perf: treat partial application and eta expansion equally for specialization (#9438)

src/Lean/Compiler/LCNF/Specialize.lean

@@ -53,6 +53,7 @@ structure Context where
   Set of let-declarations in scope that do not depend on parameters.
   -/
   ground : FVarIdSet := {}
+  underApplied : FVarIdSet := {}
   /--
   Name of the declaration being processed
   -/
@@ -77,9 +78,25 @@ def isGround [TraverseFVar α] (e : α) : SpecializeM Bool := do
 
 @[inline] def withLetDecl (decl : LetDecl) (x : SpecializeM α) : SpecializeM α := do
   let grd ← isGround decl.value
+  let isUnderApplied ←
+    match decl.value with
+    | .const fnName _ args =>
+      match ← getDecl? fnName with
+      -- This ascription to `Bool` is required to avoid this being inferred as `Prop`,
+      -- even with a type specified on the `let` binding.
+      | some { params, .. } => pure ((args.size < params.size) : Bool)
+      | none => pure false
+    | .fvar fnFVarId args =>
+      match ← findFunDecl? fnFVarId with
+      -- This ascription to `Bool` is required to avoid this being inferred as `Prop`,
+      -- even with a type specified on the `let` binding.
+      | some { params, .. } => pure ((args.size < params.size) : Bool)
+      | none => pure false
+    | _ => pure false
   let fvarId := decl.fvarId
   withReader (fun ctx => { ctx with
     scope := ctx.scope.insert fvarId
+    underApplied := if isUnderApplied then ctx.underApplied.insert fvarId else ctx.underApplied
     ground := if grd then ctx.ground.insert fvarId else ctx.ground
   }) x
 
@@ -156,7 +173,9 @@ def collect (paramsInfo : Array SpecParamInfo) (args : Array Arg) : SpecializeM
   let lctx := (← getThe CompilerM.State).lctx
   let abstract (fvarId : FVarId) : Bool :=
     -- We convert let-declarations that are not ground into parameters
-    !lctx.funDecls.contains fvarId && !ctx.ground.contains fvarId
+    !lctx.funDecls.contains fvarId &&
+    !ctx.underApplied.contains fvarId &&
+    !ctx.ground.contains fvarId
   Closure.run (inScope := ctx.scope.contains) (abstract := abstract) do
     let mut argMask := #[]
     for paramInfo in paramsInfo, arg in args do