Fix issue with nested async functions

Nested curried functions are combined into one, and the async inner function is not generated.
This PR Avoids combining nested functions when they are async.

Fixes #5980

Author: cristianoc

CHANGELOG.md

@@ -55,6 +55,7 @@ These are only breaking changes for unformatted code.
 - Fix type inference issue with uncurried functions applied to a single unit argument. The issue was introduced in https://github.com/rescript-lang/rescript-compiler/pull/5907 when adding support to `foo()` when `foo` has only optional arguments. https://github.com/rescript-lang/rescript-compiler/pull/5970
 - Fix issue where uncurried functions were incorrectly converting the type of a prop given as a default value to curried https://github.com/rescript-lang/rescript-compiler/pull/5971
 - Fix issue with error messages for uncurried functions where expected and given type were swapped https://github.com/rescript-lang/rescript-compiler/pull/5973
+- Fix issue with nested async functions, where the inner function would be emitted without `async` https://github.com/rescript-lang/rescript-compiler/pull/5983
 
 #### :nail_care: Polish
 

jscomp/ml/translcore.ml

@@ -681,6 +681,8 @@ let rec cut n l =
 
 let try_ids = Hashtbl.create 8
 
+let has_async_attribute exp = exp.exp_attributes |> List.exists (fun ({txt}, _payload) -> txt = "res.async")
+
 let rec transl_exp e =
   List.iter (Translattribute.check_attribute e) e.exp_attributes;
   transl_exp0 e
@@ -695,7 +697,7 @@ and transl_exp0 (e : Typedtree.expression) : Lambda.lambda =
   | Texp_let (rec_flag, pat_expr_list, body) ->
       transl_let rec_flag pat_expr_list (transl_exp body)
   | Texp_function { arg_label = _; param; cases; partial } ->
-      let async = e.exp_attributes |> List.exists (fun ({txt}, _payload) -> txt = "res.async") in
+      let async = has_async_attribute e in
       let params, body, return_unit =
         let pl = push_defaults e.exp_loc [] cases partial in
         transl_function e.exp_loc partial param pl
@@ -1036,7 +1038,7 @@ and transl_function loc partial param cases =
        } as exp;
    };
   ]
-    when Parmatch.inactive ~partial pat ->
+    when Parmatch.inactive ~partial pat && not (exp |> has_async_attribute) ->
       let params, body, return_unit =
         transl_function exp.exp_loc partial' param' cases
       in

jscomp/test/async_inline.js

@@ -60,6 +60,18 @@ var tui = 3;
 
 var tuia = uncurriedIdAsync(3);
 
+function nested1(param) {
+  return async function (y) {
+    return await y;
+  };
+}
+
+async function nested2(param) {
+  return async function (y) {
+    return await y;
+  };
+}
+
 var tci = 3;
 
 exports.willBeInlined = willBeInlined;
@@ -76,4 +88,6 @@ exports.tci = tci;
 exports.tcia = tcia;
 exports.tui = tui;
 exports.tuia = tuia;
+exports.nested1 = nested1;
+exports.nested2 = nested2;
 /* inlined Not a pure module */

jscomp/test/async_inline.res

@@ -45,4 +45,8 @@ let uncurriedIdAsync = async (.x ) => x
 let tci = curriedId(3)
 let tcia = curriedIdAsync(3)
 let tui = uncurriedId(. 3)
-let tuia = uncurriedIdAsync(. 3)
+let tuia = uncurriedIdAsync(. 3)
+
+let nested1 = () => async (y) => await y
+
+let nested2 = async () => async (y) => await y