fix: fixes #1576

src/Lean/Elab/Tactic/Config.lean

@@ -19,13 +19,15 @@ macro (name := configElab) doc?:(docComment)? "declare_config_elab" elabName:ide
      if optConfig.isNone then
        return { : $type }
      else
-       let c ← withoutModifyingState <| withLCtx {} {} <| withSaveInfoContext <| Term.withSynthesize do
+       let c ← withoutModifyingStateWithInfoAndMessages <| withLCtx {} {} <| withSaveInfoContext <| Term.withSynthesize do
          let c ← Term.elabTermEnsuringType optConfig[0][3] (Lean.mkConst ``$type)
          Term.synthesizeSyntheticMVarsNoPostponing
          instantiateMVars c
        eval c
   )
 
+#check @evalExpr'
+
 open Linter.MissingDocs in
 @[builtinMissingDocsHandler Elab.Tactic.configElab]
 def checkConfigElab : SimpleHandler := mkSimpleHandler "config elab"

src/Lean/Meta/Eval.lean

@@ -10,6 +10,9 @@ namespace Lean.Meta
 unsafe def evalExprCore (α) (value : Expr) (checkType : Expr → MetaM Unit) (safety := DefinitionSafety.safe) : MetaM α :=
   withoutModifyingEnv do
     let name ← mkFreshUserName `_tmp
+    let value ← instantiateMVars value
+    if value.hasMVar then
+      throwError "failed to evaluate expression, it contains metavariables{indentExpr value}"
     let type ← inferType value
     checkType type
     let decl := Declaration.defnDecl {

tests/lean/1576.lean

@@ -0,0 +1 @@
+example : True := by rw (config := non / sense) []

tests/lean/1576.lean.expected.out

@@ -0,0 +1,2 @@
+1576.lean:1:35-1:38: error: unknown identifier 'non'
+1576.lean:1:41-1:46: error: unknown identifier 'sense'