perf: skip eraseRecAppSyntaxExpr if it is not needed (#4935)

src/Lean/Elab/PreDefinition/Basic.lean

@@ -164,8 +164,11 @@ def addNonRec (preDef : PreDefinition) (applyAttrAfterCompilation := true) (all
 /--
   Eliminate recursive application annotations containing syntax. These annotations are used by the well-founded recursion module
   to produce better error messages. -/
-def eraseRecAppSyntaxExpr (e : Expr) : CoreM Expr :=
-  Core.transform e (post := fun e => pure <| TransformStep.done <| if (getRecAppSyntax? e).isSome then e.mdataExpr! else e)
+def eraseRecAppSyntaxExpr (e : Expr) : CoreM Expr := do
+  if e.find? hasRecAppSyntax |>.isSome then
+    Core.transform e (post := fun e => pure <| TransformStep.done <| if hasRecAppSyntax e then e.mdataExpr! else e)
+  else
+    return e
 
 def eraseRecAppSyntax (preDef : PreDefinition) : CoreM PreDefinition :=
   return { preDef with value := (← eraseRecAppSyntaxExpr preDef.value) }

src/Lean/Elab/RecAppSyntax.lean

@@ -11,27 +11,35 @@ namespace Lean
 private def recAppKey := `_recApp
 
 /--
-  We store the syntax at recursive applications to be able to generate better error messages
-  when performing well-founded and structural recursion.
+We store the syntax at recursive applications to be able to generate better error messages
+when performing well-founded and structural recursion.
 -/
 def mkRecAppWithSyntax (e : Expr) (stx : Syntax) : Expr :=
-  mkMData (KVMap.empty.insert recAppKey (DataValue.ofSyntax stx)) e
+  mkMData (KVMap.empty.insert recAppKey (.ofSyntax stx)) e
 
 /--
-  Retrieve (if available) the syntax object attached to a recursive application.
+Retrieve (if available) the syntax object attached to a recursive application.
 -/
 def getRecAppSyntax? (e : Expr) : Option Syntax :=
   match e with
-  | Expr.mdata d _ =>
+  | .mdata d _ =>
     match d.find recAppKey with
     | some (DataValue.ofSyntax stx) => some stx
     | _ => none
   | _                => none
 
 /--
-  Checks if the `MData` is for a recursive applciation.
+Checks if the `MData` is for a recursive applciation.
 -/
 def MData.isRecApp (d : MData) : Bool :=
   d.contains recAppKey
 
+/--
+Return `true` if `getRecAppSyntax? e` is a `some`.
+-/
+def hasRecAppSyntax (e : Expr) : Bool :=
+  match e with
+  | .mdata d _ => d.isRecApp
+  | _ => false
+
 end Lean