diff --git a/library/Init/Lean/Elaborator/Basic.lean b/library/Init/Lean/Elaborator/Basic.lean
index 5828e02aa2..04baa5ed08 100644
--- a/library/Init/Lean/Elaborator/Basic.lean
+++ b/library/Init/Lean/Elaborator/Basic.lean
@@ -435,11 +435,10 @@ modifyGet $ fun s =>
 def localContext : Elab LocalContext :=
 do scope ← getScope; pure scope.lctx
 
-def mkLocalDecl (userName : Name) (type : Expr) (bi : BinderInfo := BinderInfo.default) : Elab LocalDecl :=
+def mkLocalDecl (userName : Name) (type : Expr) (bi : BinderInfo := BinderInfo.default) : Elab Expr :=
 do idx ← mkFreshName;
-   modifyGetScope $ fun scope =>
-      let (decl, lctx) := scope.lctx.mkLocalDecl idx userName type bi;
-      (decl, { lctx := lctx, .. scope })
+   modifyScope $ fun scope => { lctx := scope.lctx.mkLocalDecl idx userName type bi, .. scope };
+   pure (Expr.fvar idx)
 
 def mkLambda (xs : Array Expr) (b : Expr) : Elab Expr :=
 do lctx ← localContext; pure $ lctx.mkLambda xs b
diff --git a/library/Init/Lean/Elaborator/PreTerm.lean b/library/Init/Lean/Elaborator/PreTerm.lean
index 1f362c4394..d7069022f0 100644
--- a/library/Init/Lean/Elaborator/PreTerm.lean
+++ b/library/Init/Lean/Elaborator/PreTerm.lean
@@ -148,8 +148,9 @@ match b.getKind with
    args.mmap $ fun arg => do
      let id := arg.getId;
      hole ← mkHoleFor arg;
-     decl ← mkLocalDecl id hole;
-     pure (mkLocal decl)
+     -- decl ← mkLocalDecl id hole;  -- HACK: this file will be deleted
+     -- pure (mkLocal decl)
+     mkLocalDecl id hole
 | `Lean.Parser.Term.explicitBinder =>
    let ids     := (b.getArg 1).getArgs;
    let optType := b.getArg 2;
@@ -165,8 +166,9 @@ match b.getKind with
           pure $ Expr.app (Expr.app (Expr.const `optParam []) type) defVal
        | `Lean.Parser.Term.binderTactic => logErrorAndThrow optDef "old elaborator does not support tactics in parameters"
        | _ => throw "unknown binder default value annotation";
-     decl ← mkLocalDecl id type;
-     pure (mkLocal decl)
+     -- decl ← mkLocalDecl id type;   -- HACK: this file will be deleted
+     -- pure (mkLocal decl)
+     mkLocalDecl id type
 | `Lean.Parser.Term.implicitBinder => do runIO (IO.println $ ">> implict " ++ (toString b)); pure #[]
 | `Lean.Parser.Term.instBinder     => do runIO (IO.println $ ">> inst " ++ (toString b)); pure #[]
 | _ => throw "unknown binder kind"
diff --git a/library/Init/Lean/LocalContext.lean b/library/Init/Lean/LocalContext.lean
index 59ec512166..a56fa3f3b3 100644
--- a/library/Init/Lean/LocalContext.lean
+++ b/library/Init/Lean/LocalContext.lean
@@ -75,20 +75,20 @@ lctx.nameToDecl.isEmpty
 
 /- Low level API for creating local declarations. It is used to implement actions in the monads `Elab` and `Tactic`. It should not be used directly since the argument `(name : Name)` is assumed to be "unique". -/
 @[export lean_local_ctx_mk_local_decl]
-def mkLocalDecl (lctx : LocalContext) (name : Name) (userName : Name) (type : Expr) (bi : BinderInfo := BinderInfo.default) : LocalDecl × LocalContext :=
+def mkLocalDecl (lctx : LocalContext) (name : Name) (userName : Name) (type : Expr) (bi : BinderInfo := BinderInfo.default) : LocalContext :=
 match lctx with
 | { nameToDecl := map, decls := decls } =>
   let idx  := decls.size;
   let decl := LocalDecl.cdecl idx name userName type bi;
-  (decl, { nameToDecl := map.insert name decl, decls := decls.push decl })
+  { nameToDecl := map.insert name decl, decls := decls.push decl }
 
 @[export lean_local_ctx_mk_let_decl]
-def mkLetDecl (lctx : LocalContext) (name : Name) (userName : Name) (type : Expr) (value : Expr) : LocalDecl × LocalContext :=
+def mkLetDecl (lctx : LocalContext) (name : Name) (userName : Name) (type : Expr) (value : Expr) : LocalContext :=
 match lctx with
 | { nameToDecl := map, decls := decls } =>
   let idx  := decls.size;
   let decl := LocalDecl.ldecl idx name userName type value;
-  (decl, { nameToDecl := map.insert name decl, decls := decls.push decl })
+  { nameToDecl := map.insert name decl, decls := decls.push decl }
 
 @[export lean_local_ctx_find]
 def find (lctx : LocalContext) (name : Name) : Option LocalDecl :=
diff --git a/library/Init/Lean/TypeClass/Synth.lean b/library/Init/Lean/TypeClass/Synth.lean
index 3ec9ab84d1..80f3f128b2 100644
--- a/library/Init/Lean/TypeClass/Synth.lean
+++ b/library/Init/Lean/TypeClass/Synth.lean
@@ -125,7 +125,7 @@ partial def introduceMVars (lctx : LocalContext) (locals : Array Expr) : Context
 partial def introduceLocals : Nat → LocalContext → Array Expr → Expr → LocalContext × Expr × Array Expr
 | nextIdx, lctx, ls, Expr.forallE name info domain body =>
   let idxName : Name := mkNumName (mkSimpleName "_tmp") nextIdx;
-  let ⟨lDecl, lctx⟩ := lctx.mkLocalDecl idxName name domain info;
+  let lctx := lctx.mkLocalDecl idxName name domain info;
   let l : Expr := Expr.fvar idxName;
   introduceLocals (nextIdx + 1) lctx (ls.push l) (body.instantiate1 l)