does **not** make any other name
+  available.
+
+* `open <any of the open shapes above> in` makes the names `open`-ed visible only in the next
+  command or expression.
+
+[scoped instance]: https://lean-lang.org/theorem_proving_in_lean4/type_classes.html#scoped-instances
+(Scoped instances in Theorem Proving in Lean)
+
+
+## Examples
+
+```lean
+/-- SKI combinators https://en.wikipedia.org/wiki/SKI_combinator_calculus -/
+namespace Combinator.Calculus
+  def I (a : α) : α := a
+  def K (a : α) : β → α := fun _ => a
+  def S (x : α → β → γ) (y : α → β) (z : α) : γ := x z (y z)
+end Combinator.Calculus
+
+section
+  -- open everything under `Combinator.Calculus`, *i.e.* `I`, `K` and `S`,
+  -- until the section ends
+  open Combinator.Calculus
+
+  theorem SKx_eq_K : S K x = I := rfl
+end
+
+-- open everything under `Combinator.Calculus` only for the next command (the next `theorem`, here)
+open Combinator.Calculus in
+theorem SKx_eq_K' : S K x = I := rfl
+
+section
+  -- open only `S` and `K` under `Combinator.Calculus`
+  open Combinator.Calculus (S K)
+
+  theorem SKxy_eq_y : S K x y = y := rfl
+
+  -- `I` is not in scope, we have to use its full path
+  theorem SKxy_eq_Iy : S K x y = Combinator.Calculus.I y := rfl
+end
+
+section
+  open Combinator.Calculus
+    renaming
+      I → identity,
+      K → konstant
+
+  #check identity
+  #check konstant
+end
+
+section
+  open Combinator.Calculus
+    hiding S
+
+  #check I
+  #check K
+end
+
+section
+  namespace Demo
+    inductive MyType
+    | val
+
+    namespace N1
+      scoped infix:68 " ≋ " => BEq.beq
+
+      scoped instance : BEq MyType where
+        beq _ _ := true
+
+      def Alias := MyType
+    end N1
+  end Demo
+
+  -- bring `≋` and the instance in scope, but not `Alias`
+  open scoped Demo.N1
+
+  #check Demo.MyType.val == Demo.MyType.val
+  #check Demo.MyType.val ≋ Demo.MyType.val
+  -- #check Alias -- unknown identifier 'Alias'
+end
+```
+-/
 @[builtin_command_elab «open»] def elabOpen : CommandElab
   | `(open $decl:openDecl) => do
     let openDecls ← elabOpenDecl decl
@@ -223,6 +419,102 @@ private def replaceBinderAnnotation (binder : TSyntax ``Parser.Term.bracketedBin
   else
     return #[binder]
 
+/-- Declares one or more typed variables, or modifies whether already-declared variables are
+implicit.
+
+Introduces variables that can be used in definitions within the same `namespace` or `section` block.
+When a definition mentions a variable, Lean will add it as an argument of the definition. The
+`variable` command is also able to add typeclass parameters. This is useful in particular when
+writing many definitions that have parameters in common (see below for an example).
+
+Variable declarations have the same flexibility as regular function paramaters. In particular they
+can be [explicit, implicit][binder docs], or [instance implicit][tpil classes] (in which case they
+can be anonymous). This can be changed, for instance one can turn explicit variable `x` into an
+implicit one with `variable {x}`. Note that currently, you should avoid changing how variables are
+bound and declare new variables at the same time; see [issue 2789] for more on this topic.
+
+See [*Variables and Sections* from Theorem Proving in Lean][tpil vars] for a more detailed
+discussion.
+
+[tpil vars]: https://lean-lang.org/theorem_proving_in_lean4/dependent_type_theory.html#variables-and-sections
+(Variables and Sections on Theorem Proving in Lean)
+[tpil classes]: https://lean-lang.org/theorem_proving_in_lean4/type_classes.html
+(Type classes on Theorem Proving in Lean)
+[binder docs]: https://leanprover-community.github.io/mathlib4_docs/Lean/Expr.html#Lean.BinderInfo
+(Documentation for the BinderInfo type)
+[issue 2789]: https://github.com/leanprover/lean4/issues/2789
+(Issue 2789 on github)
+
+## Examples
+
+```lean
+section
+  variable
+    {α : Type u}      -- implicit
+    (a : α)           -- explicit
+    [instBEq : BEq α] -- instance implicit, named
+    [Hashable α]      -- instance implicit, anonymous
+
+  def isEqual (b : α) : Bool :=
+    a == b
+
+  #check isEqual
+  -- isEqual.{u} {α : Type u} (a : α) [instBEq : BEq α] (b : α) : Bool
+
+  variable
+    {a} -- `a` is implicit now
+
+  def eqComm {b : α} := a == b ↔ b == a
+
+  #check eqComm
+  -- eqComm.{u} {α : Type u} {a : α} [instBEq : BEq α] {b : α} : Prop
+end
+```
+
+The following shows a typical use of `variable` to factor out definition arguments:
+
+```lean
+variable (Src : Type)
+
+structure Logger where
+  trace : List (Src × String)
+#check Logger
+-- Logger (Src : Type) : Type
+
+namespace Logger
+  -- switch `Src : Type` to be implicit until the `end Logger`
+  variable {Src}
+
+  def empty : Logger Src where
+    trace := []
+  #check empty
+  -- Logger.empty {Src : Type} : Logger Src
+
+  variable (log : Logger Src)
+
+  def len :=
+    log.trace.length
+  #check len
+  -- Logger.len {Src : Type} (log : Logger Src) : Nat
+
+  variable (src : Src) [BEq Src]
+
+  -- at this point all of `log`, `src`, `Src` and the `BEq` instance can all become arguments
+
+  def filterSrc :=
+    log.trace.filterMap
+      fun (src', str') => if src' == src then some str' else none
+  #check filterSrc
+  -- Logger.filterSrc {Src : Type} (log : Logger Src) (src : Src) [inst✝ : BEq Src] : List String
+
+  def lenSrc :=
+    log.filterSrc src |>.length
+  #check lenSrc
+  -- Logger.lenSrc {Src : Type} (log : Logger Src) (src : Src) [inst✝ : BEq Src] : Nat
+end Logger
+```
+
+-/
 @[builtin_command_elab «variable»] def elabVariable : CommandElab
   | `(variable $binders*) => do
     -- Try to elaborate `binders` for sanity checking

src/Lean/Elab/BuiltinNotation.lean

@@ -241,7 +241,8 @@ where
 
 /--
   Helper method for elaborating terms such as `(.+.)` where a constant name is expected.
-  This method is usually used to implement tactics that function names as arguments (e.g., `simp`).
+  This method is usually used to implement tactics that take function names as arguments
+  (e.g., `simp`).
 -/
 def elabCDotFunctionAlias? (stx : Term) : TermElabM (Option Expr) := do
   let some stx ← liftMacroM <| expandCDotArg? stx | pure none
@@ -252,11 +253,21 @@ def elabCDotFunctionAlias? (stx : Term) : TermElabM (Option Expr) := do
       try Term.resolveId? f catch _ => return none
     else
       return none
-  | `(fun $binders* => binop% $f $a $b) =>
+  | `(fun $binders* => binop% $f $a $b)
+  | `(fun $binders* => binop_lazy% $f $a $b)
+  | `(fun $binders* => leftact% $f $a $b)
+  | `(fun $binders* => rightact% $f $a $b)
+  | `(fun $binders* => binrel% $f $a $b)
+  | `(fun $binders* => binrel_no_prop% $f $a $b) =>
     if binders == #[a, b] then
       try Term.resolveId? f catch _ => return none
     else
       return none
+  | `(fun $binders* => unop% $f $a) =>
+    if binders == #[a] then
+      try Term.resolveId? f catch _ => return none
+    else
+      return none
   | _ => return none
 where
   expandCDotArg? (stx : Term) : MacroM (Option Term) :=

src/Lean/Elab/Declaration.lean

@@ -348,7 +348,7 @@ def elabMutual : CommandElab := fun stx => do
         throwErrorAt bad "invalid 'decreasing_by' in 'mutual' block, it must be used after the 'end' keyword"
     elabMutualDef stx[1].getArgs hints
   else
-    throwError "invalid mutual block"
+    throwError "invalid mutual block: either all elements of the block must be inductive declarations, or they must all be definitions/theorems/abbrevs"
 
 /- leading_parser "attribute " >> "[" >> sepBy1 (eraseAttr <|> Term.attrInstance) ", " >> "]" >> many1 ident -/
 @[builtin_command_elab «attribute»] def elabAttr : CommandElab := fun stx => do

src/Lean/Elab/Deriving/DecEq.lean

@@ -24,15 +24,15 @@ where
     | [] => ``(isTrue rfl)
     | (a, b, recField, isProof) :: todo => withFreshMacroScope do
       let rhs ← if isProof then
-        `(have h : $a = $b := rfl; by subst h; exact $(← mkSameCtorRhs todo):term)
+        `(have h : @$a = @$b := rfl; by subst h; exact $(← mkSameCtorRhs todo):term)
       else
-        `(if h : $a = $b then
+        `(if h : @$a = @$b then
            by subst h; exact $(← mkSameCtorRhs todo):term
           else
            isFalse (by intro n; injection n; apply h _; assumption))
       if let some auxFunName := recField then
         -- add local instance for `a = b` using the function being defined `auxFunName`
-        `(let inst := $(mkIdent auxFunName) $a $b; $rhs)
+        `(let inst := $(mkIdent auxFunName) @$a @$b; $rhs)
       else
         return rhs
 
@@ -113,7 +113,11 @@ def mkDecEq (declName : Name) : CommandElabM Bool := do
     return false -- nested inductive types are not supported yet
   else
     let cmds ← liftTermElabM <| mkDecEqCmds indVal
-    cmds.forM elabCommand
+    -- `cmds` can have a number of syntax nodes quadratic in the number of constructors
+    -- and thus create as many info tree nodes, which we never make use of but which can
+    -- significantly slow down e.g. the unused variables linter; avoid creating them
+    withEnableInfoTree false do
+      cmds.forM elabCommand
     return true
 
 partial def mkEnumOfNat (declName : Name) : MetaM Unit := do

src/Lean/Elab/Do.lean

@@ -1595,7 +1595,7 @@ mutual
   partial def doSeqToCode : List Syntax → M CodeBlock
     | [] => do liftMacroM mkPureUnitAction
     | doElem::doElems => withIncRecDepth <| withRef doElem do
-      checkMaxHeartbeats "`do`-expander"
+      checkSystem "`do`-expander"
       match (← liftMacroM <| expandMacro? doElem) with
       | some doElem => doSeqToCode (doElem::doElems)
       | none =>

src/Lean/Elab/Extra.lean

@@ -1,7 +1,7 @@
 /-
 Copyright (c) 2021 Microsoft Corporation. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
-Authors: Leonardo de Moura
+Authors: Leonardo de Moura, Kyle Miller, Sebastian Ullrich
 -/
 import Lean.Elab.App
 import Lean.Elab.BuiltinNotation
@@ -79,13 +79,30 @@ private def throwForInFailure (forInInstance : Expr) : TermElabM Expr :=
 namespace Op
 /-!
 
-The elaborator for `binop%`, `binop_lazy%`, and `unop%` terms.
+The elaborator for expression trees of `binop%`, `binop_lazy%`, `leftact%`, `rightact%`, and `unop%` terms.
 
-It works as follows:
+At a high level, the elaborator tries to solve for a type that each of the operands in the expression tree
+can be coerced to, while taking into account the expected type for the entire expression tree.
+Once this type is computed (and if it exists), it inserts coercions where needed.
+
+Here are brief descriptions of each of the operator types:
+- `binop% f a b` elaborates `f a b` as a binary operator with two operands `a` and `b`,
+  and each operand participates in the protocol.
+- `binop_lazy% f a b` is like `binop%` but elaborates as `f a (fun () => b)`.
+- `unop% f a` elaborates `f a` as a unary operator with one operand `a`, which participates in the protocol.
+- `leftact% f a b` elaborates `f a b` as a left action (the `a` operand "acts upon" the `b` operand).
+  Only `b` participates in the protocol since `a` can have an unrelated type, for example scalar multiplication of vectors.
+- `rightact% f a b` elaborates `f a b` as a right action (the `b` operand "acts upon" the `a` operand).
+  Only `a` participates in the protocol since `b` can have an unrelated type.
+  This is used by `HPow` since, for example, there are both `Real -> Nat -> Real` and `Real -> Real -> Real`
+  exponentiation functions, and we prefer the former in the case of `x ^ 2`, but `binop%` would choose the latter. (#2220)
+- There are also `binrel%` and `binrel_no_prop%` (see the docstring for `elabBinRelCore`).
+
+The elaborator works as follows:
 
 1- Expand macros.
-2- Convert `Syntax` object corresponding to the `binop%` (`binop_lazy%` and `unop%`) term into a `Tree`.
-   The `toTree` method visits nested `binop%` (`binop_lazy%` and `unop%`) terms and parentheses.
+2- Convert `Syntax` object corresponding to the `binop%/...` term into a `Tree`.
+   The `toTree` method visits nested `binop%/...` terms and parentheses.
 3- Synthesize pending metavariables without applying default instances and using the
    `(mayPostpone := true)`.
 4- Tries to compute a maximal type for the tree computed at step 2.
@@ -117,18 +134,29 @@ coercions inside of a `HAdd` instance.
 
 Remarks:
 
-In the new `binop%` and related elaborators the decision whether a coercion will be inserted or not
-is made at `binop%` elaboration time. This was not the case in the old elaborator.
-For example, an instance, such as `HAdd Int ?m ?n`, could be created when executing
-the `binop%` elaborator, and only resolved much later. We try to minimize this problem
-by synthesizing pending metavariables at step 3.
+* In the new `binop%` and related elaborators the decision whether a coercion will be inserted or not
+  is made at `binop%` elaboration time. This was not the case in the old elaborator.
+  For example, an instance, such as `HAdd Int ?m ?n`, could be created when executing
+  the `binop%` elaborator, and only resolved much later. We try to minimize this problem
+  by synthesizing pending metavariables at step 3.
 
-For types containing heterogeneous operators (e.g., matrix multiplication), step 4 will fail
-and we will skip coercion insertion. For example, `x : Matrix Real 5 4` and `y : Matrix Real 4 8`,
-there is no coercion `Matrix Real 5 4` from `Matrix Real 4 8` and vice-versa, but
-`x * y` is elaborated successfully and has type `Matrix Real 5 8`.
+* For types containing heterogeneous operators (e.g., matrix multiplication), step 4 will fail
+  and we will skip coercion insertion. For example, `x : Matrix Real 5 4` and `y : Matrix Real 4 8`,
+  there is no coercion `Matrix Real 5 4` from `Matrix Real 4 8` and vice-versa, but
+  `x * y` is elaborated successfully and has type `Matrix Real 5 8`.
+
+* The `leftact%` and `rightact%` elaborators are to handle binary operations where only one of
+  the arguments participates in the protocol. For example, in `2 ^ n + y` with `n : Nat` and `y : Real`,
+  we do not want to coerce `n` to be a real as well, but we do want to elaborate `2 : Real`.
 -/
 
+private inductive BinOpKind where
+  | regular   -- `binop%`
+  | lazy      -- `binop_lazy%`
+  | leftact   -- `leftact%`
+  | rightact  -- `rightact%`
+deriving BEq
+
 private inductive Tree where
   /--
   Leaf of the tree.
@@ -137,9 +165,9 @@ private inductive Tree where
   -/
   | term (ref : Syntax) (infoTrees : PersistentArray InfoTree) (val : Expr)
   /--
-  `ref` is the original syntax that expanded into `binop%`.
+  `ref` is the original syntax that expanded into `binop%/...`.
   -/
-  | binop (ref : Syntax) (lazy : Bool) (f : Expr) (lhs rhs : Tree)
+  | binop (ref : Syntax) (kind : BinOpKind) (f : Expr) (lhs rhs : Tree)
   /--
   `ref` is the original syntax that expanded into `unop%`.
   -/
@@ -164,9 +192,11 @@ private partial def toTree (s : Syntax) : TermElabM Tree := do
 where
   go (s : Syntax) := do
     match s with
-    | `(binop% $f $lhs $rhs) => processBinOp (lazy := false) s f lhs rhs
-    | `(binop_lazy% $f $lhs $rhs) => processBinOp (lazy := true) s f lhs rhs
+    | `(binop% $f $lhs $rhs) => processBinOp s .regular f lhs rhs
+    | `(binop_lazy% $f $lhs $rhs) => processBinOp s .lazy f lhs rhs
     | `(unop% $f $arg) => processUnOp s f arg
+    | `(leftact% $f $lhs $rhs) => processBinOp s .leftact f lhs rhs
+    | `(rightact% $f $lhs $rhs) => processBinOp s .rightact f lhs rhs
     | `(($e)) =>
       if hasCDot e then
         processLeaf s
@@ -181,9 +211,12 @@ where
             return .macroExpansion macroName s s' (← go s')
         | none => processLeaf s
 
-  processBinOp (ref : Syntax) (f lhs rhs : Syntax) (lazy : Bool) := do
+  processBinOp (ref : Syntax) (kind : BinOpKind) (f lhs rhs : Syntax) := do
     let some f ← resolveId? f | throwUnknownConstant f.getId
-    return .binop (lazy := lazy) ref f (← go lhs) (← go rhs)
+    -- treat corresponding argument as leaf for `leftact/rightact`
+    let lhs ← if kind == .leftact then processLeaf lhs else go lhs
+    let rhs ← if kind == .rightact then processLeaf rhs else go rhs
+    return .binop ref kind f lhs rhs
 
   processUnOp (ref : Syntax) (f arg : Syntax) := do
     let some f ← resolveId? f | throwUnknownConstant f.getId
@@ -229,6 +262,8 @@ where
      unless (← get).hasUncomparable do
        match t with
        | .macroExpansion _ _ _ nested => go nested
+       | .binop _ .leftact  _ _ rhs => go rhs
+       | .binop _ .rightact _ lhs _ => go lhs
        | .binop _ _ _ lhs rhs => go lhs; go rhs
        | .unop _ _ arg => go arg
        | .term _ _ val =>
@@ -259,9 +294,9 @@ private def toExprCore (t : Tree) : TermElabM Expr := do
   match t with
   | .term _ trees e =>
     modifyInfoState (fun s => { s with trees := s.trees ++ trees }); return e
-  | .binop ref lazy f lhs rhs =>
+  | .binop ref kind f lhs rhs =>
     withRef ref <| withInfoContext' ref (mkInfo := mkTermInfo .anonymous ref) do
-      mkBinOp lazy f (← toExprCore lhs) (← toExprCore rhs)
+      mkBinOp (kind == .lazy) f (← toExprCore lhs) (← toExprCore rhs)
   | .unop ref f arg =>
     withRef ref <| withInfoContext' ref (mkInfo := mkTermInfo .anonymous ref) do
       mkUnOp f (← toExprCore arg)
@@ -347,7 +382,11 @@ mutual
   where
     go (t : Tree) (f? : Option Expr) (lhs : Bool) (isPred : Bool) : TermElabM Tree := do
       match t with
-      | .binop ref lazy f lhs rhs =>
+      | .binop ref .leftact f lhs rhs =>
+        return .binop ref .leftact f lhs (← go rhs none false false)
+      | .binop ref .rightact f lhs rhs =>
+        return .binop ref .rightact f (← go lhs none false false) rhs
+      | .binop ref kind f lhs rhs =>
         /-
           We only keep applying coercions to `maxType` if `f` is predicate or
           `f` has a homogenous instance with `maxType`. See `hasHomogeneousInstance` for additional details.
@@ -355,10 +394,10 @@ mutual
           Remark: We assume `binrel%` elaborator is only used with homogenous predicates.
         -/
         if (← pure isPred <||> hasHomogeneousInstance f maxType) then
-          return .binop ref lazy f (← go lhs f true false) (← go rhs f false false)
+          return .binop ref kind f (← go lhs f true false) (← go rhs f false false)
         else
           let r ← withRef ref do
-            mkBinOp lazy f (← toExpr lhs none) (← toExpr rhs none)
+            mkBinOp (kind == .lazy) f (← toExpr lhs none) (← toExpr rhs none)
           let infoTrees ← getResetInfoTrees
           return .term ref infoTrees r
       | .unop ref f arg =>
@@ -396,22 +435,24 @@ end
 def elabOp : TermElab := fun stx expectedType? => do
   toExpr (← toTree stx) expectedType?
 
-@[builtin_term_elab binop]
-def elabBinOp : TermElab := elabOp
-
-@[builtin_term_elab binop_lazy]
-def elabBinOpLazy : TermElab := elabOp
-
-@[builtin_term_elab unop]
-def elabUnOp : TermElab := elabOp
+@[builtin_term_elab binop] def elabBinOp : TermElab := elabOp
+@[builtin_term_elab binop_lazy] def elabBinOpLazy : TermElab := elabOp
+@[builtin_term_elab leftact] def elabLeftact : TermElab := elabOp
+@[builtin_term_elab rightact] def elabRightact : TermElab := elabOp
+@[builtin_term_elab unop] def elabUnOp : TermElab := elabOp
 
 /--
-  Elaboration functionf for `binrel%` and `binrel_no_prop%` notations.
+  Elaboration functions for `binrel%` and `binrel_no_prop%` notations.
   We use the infrastructure for `binop%` to make sure we propagate information between the left and right hand sides
   of a binary relation.
 
-  Recall that the `binrel_no_prop%` notation is used for relations such as `==` which do not support `Prop`, but
-  we still want to be able to write `(5 > 2) == (2 > 1)`.
+  - `binrel% R x y` elaborates `R x y` using the `binop%/...` expression trees in both `x` and `y`.
+    It is similar to how `binop% R x y` elaborates but with a significant difference:
+    it does not use the expected type when computing the types of the operads.
+  - `binrel_no_prop% R x y` elaborates `R x y` like `binrel% R x y`, but if the resulting type for `x` and `y`
+    is `Prop` they are coerced to `Bool`.
+    This is used for relations such as `==` which do not support `Prop`, but we still want
+    to be able to write `(5 > 2) == (2 > 1)` for example.
 -/
 def elabBinRelCore (noProp : Bool) (stx : Syntax) (expectedType? : Option Expr) : TermElabM Expr :=  do
   match (← resolveId? stx[1]) with
@@ -448,7 +489,7 @@ def elabBinRelCore (noProp : Bool) (stx : Syntax) (expectedType? : Option Expr)
     -/
     let lhs ← withRef stx[2] <| toTree stx[2]
     let rhs ← withRef stx[3] <| toTree stx[3]
-    let tree := .binop (lazy := false) stx f lhs rhs
+    let tree := .binop stx .regular f lhs rhs
     let r ← analyze tree none
     trace[Elab.binrel] "hasUncomparable: {r.hasUncomparable}, maxType: {r.max?}"
     if r.hasUncomparable || r.max?.isNone then

src/Lean/Elab/Frontend.lean

@@ -102,7 +102,8 @@ def runFrontend
     : IO (Environment × Bool) := do
   let inputCtx := Parser.mkInputContext input fileName
   let (header, parserState, messages) ← Parser.parseHeader inputCtx
-  let (env, messages) ← processHeader header opts messages inputCtx trustLevel
+  -- allow `env` to be leaked, which would live until the end of the process anyway
+  let (env, messages) ← processHeader (leakEnv := true) header opts messages inputCtx trustLevel
   let env := env.setMainModule mainModuleName
   let mut commandState := Command.mkState env messages opts
 

src/Lean/Elab/Import.lean

@@ -16,10 +16,11 @@ def headerToImports (header : Syntax) : Array Import :=
     let id      := stx[2].getId
     { module := id, runtimeOnly := runtime }
 
-def processHeader (header : Syntax) (opts : Options) (messages : MessageLog) (inputCtx : Parser.InputContext) (trustLevel : UInt32 := 0)
+def processHeader (header : Syntax) (opts : Options) (messages : MessageLog)
+    (inputCtx : Parser.InputContext) (trustLevel : UInt32 := 0) (leakEnv := false)
     : IO (Environment × MessageLog) := do
   try
-    let env ← importModules (headerToImports header) opts trustLevel
+    let env ← importModules (leakEnv := leakEnv) (headerToImports header) opts trustLevel
     pure (env, messages)
   catch e =>
     let env ← mkEmptyEnvironment

src/Lean/Elab/InfoTree/Main.lean

@@ -142,7 +142,7 @@ def MacroExpansionInfo.format (ctx : ContextInfo) (info : MacroExpansionInfo) :
   return f!"Macro expansion\n{stx}\n===>\n{output}"
 
 def UserWidgetInfo.format (info : UserWidgetInfo) : Format :=
-  f!"UserWidget {info.widgetId}\n{Std.ToFormat.format info.props}"
+  f!"UserWidget {info.id}\n{Std.ToFormat.format <| info.props.run' {}}"
 
 def FVarAliasInfo.format (info : FVarAliasInfo) : Format :=
   f!"FVarAlias {info.userName.eraseMacroScopes}"

src/Lean/Elab/InfoTree/Types.lean

@@ -9,6 +9,8 @@ import Lean.Data.OpenDecl
 import Lean.MetavarContext
 import Lean.Environment
 import Lean.Data.Json
+import Lean.Server.Rpc.Basic
+import Lean.Widget.Types
 
 namespace Lean.Elab
 
@@ -95,17 +97,12 @@ structure CustomInfo where
   stx : Syntax
   value : Dynamic
 
-/-- An info that represents a user-widget.
-User-widgets are custom pieces of code that run on the editor client.
-You can learn about user widgets at `src/Lean/Widget/UserWidget`
--/
-structure UserWidgetInfo where
+/-- Information about a user widget associated with a syntactic span.
+This must be a panel widget.
+A panel widget is a widget that can be displayed
+in the infoview alongside the goal state. -/
+structure UserWidgetInfo extends Widget.WidgetInstance where
   stx : Syntax
-  /-- Id of `WidgetSource` object to use. -/
-  widgetId : Name
-  /-- Json representing the props to be loaded in to the component. -/
-  props : Json
-  deriving Inhabited
 
 /--
 Specifies that the given free variables should be considered semantically identical.

src/Lean/Elab/Open.lean

@@ -57,7 +57,7 @@ private def resolveNameUsingNamespacesCore (nss : List Name) (idStx : Syntax) :
   else
     withRef idStx do throwError "ambiguous identifier '{idStx.getId}', possible interpretations: {result.map mkConst}"
 
-def elabOpenDecl [MonadResolveName m] (stx : TSyntax ``Parser.Command.openDecl) : m (List OpenDecl) := do
+def elabOpenDecl [MonadResolveName m] [MonadInfoTree m] (stx : TSyntax ``Parser.Command.openDecl) : m (List OpenDecl) := do
   StateRefT'.run' (s := { openDecls := (← getOpenDecls), currNamespace := (← getCurrNamespace) }) do
     match stx with
     | `(Parser.Command.openDecl| $nss*) =>
@@ -73,18 +73,25 @@ def elabOpenDecl [MonadResolveName m] (stx : TSyntax ``Parser.Command.openDecl)
       let nss ← resolveNamespace ns
       for idStx in ids do
         let declName ← resolveNameUsingNamespacesCore nss idStx
+        if (← getInfoState).enabled then
+          addConstInfo idStx declName
         addOpenDecl (OpenDecl.explicit idStx.getId declName)
     | `(Parser.Command.openDecl| $ns hiding $ids*) =>
       let ns ← resolveUniqueNamespace ns
       activateScoped ns
       for id in ids do
-        let _ ← resolveId ns id
+        let declName ← resolveId ns id
+        if (← getInfoState).enabled then
+          addConstInfo id declName
       let ids := ids.map (·.getId) |>.toList
       addOpenDecl (OpenDecl.simple ns ids)
     | `(Parser.Command.openDecl| $ns renaming $[$froms -> $tos],*) =>
       let ns ← resolveUniqueNamespace ns
       for («from», to) in froms.zip tos do
         let declName ← resolveId ns «from»
+        if (← getInfoState).enabled then
+          addConstInfo «from» declName
+          addConstInfo to declName
         addOpenDecl (OpenDecl.explicit to.getId declName)
     | _ => throwUnsupportedSyntax
     return (← get).openDecls

src/Lean/Elab/PatternVar.lean

@@ -146,7 +146,6 @@ partial def collect (stx : Syntax) : M Syntax := withRef stx <| withFreshMacroSc
       ```
       def namedPattern := check... >> trailing_parser "@" >> optional (atomic (ident >> ":")) >> termParser
       ```
-      TODO: pattern variable for equality proof
      -/
     let id := stx[0]
     discard <| processVar id

src/Lean/Elab/PreDefinition/Basic.lean

@@ -62,7 +62,7 @@ def applyAttributesOf (preDefs : Array PreDefinition) (applicationTime : Attribu
   for preDef in preDefs do
     applyAttributesAt preDef.declName preDef.modifiers.attrs applicationTime
 
-def abstractNestedProofs (preDef : PreDefinition) : MetaM PreDefinition :=
+def abstractNestedProofs (preDef : PreDefinition) : MetaM PreDefinition := withRef preDef.ref do
   if preDef.kind.isTheorem || preDef.kind.isExample then
     pure preDef
   else do

src/Lean/Elab/PreDefinition/Eqns.lean

@@ -21,16 +21,25 @@ structure EqnInfoCore where
   value       : Expr
   deriving Inhabited
 
-partial def expand : Expr → Expr
-  | Expr.letE _ _ v b _ => expand (b.instantiate1 v)
-  | Expr.mdata _ b      => expand b
-  | e => e
+/--
+Zeta reduces `let` and `let_fun` while consuming metadata.
+Returns true if progress is made.
+-/
+partial def expand (progress : Bool) (e : Expr) : Bool × Expr :=
+  match e with
+  | Expr.letE _ _ v b _ => expand true (b.instantiate1 v)
+  | Expr.mdata _ b      => expand true b
+  | e =>
+    if let some (_, _, v, b) := e.letFun? then
+      expand true (b.instantiate1 v)
+    else
+      (progress, e)
 
 def expandRHS? (mvarId : MVarId) : MetaM (Option MVarId) := do
   let target ← mvarId.getType'
   let some (_, lhs, rhs) := target.eq? | return none
-  unless rhs.isLet || rhs.isMData do return none
-  return some (← mvarId.replaceTargetDefEq (← mkEq lhs (expand rhs)))
+  let (true, rhs') := expand false rhs | return none
+  return some (← mvarId.replaceTargetDefEq (← mkEq lhs rhs'))
 
 def funext? (mvarId : MVarId) : MetaM (Option MVarId) := do
   let target ← mvarId.getType'

src/Lean/Elab/PreDefinition/Main.lean

@@ -100,8 +100,8 @@ def addPreDefinitions (preDefs : Array PreDefinition) (hints : TerminationHints)
   let preDefs ← preDefs.mapM ensureNoUnassignedMVarsAtPreDef
   let preDefs ← betaReduceLetRecApps preDefs
   let cliques := partitionPreDefs preDefs
-  let mut terminationBy ← liftMacroM <| WF.expandTerminationBy hints.terminationBy? (cliques.map fun ds => ds.map (·.declName))
-  let mut decreasingBy  ← liftMacroM <| WF.expandTerminationHint hints.decreasingBy? (cliques.map fun ds => ds.map (·.declName))
+  let mut terminationBy ← liftMacroM <| WF.expandTerminationBy? hints.terminationBy? (cliques.map fun ds => ds.map (·.declName))
+  let mut decreasingBy  ← liftMacroM <| WF.expandDecreasingBy? hints.decreasingBy? (cliques.map fun ds => ds.map (·.declName))
   let mut hasErrors := false
   for preDefs in cliques do
     trace[Elab.definition.scc] "{preDefs.map (·.declName)}"

src/Lean/Elab/PreDefinition/Structural/BRecOn.lean

@@ -113,12 +113,12 @@ private partial def replaceRecApps (recFnName : Name) (recArgInfo : RecArgInfo)
     | Expr.letE n type val body _ =>
       withLetDecl n (← loop below type) (← loop below val) fun x => do
         mkLetFVars #[x] (← loop below (body.instantiate1 x)) (usedLetOnly := false)
-    | Expr.mdata d b     =>
-      if let some _ := getRecAppSyntax? e then
-        loop below b
+    | Expr.mdata d b =>
+      if let some stx := getRecAppSyntax? e then
+        withRef stx <| loop below b
       else
         return mkMData d (← loop below b)
-    | Expr.proj n i e    => return mkProj n i (← loop below e)
+    | Expr.proj n i e => return mkProj n i (← loop below e)
     | Expr.app _ _ =>
       let processApp (e : Expr) : StateRefT (HasConstCache recFnName) M Expr :=
         e.withApp fun f args => do

src/Lean/Elab/PreDefinition/Structural/IndPred.lean

@@ -23,7 +23,11 @@ private partial def replaceIndPredRecApps (recFnName : Name) (recArgInfo : RecAr
     | Expr.letE n type val body _ =>
       withLetDecl n (← loop type) (← loop val) fun x => do
         mkLetFVars #[x] (← loop (body.instantiate1 x))
-    | Expr.mdata d e     => return mkMData d (← loop e)
+    | Expr.mdata d b => do
+      if let some stx := getRecAppSyntax? e then
+        withRef stx <| loop b
+      else
+        return mkMData d (← loop b)
     | Expr.proj n i e    => return mkProj n i (← loop e)
     | Expr.app _ _ =>
       let processApp (e : Expr) : M Expr := do

src/Lean/Elab/PreDefinition/Structural/Main.lean

@@ -83,19 +83,22 @@ def structuralRecursion (preDefs : Array PreDefinition) : TermElabM Unit :=
   else do
     let ((recArgPos, preDefNonRec), state) ← run <| elimRecursion preDefs[0]!
     let preDefNonRec ← eraseRecAppSyntax preDefNonRec
-    let preDef ← eraseRecAppSyntax preDefs[0]!
+    let mut preDef ← eraseRecAppSyntax preDefs[0]!
     state.addMatchers.forM liftM
+    mapError (addNonRec preDefNonRec (applyAttrAfterCompilation := false)) fun msg =>
+      m!"structural recursion failed, produced type incorrect term{indentD msg}"
+    -- We create the `_unsafe_rec` before we abstract nested proofs.
+    -- Reason: the nested proofs may be referring to the _unsafe_rec.
+    addAndCompilePartialRec #[preDef]
     unless preDef.kind.isTheorem do
       unless (← isProp preDef.type) do
+        preDef ← abstractNestedProofs preDef
         /-
         Don't save predefinition info for equation generator
         for theorems and definitions that are propositions.
         See issue #2327
         -/
         registerEqnsInfo preDef recArgPos
-    mapError (addNonRec preDefNonRec (applyAttrAfterCompilation := false)) fun msg =>
-      m!"structural recursion failed, produced type incorrect term{indentD msg}"
-    addAndCompilePartialRec #[preDef]
     addSmartUnfoldingDef preDef recArgPos
     applyAttributesOf #[preDefNonRec] AttributeApplicationTime.afterCompilation
 

src/Lean/Elab/PreDefinition/Structural/Preprocess.lean

@@ -4,6 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Leonardo de Moura
 -/
 import Lean.Meta.Transform
+import Lean.Elab.RecAppSyntax
 
 namespace Lean.Elab.Structural
 open Meta
@@ -15,21 +16,39 @@ private def shouldBetaReduce (e : Expr) (recFnName : Name) : Bool :=
     false
 
 /--
-  Beta reduce terms where the recursive function occurs in the lambda term.
-  This is useful to improve the effectiveness of `elimRecursion`.
+Preprocesses the expessions to improve the effectiveness of `elimRecursion`.
+
+* Beta reduce terms where the recursive function occurs in the lambda term.
   Example:
   ```
   def f : Nat → Nat
     | 0 => 1
     | i+1 => (fun x => f x) i
   ```
+
+* Floats out the RecApp markers.
+  Example:
+  ```
+  def f : Nat → Nat
+    | 0 => 1
+    | i+1 => (f x) i
+  ```
 -/
 def preprocess (e : Expr) (recFnName : Name) : CoreM Expr :=
   Core.transform e
-   fun e =>
-     if shouldBetaReduce e recFnName then
-       return .visit e.headBeta
-     else
-       return .continue
+    (pre := fun e =>
+      if shouldBetaReduce e recFnName then
+        return .visit e.headBeta
+      else
+        return .continue)
+    (post := fun e =>
+      match e with
+      | .app (.mdata m f) a =>
+        if m.isRecApp then
+          return .done (.mdata m (.app f a))
+        else
+          return .done e
+      | _ => return .done e)
+
 
 end Lean.Elab.Structural

src/Lean/Elab/PreDefinition/WF/Eqns.lean

@@ -122,7 +122,7 @@ where
     match (← reduceRecMatcher? e) with
     | some e' => return Simp.Step.done { expr := e' }
     | none    =>
-      match (← Simp.simpMatchCore? app e SplitIf.discharge?) with
+      match (← Simp.simpMatchCore? app.matcherName e SplitIf.discharge?) with
       | some r => return r
       | none => return Simp.Step.visit { expr := e }
 

src/Lean/Elab/PreDefinition/WF/Fix.lean

@@ -13,28 +13,25 @@ import Lean.Elab.RecAppSyntax
 import Lean.Elab.PreDefinition.Basic
 import Lean.Elab.PreDefinition.Structural.Basic
 import Lean.Elab.PreDefinition.Structural.BRecOn
+import Lean.Data.Array
 
 namespace Lean.Elab.WF
 open Meta
 
-private def applyDefaultDecrTactic (mvarId : MVarId) : TermElabM Unit := do
-  let remainingGoals ← Tactic.run mvarId do
-    Tactic.evalTactic (← `(tactic| decreasing_tactic))
-  remainingGoals.forM fun mvarId => Term.reportUnsolvedGoals [mvarId]
-
-private def mkDecreasingProof (decreasingProp : Expr) (decrTactic? : Option Syntax) : TermElabM Expr := do
-  let mvar ← mkFreshExprSyntheticOpaqueMVar decreasingProp
+/-
+Creates a subgoal for a recursive call, as an unsolved `MVar`. The goal is cleaned up, and
+the current syntax reference is stored in the `MVar`’s type as a `RecApp` marker, for
+use by `solveDecreasingGoals` below.
+-/
+private def mkDecreasingProof (decreasingProp : Expr) : TermElabM Expr := do
+  -- We store the current Ref in the MVar as a RecApp annotation around the type
+  let ref ← getRef
+  let mvar ← mkFreshExprSyntheticOpaqueMVar (mkRecAppWithSyntax decreasingProp ref)
   let mvarId := mvar.mvarId!
-  let mvarId ← mvarId.cleanup
-  match decrTactic? with
-  | none => applyDefaultDecrTactic mvarId
-  | some decrTactic =>
-    -- make info from `runTactic` available
-    pushInfoTree (.hole mvarId)
-    Term.runTactic mvarId decrTactic
-  instantiateMVars mvar
+  let _mvarId ← mvarId.cleanup
+  return mvar
 
-private partial def replaceRecApps (recFnName : Name) (fixedPrefixSize : Nat) (decrTactic? : Option Syntax) (F : Expr) (e : Expr) : TermElabM Expr := do
+private partial def replaceRecApps (recFnName : Name) (fixedPrefixSize : Nat) (F : Expr) (e : Expr) : TermElabM Expr := do
   trace[Elab.definition.wf] "replaceRecApps:{indentExpr e}"
   trace[Elab.definition.wf] "{F} : {← inferType F}"
   loop F e |>.run' {}
@@ -46,7 +43,7 @@ where
       let args := e.getAppArgs
       let r := mkApp F (← loop F args[fixedPrefixSize]!)
       let decreasingProp := (← whnf (← inferType r)).bindingDomain!
-      let r := mkApp r (← mkDecreasingProof decreasingProp decrTactic?)
+      let r := mkApp r (← mkDecreasingProof decreasingProp)
       return mkAppN r (← args[fixedPrefixSize+1:].toArray.mapM (loop F))
 
   processApp (F : Expr) (e : Expr) : StateRefT (HasConstCache recFnName) TermElabM Expr := do
@@ -163,6 +160,47 @@ private partial def processPSigmaCasesOn (x F val : Expr) (k : (F : Expr) → (v
   else
     k F val
 
+private def applyDefaultDecrTactic (mvarId : MVarId) : TermElabM Unit := do
+  let remainingGoals ← Tactic.run mvarId do
+    Tactic.evalTactic (← `(tactic| decreasing_tactic))
+  unless remainingGoals.isEmpty do
+    Term.reportUnsolvedGoals remainingGoals
+
+/-
+Given an array of MVars, assign MVars with equal type and subsumed local context to each other.
+Returns those MVar that did not get assigned.
+-/
+def assignSubsumed (mvars : Array MVarId) : MetaM (Array MVarId) :=
+  mvars.filterPairsM fun mv₁ mv₂ => do
+    let mvdecl₁ ← mv₁.getDecl
+    let mvdecl₂ ← mv₂.getDecl
+    if mvdecl₁.type == mvdecl₂.type then
+      -- same goals; check contexts.
+        if mvdecl₁.lctx.isSubPrefixOf mvdecl₂.lctx then
+          -- mv₁ is better
+          mv₂.assign (.mvar mv₁)
+          return (true, false)
+        if mvdecl₂.lctx.isSubPrefixOf mvdecl₁.lctx then
+          -- mv₂ is better
+          mv₁.assign (.mvar mv₂)
+          return (false, true)
+    return (true, true)
+
+def solveDecreasingGoals (decrTactic? : Option Syntax) (value : Expr) : MetaM Expr := do
+  let goals ← getMVarsNoDelayed value
+  let goals ← assignSubsumed goals
+  goals.forM fun goal => Lean.Elab.Term.TermElabM.run' <|
+    match decrTactic? with
+    | none => do
+      let some ref := getRecAppSyntax? (← goal.getType)
+        | throwError "MVar does not look like like a recursive call"
+        withRef ref <| applyDefaultDecrTactic goal
+    | some decrTactic => do
+      -- make info from `runTactic` available
+      pushInfoTree (.hole goal)
+      Term.runTactic goal decrTactic
+  instantiateMVars value
+
 def mkFix (preDef : PreDefinition) (prefixArgs : Array Expr) (wfRel : Expr) (decrTactic? : Option Syntax) : TermElabM Expr := do
   let type ← instantiateForall preDef.type prefixArgs
   let (wfFix, varName) ← forallBoundedTelescope type (some 1) fun x type => do
@@ -185,7 +223,8 @@ def mkFix (preDef : PreDefinition) (prefixArgs : Array Expr) (wfRel : Expr) (dec
       let F   := xs[1]!
       let val := preDef.value.beta (prefixArgs.push x)
       let val ← processSumCasesOn x F val fun x F val => do
-        processPSigmaCasesOn x F val (replaceRecApps preDef.declName prefixArgs.size decrTactic?)
+        processPSigmaCasesOn x F val (replaceRecApps preDef.declName prefixArgs.size)
+      let val ← solveDecreasingGoals decrTactic? val
       mkLambdaFVars prefixArgs (mkApp wfFix (← mkLambdaFVars #[x, F] val))
 
 end Lean.Elab.WF

src/Lean/Elab/PreDefinition/WF/GuessLex.lean

@@ -0,0 +1,711 @@
+/-
+Copyright (c) 2023 Lean FRO, LLC. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Joachim Breitner
+-/
+
+import Lean.Util.HasConstCache
+import Lean.Meta.CasesOn
+import Lean.Meta.Match.Match
+import Lean.Meta.Tactic.Cleanup
+import Lean.Meta.Tactic.Refl
+import Lean.Elab.Quotation
+import Lean.Elab.RecAppSyntax
+import Lean.Elab.PreDefinition.Basic
+import Lean.Elab.PreDefinition.Structural.Basic
+import Lean.Elab.PreDefinition.WF.TerminationHint
+import Lean.Elab.PreDefinition.WF.PackMutual
+import Lean.Data.Array
+
+
+/-!
+This module finds lexicographic termination arguments for well-founded recursion.
+
+Starting with basic measures (`sizeOf xᵢ` for all parameters `xᵢ`), it tries all combinations
+until it finds one where all proof obligations go through with the given tactic (`decerasing_by`),
+if given, or the default `decreasing_tactic`.
+
+For mutual recursion, a single measure is not just one parameter, but one from each recursive
+function. Enumerating these can lead to a combinatoric explosion, so we bound
+the nubmer of measures tried.
+
+In addition to measures derived from `sizeOf xᵢ`, it also considers measures
+that assign an order to the functions themselves. This way we can support mutual
+function definitions where no arguments decrease from one function to another.
+
+The result of this module is a `TerminationWF`, which is then passed on to `wfRecursion`; this
+design is crucial so that whatever we infer in this module could also be written manually by the
+user. It would be bad if there are function definitions that can only be processed with the
+guessed lexicographic order.
+
+The following optimizations are applied to make this feasible:
+
+1. The crucial optimiziation is to look at each argument of each recursive call
+   _once_, try to prove `<` and (if that fails `≤`), and then look at that table to
+   pick a suitable measure.
+
+2. The next-crucial optimization is to fill that table lazily.  This way, we run the (likely
+   expensive) tactics as few times as possible, while still being able to consider a possibly
+   large number of combinations.
+
+3. Before we even try to prove `<`, we check if the arguments are equal (`=`). No well-founded
+   measure will relate equal terms, likely this check is faster than firing up the tactic engine,
+   and it adds more signal to the output.
+
+4. Instead of traversing the whole function body over and over, we traverse it once and store
+   the arguments (in unpacked form) and the local `MetaM` state at each recursive call
+   (see `collectRecCalls`), which we then re-use for the possibly many proof attempts.
+
+The logic here is based on “Finding Lexicographic Orders for Termination Proofs in Isabelle/HOL”
+by Lukas Bulwahn, Alexander Krauss, and Tobias Nipkow, 10.1007/978-3-540-74591-4_5
+<https://www21.in.tum.de/~nipkow/pubs/tphols07.pdf>.
+-/
+
+set_option autoImplicit false
+
+open Lean Meta Elab
+
+namespace Lean.Elab.WF.GuessLex
+
+register_builtin_option showInferredTerminationBy : Bool := {
+  defValue := false
+  descr    := "In recursive definitions, show the inferred `termination_by` measure."
+}
+
+/--
+Given a predefinition, find good variable names for its parameters.
+Use user-given parameter names if present; use x1...xn otherwise.
+
+The length of the returned array is also used to determine the arity
+of the function, so it should match what `packDomain` does.
+
+The names ought to accessible (no macro scopes) and still fresh wrt to the current environment,
+so that with `showInferredTerminationBy` we can print them to the user reliably.
+We do that by appending `'` as needed.
+-/
+partial
+def naryVarNames (fixedPrefixSize : Nat) (preDef : PreDefinition) : MetaM (Array Name):= do
+  lambdaTelescope preDef.value fun xs _ => do
+    let xs := xs.extract fixedPrefixSize xs.size
+    let mut ns : Array Name := #[]
+    for h : i in [:xs.size] do
+      let n ← (xs[i]'h.2).fvarId!.getUserName
+      let n := if n.hasMacroScopes then .mkSimple s!"x{i+1}" else n
+      ns := ns.push (← freshen ns n)
+    return ns
+  where
+    freshen  (ns : Array Name) (n : Name): MetaM Name := do
+      if !(ns.elem n) && (← resolveGlobalName n).isEmpty then
+        return n
+      else
+        freshen ns (n.appendAfter "'")
+
+
+/-- Internal monad used by `withRecApps` -/
+abbrev M (recFnName : Name) (α β : Type) : Type :=
+  StateRefT (Array α) (StateRefT (HasConstCache recFnName) MetaM) β
+
+/--
+Traverses the given expression `e`, and invokes the continuation `k`
+at every saturated call to `recFnName`.
+
+The expression `param` is passed along, and refined when going under a matcher
+or `casesOn` application.
+-/
+partial def withRecApps {α} (recFnName : Name) (fixedPrefixSize : Nat) (param : Expr) (e : Expr)
+    (k : Expr → Array Expr → MetaM α) : MetaM (Array α) := do
+  trace[Elab.definition.wf] "withRecApps: {indentExpr e}"
+  let (_, as) ← loop param e |>.run #[] |>.run' {}
+  return as
+where
+  processRec (param : Expr) (e : Expr) : M recFnName α Unit := do
+    if e.getAppNumArgs < fixedPrefixSize + 1 then
+      loop param (← etaExpand e)
+    else
+      let a ← k param e.getAppArgs
+      modifyThe (Array α) (·.push a)
+
+  processApp (param : Expr) (e : Expr) : M recFnName α Unit := do
+    e.withApp fun f args => do
+      args.forM (loop param)
+      if f.isConstOf recFnName then
+        processRec param e
+      else
+        loop param f
+
+  containsRecFn (e : Expr) : M recFnName α Bool := do
+    modifyGetThe (HasConstCache recFnName) (·.contains e)
+
+  loop (param : Expr) (e : Expr) : M recFnName α Unit := do
+    if !(← containsRecFn e) then
+      return
+    match e with
+    | Expr.lam n d b c =>
+      loop param d
+      withLocalDecl n c d fun x => do
+        loop param (b.instantiate1 x)
+    | Expr.forallE n d b c =>
+      loop param d
+      withLocalDecl n c d fun x => do
+        loop param (b.instantiate1 x)
+    | Expr.letE n type val body _ =>
+      loop param type
+      loop param val
+      withLetDecl n type val fun x => do
+        loop param (body.instantiate1 x)
+    | Expr.mdata _d b =>
+      if let some stx := getRecAppSyntax? e then
+        withRef stx <| loop param b
+      else
+        loop param b
+    | Expr.proj _n _i e => loop param e
+    | Expr.const .. => if e.isConstOf recFnName then processRec param e
+    | Expr.app .. =>
+      match (← matchMatcherApp? e) with
+      | some matcherApp =>
+        if !Structural.recArgHasLooseBVarsAt recFnName fixedPrefixSize e then
+          processApp param e
+        else
+          if let some altParams ← matcherApp.refineThrough? param then
+            (Array.zip matcherApp.alts (Array.zip matcherApp.altNumParams altParams)).forM
+              fun (alt, altNumParam, altParam) =>
+                lambdaTelescope altParam fun xs altParam => do
+                  -- TODO: Use boundedLambdaTelescope
+                  unless altNumParam = xs.size do
+                    throwError "unexpected `casesOn` application alternative{indentExpr alt}\nat application{indentExpr e}"
+                  let altBody := alt.beta xs
+                  loop altParam altBody
+          else
+            processApp param e
+      | none =>
+      match (← toCasesOnApp? e) with
+      | some casesOnApp =>
+        if !Structural.recArgHasLooseBVarsAt recFnName fixedPrefixSize e then
+          processApp param e
+        else
+          if let some altParams ← casesOnApp.refineThrough? param then
+          (Array.zip casesOnApp.alts (Array.zip casesOnApp.altNumParams altParams)).forM
+            fun (alt, altNumParam, altParam) =>
+              lambdaTelescope altParam fun xs altParam => do
+                -- TODO: Use boundedLambdaTelescope
+                unless altNumParam = xs.size do
+                  throwError "unexpected `casesOn` application alternative{indentExpr alt}\nat application{indentExpr e}"
+                let altBody := alt.beta xs
+                loop altParam altBody
+          else
+            processApp param e
+      | none => processApp param e
+    | e => do
+      let _ ← ensureNoRecFn recFnName e
+
+/--
+A `SavedLocalContext` captures the state and local context of a `MetaM`, to be continued later.
+-/
+structure SavedLocalContext where
+  savedLocalContext : LocalContext
+  savedLocalInstances : LocalInstances
+  savedState : Meta.SavedState
+
+/-- Capture the `MetaM` state including local context. -/
+def SavedLocalContext.create : MetaM SavedLocalContext := do
+  let savedLocalContext ← getLCtx
+  let savedLocalInstances ← getLocalInstances
+  let savedState ← saveState
+  return { savedLocalContext, savedLocalInstances, savedState }
+
+/-- Run a `MetaM` action in the saved state. -/
+def SavedLocalContext.run {α} (slc : SavedLocalContext) (k : MetaM α) :
+    MetaM α :=
+  withoutModifyingState $ do
+    withLCtx slc.savedLocalContext slc.savedLocalInstances do
+      slc.savedState.restore
+      k
+
+/-- A `RecCallWithContext` focuses on a single recursive call in a unary predefinition,
+and runs the given action in the context of that call.  -/
+structure RecCallWithContext where
+  /-- Syntax location of reursive call -/
+  ref : Syntax
+  /-- Function index of caller -/
+  caller : Nat
+  /-- Parameters of caller -/
+  params : Array Expr
+  /-- Function index of callee -/
+  callee : Nat
+  /-- Arguments to callee -/
+  args : Array Expr
+  ctxt : SavedLocalContext
+
+/-- Store the current recursive call and its context. -/
+def RecCallWithContext.create (ref : Syntax) (caller : Nat) (params : Array Expr) (callee : Nat)
+    (args : Array Expr) : MetaM RecCallWithContext := do
+  return { ref, caller, params, callee, args, ctxt := (← SavedLocalContext.create) }
+
+
+/--
+The elaborator is prone to duplicate terms, including recursive calls, even if the user
+only wrote a single one. This duplication is wasteful if we run the tactics on duplicated
+calls, and confusing in the output of GuessLex. So prune the list of recursive calls,
+and remove those where another call exists that has the same goal and context that is no more
+specific.
+-/
+def filterSubsumed (rcs : Array RecCallWithContext ) : Array RecCallWithContext := Id.run do
+  rcs.filterPairsM fun rci rcj => do
+    if rci.caller == rcj.caller && rci.callee == rcj.callee &&
+      rci.params == rcj.params && rci.args == rcj.args then
+      -- same goals; check contexts.
+        let lci := rci.ctxt.savedLocalContext
+        let lcj := rcj.ctxt.savedLocalContext
+        if lci.isSubPrefixOf lcj then
+          -- rci is better
+          return (true, false)
+        else if lcj.isSubPrefixOf lci then
+          -- rcj is better
+          return (false, true)
+    return (true, true)
+
+/-- Traverse a unary PreDefinition, and returns a `WithRecCall` closure for each recursive
+call site.
+-/
+def collectRecCalls (unaryPreDef : PreDefinition) (fixedPrefixSize : Nat) (arities : Array Nat)
+    : MetaM (Array RecCallWithContext) := withoutModifyingState do
+  addAsAxiom unaryPreDef
+  lambdaTelescope unaryPreDef.value fun xs body => do
+    unless xs.size == fixedPrefixSize + 1 do
+      -- Maybe cleaner to have lambdaBoundedTelescope?
+      throwError "Unexpected number of lambdas in unary pre-definition"
+    -- trace[Elab.definition.wf] "collectRecCalls: {xs} {body}"
+    let param := xs[fixedPrefixSize]!
+    withRecApps unaryPreDef.declName fixedPrefixSize param body fun param args => do
+      unless args.size ≥ fixedPrefixSize + 1 do
+        throwError "Insufficient arguments in recursive call"
+      let arg := args[fixedPrefixSize]!
+      let (caller, params) ← unpackArg arities param
+      let (callee, args) ← unpackArg arities arg
+      RecCallWithContext.create (← getRef) caller params callee args
+
+/-- A `GuessLexRel` described how a recursive call affects a measure; whether it
+decreases strictly, non-strictly, is equal, or else.  -/
+inductive GuessLexRel | lt | eq | le | no_idea
+deriving Repr, DecidableEq
+
+instance : ToString GuessLexRel where
+  toString | .lt => "<"
+           | .eq => "="
+           | .le => "≤"
+           | .no_idea => "?"
+
+instance : ToFormat GuessLexRel where
+  format r := toString r
+
+/-- Given a `GuessLexRel`, produce a binary `Expr` that relates two `Nat` values accordingly. -/
+def GuessLexRel.toNatRel : GuessLexRel → Expr
+  | lt => mkAppN (mkConst ``LT.lt [levelZero]) #[mkConst ``Nat, mkConst ``instLTNat]
+  | eq => mkAppN (mkConst ``Eq [levelOne]) #[mkConst ``Nat]
+  | le => mkAppN (mkConst ``LE.le [levelZero]) #[mkConst ``Nat, mkConst ``instLENat]
+  | no_idea => unreachable!
+
+/-- Given an expression `e`, produce `sizeOf e` with a suitable instance. -/
+def mkSizeOf (e : Expr) : MetaM Expr := do
+  let ty ← inferType e
+  let lvl ← getLevel ty
+  let inst ← synthInstance (mkAppN (mkConst ``SizeOf [lvl]) #[ty])
+  let res := mkAppN (mkConst ``sizeOf [lvl]) #[ty,  inst, e]
+  check res
+  return res
+
+/--
+For a given recursive call, and a choice of parameter and argument index,
+try to prove equality, < or ≤.
+-/
+def evalRecCall (decrTactic? : Option Syntax) (rcc : RecCallWithContext) (paramIdx argIdx : Nat) :
+    MetaM GuessLexRel := do
+  rcc.ctxt.run do
+    let param := rcc.params[paramIdx]!
+    let arg := rcc.args[argIdx]!
+    trace[Elab.definition.wf] "inspectRecCall: {rcc.caller} ({param}) → {rcc.callee} ({arg})"
+    let arg ← mkSizeOf rcc.args[argIdx]!
+    let param ← mkSizeOf rcc.params[paramIdx]!
+    for rel in [GuessLexRel.eq, .lt, .le] do
+      let goalExpr := mkAppN rel.toNatRel #[arg, param]
+      trace[Elab.definition.wf] "Goal for {rel}: {goalExpr}"
+      check goalExpr
+
+      let mvar ← mkFreshExprSyntheticOpaqueMVar goalExpr
+      let mvarId := mvar.mvarId!
+      let mvarId ← mvarId.cleanup
+      -- logInfo m!"Remaining goals: {goalsToMessageData [mvarId]}"
+      try
+        if rel = .eq then
+          MVarId.refl mvarId
+        else do
+          Lean.Elab.Term.TermElabM.run' do
+            match decrTactic? with
+            | none =>
+              let remainingGoals ← Tactic.run mvarId do
+                Tactic.evalTactic (← `(tactic| decreasing_tactic))
+              remainingGoals.forM fun mvarId => Term.reportUnsolvedGoals [mvarId]
+              -- trace[Elab.definition.wf] "Found {rel} proof: {← instantiateMVars mvar}"
+              pure ()
+            | some decrTactic => Term.withoutErrToSorry do
+              -- make info from `runTactic` available
+              pushInfoTree (.hole mvarId)
+              Term.runTactic mvarId decrTactic
+              -- trace[Elab.definition.wf] "Found {rel} proof: {← instantiateMVars mvar}"
+              pure ()
+        trace[Elab.definition.wf] "inspectRecCall: success!"
+        return rel
+      catch _e =>
+        trace[Elab.definition.wf] "Did not find {rel} proof: {goalsToMessageData [mvarId]}"
+        continue
+    return .no_idea
+
+/- A cache for `evalRecCall` -/
+structure RecCallCache where mk'' ::
+  decrTactic? : Option Syntax
+  rcc : RecCallWithContext
+  cache : IO.Ref (Array (Array (Option GuessLexRel)))
+
+/-- Create a cache to memoize calls to `evalRecCall descTactic? rcc` -/
+def RecCallCache.mk (decrTactic? : Option Syntax) (rcc : RecCallWithContext) :
+    BaseIO RecCallCache := do
+  let cache ← IO.mkRef <| Array.mkArray rcc.params.size (Array.mkArray rcc.args.size Option.none)
+  return { decrTactic?, rcc, cache }
+
+/-- Run `evalRecCall` and cache there result -/
+def RecCallCache.eval (rc: RecCallCache) (paramIdx argIdx : Nat) : MetaM GuessLexRel := do
+  -- Check the cache first
+  if let Option.some res := (← rc.cache.get)[paramIdx]![argIdx]! then
+    return res
+  else
+    let res ← evalRecCall rc.decrTactic? rc.rcc paramIdx argIdx
+    rc.cache.modify (·.modify paramIdx (·.set! argIdx res))
+    return res
+
+
+/-- Print a single cache entry as a string, without forcing it -/
+def RecCallCache.prettyEntry (rcc : RecCallCache) (paramIdx argIdx : Nat) : MetaM String := do
+  let cachedEntries ← rcc.cache.get
+  return match cachedEntries[paramIdx]![argIdx]! with
+  | .some rel => toString rel
+  | .none => "_"
+
+/-- The measures that we order lexicographically can be comparing arguments,
+or numbering the functions -/
+inductive MutualMeasure where
+  /-- For every function, the given argument index -/
+  | args : Array Nat → MutualMeasure
+  /-- The given function index is assigned 1, the rest 0 -/
+  | func : Nat → MutualMeasure
+
+/-- Evaluate a recursive call at a given `MutualMeasure` -/
+def inspectCall (rc : RecCallCache) : MutualMeasure → MetaM GuessLexRel
+  | .args argIdxs => do
+    let paramIdx := argIdxs[rc.rcc.caller]!
+    let argIdx := argIdxs[rc.rcc.callee]!
+    rc.eval paramIdx argIdx
+  | .func funIdx => do
+    if rc.rcc.caller == funIdx && rc.rcc.callee != funIdx then
+      return .lt
+    if rc.rcc.caller != funIdx && rc.rcc.callee == funIdx then
+      return .no_idea
+    else
+      return .eq
+
+/--
+Given a predefinition with value `fun (x_₁ ... xₙ) (y_₁ : α₁)... (yₘ : αₘ) => ...`,
+where `n = fixedPrefixSize`, return an array `A` s.t. `i ∈ A` iff `sizeOf yᵢ` reduces to a literal.
+This is the case for types such as `Prop`, `Type u`, etc.
+These arguments should not be considered when guessing a well-founded relation.
+See `generateCombinations?`
+-/
+def getForbiddenByTrivialSizeOf (fixedPrefixSize : Nat) (preDef : PreDefinition) : MetaM (Array Nat) :=
+  lambdaTelescope preDef.value fun xs _ => do
+    let mut result := #[]
+    for x in xs[fixedPrefixSize:], i in [:xs.size] do
+      try
+        let sizeOf ← whnfD (← mkAppM ``sizeOf #[x])
+        if sizeOf.isLit then
+         result := result.push i
+      catch _ =>
+        result := result.push i
+    return result
+
+
+/--
+Generate all combination of arguments, skipping those that are forbidden.
+
+Sorts the uniform combinations ([0,0,0], [1,1,1]) to the front; they are commonly most useful to
+try first, when the mutually recursive functions have similar argument structures
+-/
+partial def generateCombinations? (forbiddenArgs : Array (Array Nat)) (numArgs : Array Nat)
+    (threshold : Nat := 32) : Option (Array (Array Nat)) :=
+  (do goUniform 0; go 0 #[]) |>.run #[] |>.2
+where
+  isForbidden (fidx : Nat) (argIdx : Nat) : Bool :=
+    if h : fidx < forbiddenArgs.size then
+       forbiddenArgs[fidx] |>.contains argIdx
+    else
+      false
+
+  -- Enumerate all permissible uniform combinations
+  goUniform (argIdx : Nat) : OptionT (StateM (Array (Array Nat))) Unit  := do
+    if numArgs.all (argIdx < ·) then
+      unless forbiddenArgs.any (·.contains argIdx) do
+        modify (·.push (Array.mkArray numArgs.size argIdx))
+      goUniform (argIdx + 1)
+
+  -- Enumerate all other permissible combinations
+  go (fidx : Nat) : OptionT (ReaderT (Array Nat) (StateM (Array (Array Nat)))) Unit := do
+    if h : fidx < numArgs.size then
+      let n := numArgs[fidx]
+      for argIdx in [:n] do
+        unless isForbidden fidx argIdx do
+          withReader (·.push argIdx) (go (fidx + 1))
+    else
+      let comb ← read
+      unless comb.all (· == comb[0]!) do
+        modify (·.push comb)
+      if (← get).size > threshold then
+        failure
+
+
+/--
+Enumerate all meausures we want to try: All arguments (resp. combinations thereof) and
+possible orderings of functions (if more than one)
+-/
+def generateMeasures (forbiddenArgs : Array (Array Nat)) (arities : Array Nat) :
+    MetaM (Array MutualMeasure) := do
+  let some arg_measures := generateCombinations? forbiddenArgs arities
+      | throwError "Too many combinations"
+
+  let func_measures :=
+    if arities.size > 1 then
+      (List.range arities.size).toArray
+    else
+      #[]
+
+  return arg_measures.map .args ++ func_measures.map .func
+
+/--
+The core logic of guessing the lexicographic order
+Given a matrix that for each call and measure indicates whether that measure is
+decreasing, equal, less-or-equal or unknown, It finds a sequence of measures
+that is lexicographically decreasing.
+
+The matrix is implemented here as an array of monadic query methods only so that
+we can fill is lazily. Morally, this is a pure function
+-/
+partial def solve {m} {α} [Monad m] (measures : Array α)
+  (calls : Array (α → m GuessLexRel)) : m (Option (Array α)) := do
+  go measures calls #[]
+  where
+  go (measures : Array α) (calls : Array (α → m GuessLexRel)) (acc : Array α) := do
+    if calls.isEmpty then return .some acc
+
+    -- Find the first measure that has at least one < and otherwise only = or <=
+    for h : measureIdx in [:measures.size] do
+      let measure := measures[measureIdx]'h.2
+      let mut has_lt := false
+      let mut all_le := true
+      let mut todo := #[]
+      for call in calls do
+        let entry ← call measure
+        if entry = .lt then
+          has_lt := true
+        else
+          todo := todo.push call
+          if entry != .le && entry != .eq then
+            all_le := false
+            break
+      -- No progress here? Try the next measure
+      if not (has_lt && all_le) then continue
+      -- We found a suitable measure, remove it from the list (mild optimization)
+      let measures' := measures.eraseIdx measureIdx
+      return ← go measures' todo (acc.push measure)
+    -- None found, we have to give up
+    return .none
+
+/--
+Create Tuple syntax (`()` if the array is empty, and just the value if its a singleton)
+-/
+def mkTupleSyntax : Array Term → MetaM Term
+  | #[]  => `(())
+  | #[e] => return e
+  | es   => `(($(es[0]!), $(es[1:]),*))
+
+/--
+Given an array of `MutualMeasures`, creates a `TerminationWF` that specifies the lexicographic
+combination of these measures.
+-/
+def buildTermWF (declNames : Array Name) (varNamess : Array (Array Name))
+    (measures : Array MutualMeasure) : MetaM TerminationWF := do
+  let mut termByElements := #[]
+  for h : funIdx in [:varNamess.size] do
+    let vars := (varNamess[funIdx]'h.2).map mkIdent
+    let body ← mkTupleSyntax (← measures.mapM fun
+      | .args varIdxs => do
+          let v := vars.get! (varIdxs[funIdx]!)
+          let sizeOfIdent := mkIdent (← unresolveNameGlobal ``sizeOf)
+          `($sizeOfIdent $v)
+      | .func funIdx' => if funIdx' == funIdx then `(1) else `(0)
+      )
+    let declName := declNames[funIdx]!
+
+    termByElements := termByElements.push
+      { ref := .missing
+        declName, vars, body,
+        implicit := true
+      }
+  return termByElements
+
+
+/--
+Given a matrix (row-major) of strings, arranges them in tabular form.
+First column is left-aligned, others right-aligned.
+Single space as column separator.
+-/
+def formatTable : Array (Array String) → String := fun xss => Id.run do
+  let mut colWidths := xss[0]!.map (fun _ => 0)
+  for i in [:xss.size] do
+    for j in [:xss[i]!.size] do
+      if xss[i]![j]!.length > colWidths[j]! then
+        colWidths := colWidths.set! j xss[i]![j]!.length
+  let mut str := ""
+  for i in [:xss.size] do
+    for j in [:xss[i]!.size] do
+      let s := xss[i]![j]!
+      if j > 0 then -- right-align
+        for _ in [:colWidths[j]! - s.length] do
+          str := str ++ " "
+      str := str ++ s
+      if j = 0 then -- left-align
+        for _ in [:colWidths[j]! - s.length] do
+          str := str ++ " "
+      if j + 1 < xss[i]!.size then
+        str := str ++ " "
+    if i + 1 < xss.size then
+      str := str ++ "\n"
+  return str
+
+/-- Concise textual representation of the source location of a recursive call  -/
+def RecCallWithContext.posString (rcc : RecCallWithContext) : MetaM String := do
+  let fileMap ← getFileMap
+  let .some pos := rcc.ref.getPos? | return ""
+  let position := fileMap.toPosition pos
+  let endPosStr := match rcc.ref.getTailPos? with
+  | some endPos =>
+    let endPosition := fileMap.toPosition endPos
+    if endPosition.line = position.line then
+      s!"-{endPosition.column}"
+    else
+      s!"-{endPosition.line}:{endPosition.column}"
+  | none        => ""
+  return s!"{position.line}:{position.column}{endPosStr}"
+
+
+/-- Explain what we found out about the recursive calls (non-mutual case) -/
+def explainNonMutualFailure (varNames : Array Name) (rcs : Array RecCallCache) : MetaM Format := do
+  let header := varNames.map (·.eraseMacroScopes.toString)
+  let mut table : Array (Array String) := #[#[""] ++ header]
+  for i in [:rcs.size], rc in rcs do
+    let mut row := #[s!"{i+1}) {← rc.rcc.posString}"]
+    for argIdx in [:varNames.size] do
+      row := row.push (← rc.prettyEntry argIdx argIdx)
+    table := table.push row
+
+  return formatTable table
+
+/-- Explain what we found out about the recursive calls (mutual case) -/
+def explainMutualFailure (declNames : Array Name) (varNamess : Array (Array Name))
+    (rcs : Array RecCallCache) : MetaM Format := do
+  let mut r := Format.nil
+
+  for rc in rcs do
+    let caller := rc.rcc.caller
+    let callee := rc.rcc.callee
+    r := r ++ f!"Call from {declNames[caller]!} to {declNames[callee]!} " ++
+      f!"at {← rc.rcc.posString}:\n"
+
+    let header := varNamess[caller]!.map (·.eraseMacroScopes.toString)
+    let mut table : Array (Array String) := #[#[""] ++ header]
+    if caller = callee then
+      -- For self-calls, only the diagonal is interesting, so put it into one row
+      let mut row := #[""]
+      for argIdx in [:varNamess[caller]!.size] do
+        row := row.push (← rc.prettyEntry argIdx argIdx)
+      table := table.push row
+    else
+      for argIdx in [:varNamess[callee]!.size] do
+        let mut row := #[]
+        row := row.push varNamess[callee]![argIdx]!.eraseMacroScopes.toString
+        for paramIdx in [:varNamess[caller]!.size] do
+          row := row.push (← rc.prettyEntry paramIdx argIdx)
+        table := table.push row
+    r := r ++ formatTable table ++ "\n"
+
+  return r
+
+def explainFailure (declNames : Array Name) (varNamess : Array (Array Name))
+    (rcs : Array RecCallCache) : MetaM Format := do
+  let mut r : Format := "The arguments relate at each recursive call as follows:\n" ++
+    "(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)\n"
+  if declNames.size = 1 then
+    r := r ++ (← explainNonMutualFailure varNamess[0]! rcs)
+  else
+    r := r ++ (← explainMutualFailure declNames varNamess rcs)
+  return r
+
+end Lean.Elab.WF.GuessLex
+
+namespace Lean.Elab.WF
+
+open Lean.Elab.WF.GuessLex
+
+/--
+Main entry point of this module:
+
+Try to find a lexicographic ordering of the arguments for which the recursive definition
+terminates. See the module doc string for a high-level overview.
+-/
+def guessLex (preDefs : Array PreDefinition)  (unaryPreDef : PreDefinition)
+    (fixedPrefixSize : Nat) (decrTactic? : Option Syntax) :
+    MetaM TerminationWF := do
+  let varNamess ← preDefs.mapM (naryVarNames fixedPrefixSize ·)
+  let arities := varNamess.map (·.size)
+  trace[Elab.definition.wf] "varNames is: {varNamess}"
+
+  let forbiddenArgs ← preDefs.mapM fun preDef =>
+    getForbiddenByTrivialSizeOf fixedPrefixSize preDef
+
+  -- The list of measures, including the measures that order functions.
+  -- The function ordering measures come last
+  let measures ← generateMeasures forbiddenArgs arities
+
+  -- If there is only one plausible measure, use that
+  if let #[solution] := measures then
+    return ← buildTermWF (preDefs.map (·.declName)) varNamess #[solution]
+
+  -- Collect all recursive calls and extract their context
+  let recCalls ← collectRecCalls unaryPreDef fixedPrefixSize arities
+  let recCalls := filterSubsumed recCalls
+  let rcs ← recCalls.mapM (RecCallCache.mk decrTactic? ·)
+  let callMatrix := rcs.map (inspectCall ·)
+
+  match ← liftMetaM <| solve measures callMatrix with
+  | .some solution => do
+    let wf ← buildTermWF (preDefs.map (·.declName)) varNamess solution
+
+    let wfStx ← withoutModifyingState do
+      preDefs.forM (addAsAxiom ·)
+      wf.unexpand
+
+    if showInferredTerminationBy.get (← getOptions) then
+      logInfo m!"Inferred termination argument:{wfStx}"
+
+    return wf
+  | .none =>
+    let explanation ← explainFailure (preDefs.map (·.declName)) varNamess rcs
+    Lean.throwError <| "Could not find a decreasing measure.\n" ++
+      explanation ++ "\n" ++
+      "Please use `termination_by` to specify a decreasing measure."

src/Lean/Elab/PreDefinition/WF/Main.lean

@@ -7,10 +7,12 @@ import Lean.Elab.PreDefinition.Basic
 import Lean.Elab.PreDefinition.WF.TerminationHint
 import Lean.Elab.PreDefinition.WF.PackDomain
 import Lean.Elab.PreDefinition.WF.PackMutual
+import Lean.Elab.PreDefinition.WF.Preprocess
 import Lean.Elab.PreDefinition.WF.Rel
 import Lean.Elab.PreDefinition.WF.Fix
 import Lean.Elab.PreDefinition.WF.Eqns
 import Lean.Elab.PreDefinition.WF.Ite
+import Lean.Elab.PreDefinition.WF.GuessLex
 
 namespace Lean.Elab
 open WF
@@ -79,6 +81,7 @@ private def isOnlyOneUnaryDef (preDefs : Array PreDefinition) (fixedPrefixSize :
     return false
 
 def wfRecursion (preDefs : Array PreDefinition) (wf? : Option TerminationWF) (decrTactic? : Option Syntax) : TermElabM Unit := do
+  let preDefs ← preDefs.mapM fun preDef => return { preDef with value := (← preprocess preDef.value) }
   let (unaryPreDef, fixedPrefixSize) ← withoutModifyingEnv do
     for preDef in preDefs do
       addAsAxiom preDef
@@ -87,10 +90,17 @@ def wfRecursion (preDefs : Array PreDefinition) (wf? : Option TerminationWF) (de
     let preDefsDIte ← preDefs.mapM fun preDef => return { preDef with value := (← iteToDIte preDef.value) }
     let unaryPreDefs ← packDomain fixedPrefixSize preDefsDIte
     return (← packMutual fixedPrefixSize preDefs unaryPreDefs, fixedPrefixSize)
+
+  let wf ←
+    if let .some wf := wf? then
+      pure wf
+    else
+      guessLex preDefs unaryPreDef fixedPrefixSize decrTactic?
+
   let preDefNonRec ← forallBoundedTelescope unaryPreDef.type fixedPrefixSize fun prefixArgs type => do
     let type ← whnfForall type
     let packedArgType := type.bindingDomain!
-    elabWFRel preDefs unaryPreDef.declName fixedPrefixSize packedArgType wf? fun wfRel => do
+    elabWFRel preDefs unaryPreDef.declName fixedPrefixSize packedArgType wf fun wfRel => do
       trace[Elab.definition.wf] "wfRel: {wfRel}"
       let (value, envNew) ← withoutModifyingEnv' do
         addAsAxiom unaryPreDef
@@ -107,10 +117,13 @@ def wfRecursion (preDefs : Array PreDefinition) (wf? : Option TerminationWF) (de
     withEnableInfoTree false do
       addNonRec preDefNonRec (applyAttrAfterCompilation := false)
     addNonRecPreDefs preDefs preDefNonRec fixedPrefixSize
+  -- We create the `_unsafe_rec` before we abstract nested proofs.
+  -- Reason: the nested proofs may be referring to the _unsafe_rec.
+  addAndCompilePartialRec preDefs
+  let preDefs ← preDefs.mapM (abstractNestedProofs ·)
   registerEqnsInfo preDefs preDefNonRec.declName fixedPrefixSize
   for preDef in preDefs do
     applyAttributesOf #[preDef] AttributeApplicationTime.afterCompilation
-  addAndCompilePartialRec preDefs
 
 builtin_initialize registerTraceClass `Elab.definition.wf