Error message for when trying to await something that is not a promise

CHANGELOG.md

@@ -12,6 +12,10 @@
 
 # 12.0.0-alpha.15 (Unreleased)
 
+#### :nail_care: Polish
+
+- Better error message for when trying to await something that is not a promise. https://github.com/rescript-lang/rescript/pull/7561
+
 # 12.0.0-alpha.14
 
 #### :boom: Breaking Change

compiler/ml/ast_await.ml

@@ -9,6 +9,25 @@ let create_await_expression (e : Parsetree.expression) =
   in
   Ast_helper.Exp.apply ~loc unsafe_await [(Nolabel, e)]
 
+let is_await_expr (e : Parsetree.expression) =
+  match e with
+  | {
+   pexp_loc = {loc_ghost = true};
+   pexp_desc =
+     Pexp_apply
+       {
+         funct =
+           {
+             pexp_loc = {loc_ghost = true};
+             pexp_desc = Pexp_ident {txt = Ldot (Lident ident, "unsafe_await")};
+           };
+         args = [(Nolabel, _)];
+       };
+  }
+    when ident = Primitive_modules.promise ->
+    true
+  | _ -> false
+
 (* Transform `@res.await M` to unpack(@res.await Js.import(module(M: __M0__))) *)
 let create_await_module_expression ~module_type_lid (e : Parsetree.module_expr)
     =

compiler/ml/error_message_utils.ml

@@ -102,6 +102,7 @@ type type_clash_context =
   | FunctionArgument of {optional: bool; name: string option}
   | Statement of type_clash_statement
   | ForLoopCondition
+  | Await
 
 let context_to_string = function
   | Some WhileCondition -> "WhileCondition"
@@ -120,6 +121,7 @@ let context_to_string = function
   | Some (FunctionArgument _) -> "FunctionArgument"
   | Some ComparisonOperator -> "ComparisonOperator"
   | Some IfReturn -> "IfReturn"
+  | Some Await -> "Await"
   | None -> "None"
 
 let fprintf = Format.fprintf
@@ -185,6 +187,10 @@ let error_expected_type_text ppf type_clash_context =
       "But it's being used with the @{<info>%s@} operator, which works on:"
       operator
   | Some StringConcat -> fprintf ppf "But string concatenation is expecting:"
+  | Some Await ->
+    fprintf ppf
+      "But you're using @{<info>await@} on this expression, so it is expected \
+       to be of type:"
   | Some MaybeUnwrapOption | None ->
     fprintf ppf "But it's expected to have type:"
 
@@ -282,6 +288,14 @@ let print_extra_type_clash_help ~extract_concrete_typedecl ~env loc ppf
       "\n\n\
       \  To fix this, change the highlighted code so it evaluates to a \
        @{<info>bool@}."
+  | Some Await, _ ->
+    fprintf ppf
+      "\n\n\
+      \  You're trying to await something that is not a promise.\n\n\
+       Possible solutions:\n\
+      \  - Remove the @{<info>await@} if this is not expected to be a promise\n\
+      \  - Wrap the expression in @{<info>Promise.resolve@} to convert the \
+       expression to a promise"
   | Some IfReturn, _ ->
     fprintf ppf
       "\n\n\
@@ -533,7 +547,7 @@ let type_clash_context_from_function sexp sfunct =
     Some (MathOperator {for_float = true; operator; is_constant})
   | Pexp_ident {txt = Lident (("/" | "*" | "+" | "-") as operator)} ->
     Some (MathOperator {for_float = false; operator; is_constant})
-  | _ -> None
+  | _ -> if Ast_await.is_await_expr sexp then Some Await else None
 
 let type_clash_context_for_function_argument ~label type_clash_context sarg0 =
   match type_clash_context with

tests/build_tests/super_errors/expected/awaiting_non_promise.res.expected

@@ -0,0 +1,18 @@
+
+  We've found a bug for you!
+  /.../fixtures/awaiting_non_promise.res:3:15
+
+  1 │ let x = 1
+  2 │ 
+  3 │ let f = await x
+  4 │ 
+
+  This has type: int
+  But you're using await on this expression, so it is expected to be of type:
+    promise<'a>
+
+  You're trying to await something that is not a promise.
+
+Possible solutions:
+  - Remove the await if this is not expected to be a promise
+  - Wrap the expression in Promise.resolve to convert the expression to a promise

tests/build_tests/super_errors/fixtures/awaiting_non_promise.res

@@ -0,0 +1,3 @@
+let x = 1
+
+let f = await x