chore: rewrite mkFieldParamsForCtorType in a more readable style (#8755)

src/Lean/Compiler/LCNF/StructProjCases.lean

@@ -19,17 +19,23 @@ def findStructCtorInfo? (typeName : Name) : CoreM (Option ConstructorVal) := do
   let some (.ctorInfo ctorInfo) := (← getEnv).find? ctorName | return none
   return ctorInfo
 
-def mkFieldParamsForCtorType (e : Expr) (numParams : Nat): CompilerM (Array Param) := do
-  let rec loop (params : Array Param) (e : Expr) (numParams : Nat): CompilerM (Array Param) := do
-    match e with
-    | .forallE name type body _ =>
-      if numParams == 0 then
-        let param ← mkParam name (← toMonoType type) false
-        loop (params.push param) body numParams
-      else
-        loop params body (numParams - 1)
-    | _ => return params
-  loop #[] e numParams
+def mkFieldParamsForCtorType (ctorType : Expr) (numParams : Nat) (numFields : Nat)
+    : CompilerM (Array Param) := do
+  let mut type := ctorType
+  for _ in [0:numParams] do
+    match type with
+    | .forallE _ _ body _ =>
+      type := body
+    | _ => unreachable!
+  let mut fields := Array.emptyWithCapacity numFields
+  for _ in [0:numFields] do
+    match type with
+    | .forallE name fieldType body _ =>
+      let param ← mkParam name (← toMonoType fieldType) false
+      fields := fields.push param
+      type := body
+    | _ => unreachable!
+  return fields
 
 structure State where
   projMap : Std.HashMap FVarId (Array FVarId) := {}
@@ -57,8 +63,7 @@ partial def visitCode (code : Code) : M Code := do
         visitCode k
       else
         let some ctorInfo ← findStructCtorInfo? typeName | panic! "expected struct constructor"
-        let params ← mkFieldParamsForCtorType ctorInfo.type ctorInfo.numParams
-        assert! params.size == ctorInfo.numFields
+        let params ← mkFieldParamsForCtorType ctorInfo.type ctorInfo.numParams ctorInfo.numFields
         let fvars := params.map (·.fvarId)
         modify fun s => { s with projMap := s.projMap.insert base fvars,
                                  fvarMap := s.fvarMap.insert decl.fvarId fvars[i]! }