feat: allow erasing cbv_eval attributes (#12851)

This PR add support for erasing `@[cbv_eval]` annotations using
`attribute [-cbv_eval]`, mirroring the existing `@[-simp]` mechanism for
simp lemmas.

The `CbvEvalEntry` now tracks the original declaration name (`origin`)
so that inverted theorems (`@[cbv_eval ←]`) can be erased by their
original name. The `CbvEvalState` stores individual entries alongside
the composed `Theorems` discrimination tree, allowing the tree to be
rebuilt from remaining entries after erasure. Erasure is properly scoped
via `modifyState`, so `attribute [-cbv_eval]` inside a `section` is
reverted when the section ends.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

src/Lean/Meta/Tactic/Cbv/CbvEvalExt.lean

@@ -35,11 +35,13 @@ open Lean.Meta.Sym.Simp
 
 /--
   Entry of the `CbvEvalExtension`.
-  Consists of the precomputed `Theorem` object and a name of the head function appearing on the left-hand side of the theorem.
+  Consists of name of the theorem used, the precomputed `Theorem` object 
+  and a name of the head function appearing on the left-hand side of the theorem.
 -/
 structure CbvEvalEntry where
-  appFn : Name
-  thm  : Theorem
+  origin : Name
+  appFn  : Name
+  thm    : Theorem
   deriving BEq, Inhabited
 
 /-- Create a `CbvEvalEntry` from a theorem declaration. When `inv = true`, creates an
@@ -62,15 +64,38 @@ def mkCbvTheoremFromConst (declName : Name) (inv : Bool := false) : MetaM CbvEva
       thmDeclName ← mkAuxLemma (kind? := `_cbv_eval) cinfo.levelParams invType invVal
     return (constName, thmDeclName)
   let thm ← mkTheoremFromDecl thmDeclName
-  return ⟨fnName, thm⟩
+  return ⟨declName, fnName, thm⟩
 
 structure CbvEvalState where
-  lemmas : NameMap Theorems := {}
+  lemmas  : NameMap Theorems := {}
+  entries : NameMap <| Array CbvEvalEntry := {}
   deriving Inhabited
 
 def CbvEvalState.addEntry (s : CbvEvalState) (e : CbvEvalEntry) : CbvEvalState :=
-  let existing := (s.lemmas.find? e.appFn).getD {}
-  { s with lemmas := s.lemmas.insert e.appFn (existing.insert e.thm) }
+  let lemmas := (s.lemmas.find? e.appFn).getD {}
+  let entries := (s.entries.find? e.appFn).getD {}
+  { s with
+    lemmas  := s.lemmas.insert e.appFn (lemmas.insert e.thm)
+    entries := s.entries.insert e.appFn <| entries.push e}
+
+/-- Rebuild the `Theorems` for a given `appFn` from the entries that target it. -/
+private def CbvEvalState.rebuildLemmas (entries : NameMap <| Array CbvEvalEntry) (appFn : Name) (lemmas : NameMap Theorems) : NameMap Theorems := 
+  let appFnEntries := entries.getD appFn #[] 
+  if appFnEntries.isEmpty then
+    lemmas.erase appFn
+  else
+    lemmas.insert appFn (appFnEntries.foldl (fun thms e => thms.insert e.thm) {})
+
+/-- Erase a theorem from the state. Returns `none` if the theorem is not found. -/
+def CbvEvalState.erase (s : CbvEvalState) (declName : Name) : Option CbvEvalState := do
+  let (appFn, oldEntries) ← s.entries.foldl (init := none) fun acc appFn entries =>
+    if acc.isSome then acc
+    else if entries.any (·.origin == declName) then some (appFn, entries)
+    else none
+  let newEntries := oldEntries.filter (·.origin != declName)
+  let entries := if newEntries.isEmpty then s.entries.erase appFn
+    else s.entries.insert appFn newEntries
+  return { lemmas := rebuildLemmas entries appFn s.lemmas, entries }
 
 abbrev CbvEvalExtension := SimpleScopedEnvExtension CbvEvalEntry CbvEvalState
 
@@ -99,6 +124,11 @@ builtin_initialize
       let inv := !stx[1].isNone
       let (entry, _) ← MetaM.run (mkCbvTheoremFromConst lemmaName (inv := inv)) {}
       cbvEvalExt.add entry kind
+    erase := fun declName => do
+      let s := cbvEvalExt.getState (← getEnv)
+      match s.erase declName with
+      | some s' => modifyEnv fun env => cbvEvalExt.modifyState env fun _ => s'
+      | none => logWarning m!"`{.ofConstName declName}` does not have the `[cbv_eval]` attribute"
   }
 
 end Lean.Meta.Tactic.Cbv

tests/elab/cbv_eval_erase.lean

@@ -0,0 +1,100 @@
+import Std
+set_option cbv.warning false
+-- Use opaque constants so that cbv_eval is the ONLY way cbv can reduce them.
+-- After erasure, cbv makes no progress and the goal must be closed manually.
+
+opaque myConst : Nat
+@[cbv_eval] theorem myConst_eq : myConst = 42 := sorry
+
+-- Before erasure: cbv reduces myConst to 42 via cbv_eval
+example : myConst = 42 := by cbv
+
+-- Warning when erasing a theorem that doesn't have [cbv_eval]
+theorem not_cbv (n : Nat) : n = n := rfl
+
+/-- warning: `not_cbv` does not have the `[cbv_eval]` attribute -/
+#guard_msgs in
+attribute [-cbv_eval] not_cbv
+
+-- Basic erasure (no section — permanent)
+attribute [-cbv_eval] myConst_eq
+
+-- After erasure: cbv can't reduce myConst, so the goal isn't closed
+example : myConst = 42 := by
+  cbv -- makes no progress
+  exact myConst_eq
+
+-- Scoping: erasure inside a section is reverted
+opaque myConst2 : Nat
+@[cbv_eval] theorem myConst2_eq : myConst2 = 100 := sorry
+
+section
+attribute [-cbv_eval] myConst2_eq
+
+-- Inside section: cbv can't reduce
+/--
+trace: ⊢ myConst2 = 100
+---
+warning: declaration uses `sorry`
+-/
+#guard_msgs in
+example : myConst2 = 100 := by
+  cbv
+  trace_state
+  sorry
+end
+
+-- Outside section: cbv_eval is back
+example : myConst2 = 100 := by cbv
+
+-- Erasure of inverted theorem
+opaque myConst3 : Nat
+@[cbv_eval ←] theorem myConst3_eq : 7 = myConst3 := sorry
+
+example : myConst3 = 7 := by cbv
+
+section
+attribute [-cbv_eval] myConst3_eq
+
+/--
+trace: ⊢ myConst3 = 7
+---
+warning: declaration uses `sorry`
+-/
+#guard_msgs in
+example : myConst3 = 7 := by
+  cbv
+  trace_state
+  sorry
+end
+
+-- Reverted after section
+example : myConst3 = 7 := by cbv
+
+-- Erasure with multiple cbv_eval rules: erase only one
+opaque myFn : Nat → Nat
+@[cbv_eval] theorem myFn_zero : myFn 0 = 1 := sorry
+@[cbv_eval] theorem myFn_one  : myFn 1 = 0 := sorry
+
+example : myFn 0 = 1 := by cbv
+example : myFn 1 = 0 := by cbv
+
+section
+attribute [-cbv_eval] myFn_zero
+
+-- myFn_zero is erased, so cbv can't reduce myFn 0
+
+/--
+trace: ⊢ myFn 0 = 1
+---
+warning: declaration uses `sorry`
+-/
+#guard_msgs in
+example : myFn 0 = 1 := by
+  cbv
+  trace_state
+  sorry
+
+-- myFn_one is still active
+example : myFn 1 = 0 := by cbv
+end

tests/elab/cbv_eval_erase.lean.out.expected

@@ -0,0 +1,5 @@
+cbv_eval_erase.lean:7:20-7:30: warning: declaration uses `sorry`
+cbv_eval_erase.lean:29:20-29:31: warning: declaration uses `sorry`
+cbv_eval_erase.lean:52:22-52:33: warning: declaration uses `sorry`
+cbv_eval_erase.lean:76:20-76:29: warning: declaration uses `sorry`
+cbv_eval_erase.lean:77:20-77:28: warning: declaration uses `sorry`