Propagate expected dict and async value type (#8359)

* propagate expected dict value type into dict literal typing

* cleaner impl

* change approach back to structural, and fix async as well

* changelog

Author: zth

CHANGELOG.md

@@ -29,6 +29,7 @@
 - Rewatch: preserve warnings after atomic-save full rebuilds. https://github.com/rescript-lang/rescript/pull/8358
 
 - Preserve JSX prop locations across the AST0 translation layer, fixing `0:0` editor diagnostics in PPX-related flows. https://github.com/rescript-lang/rescript/pull/8350
+- Fix type lowering for `dict{}` and `async`, so you don't need to annotate one extra time when the type is known. https://github.com/rescript-lang/rescript/pull/8359
 
 #### :memo: Documentation
 

compiler/ml/typecore.ml

@@ -2259,6 +2259,19 @@ let extract_function_name funct =
   | Texp_ident (path, _, _) -> Some (Longident.parse (Path.name path))
   | _ -> None
 
+let should_unify_expected_result_before_typing_lowered_apply funct sargs =
+  match (extract_function_name funct, sargs) with
+  | ( Some (Longident.Ldot (Longident.Lident "Primitive_dict", "make")),
+      [(Asttypes.Nolabel, {Parsetree.pexp_desc = Parsetree.Pexp_array _})] ) ->
+    (* Dict literals *)
+    true
+  | ( Some
+        (Longident.Ldot (Longident.Lident "Primitive_promise", "unsafe_async")),
+      [(Asttypes.Nolabel, _)] ) ->
+    (* Async wrapper *)
+    true
+  | _ -> false
+
 type lazy_args =
   (Asttypes.arg_label * (unit -> Typedtree.expression) option) list
 
@@ -2460,6 +2473,17 @@ and type_expect_ ?deprecated_context ~context ?in_function ?(recarg = Rejected)
     let funct =
       type_exp ~deprecated_context:FunctionCall ~context:None env sfunct
     in
+    (if should_unify_expected_result_before_typing_lowered_apply funct sargs
+     then
+       (* Lowered syntax like dict literals and async wrappers becomes a regular
+          application, so thread the expected result type into the application
+          before typing its arguments. *)
+       let _, ty_res =
+         filter_arrow ~env
+           ~arity:(Some (List.length sargs))
+           funct.exp_type Nolabel
+       in
+       unify_exp_types ~context:None loc env ty_res (instance env ty_expected));
     let ty = instance env funct.exp_type in
     end_def ();
     wrap_trace_gadt_instances env (lower_args env []) ty;

tests/build_tests/super_errors/expected/dict_show_no_coercion.res.expected

@@ -1,15 +1,12 @@
 
   We've found a bug for you!
-  /.../fixtures/dict_show_no_coercion.res:2:23-35
+  /.../fixtures/dict_show_no_coercion.res:2:33-34
 
   1 │ // This should not show coercion suggestion since just the inner types a
     │ re coercable, not the full type + expression (dict<float> -> dict<JSON.t
     │ >)
-  2 │ let x: dict<JSON.t> = dict{"1": 1.}
+  2 │ let x: dict<JSON.t> = dict{"1": 1.}
   3 │ 
 
-  This has type: dict<float>
-  But it's expected to have type: dict<JSON.t>
-  
-  The incompatible parts:
-    float vs JSON.t (defined as JSON.t)
+  This has type: float
+  But it's expected to have type: JSON.t (defined as JSON.t)

tests/tests/src/DictScopedRecordLiteral.mjs

@@ -0,0 +1,30 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+
+let Hidden = {};
+
+let dictValueInference = {
+  health: {
+    get: 200
+  }
+};
+
+async function asyncValueInference() {
+  return {
+    get: 200
+  };
+}
+
+let primitiveMakeValueInference = {
+  health: {
+    get: 200
+  }
+};
+
+export {
+  Hidden,
+  dictValueInference,
+  asyncValueInference,
+  primitiveMakeValueInference,
+}
+/* No side effect */

tests/tests/src/DictScopedRecordLiteral.res

@@ -0,0 +1,13 @@
+module Hidden = {
+  type routeHandlerObject = {get: int}
+}
+
+let dictValueInference: Dict.t<Hidden.routeHandlerObject> = dict{
+  "health": {get: 200},
+}
+
+let asyncValueInference: unit => promise<Hidden.routeHandlerObject> = async () => {
+  get: 200,
+}
+
+let primitiveMakeValueInference: Dict.t<Hidden.routeHandlerObject> = dict{"health": {get: 200}}