Fix: Inline auto curried async functions produce invalid javascript

CHANGELOG.md

@@ -64,6 +64,7 @@ These are only breaking changes for unformatted code.
 - GenType: fix issue with V3 compatibility mode (see https://github.com/rescript-lang/rescript-compiler/issues/5990) https://github.com/rescript-lang/rescript-compiler/pull/5991
 - Fix issue in `Js.Promise2` where `then` and `catch` were returning `undefined` https://github.com/rescript-lang/rescript-compiler/pull/5997
 - Fix formatting of props spread for multiline JSX expression https://github.com/rescript-lang/rescript-compiler/pull/6006
+- Fix issue in the compiler back-end where async functions passed to an `@uncurry` external would be inlined and transformed in a way that loses async https://github.com/rescript-lang/rescript-compiler/pull/6010
 
 #### :nail_care: Polish
 

jscomp/core/lam_eta_conversion.ml

@@ -113,13 +113,16 @@ let unsafe_adjust_to_arity loc ~(to_ : int) ?(from : int option) (fn : Lam.t) :
   let ap_info : Lam.ap_info =
     { ap_loc = loc; ap_inlined = Default_inline; ap_status = App_na }
   in
+  let is_async_fn = match fn with
+    | Lfunction { attr = {async}} -> async 
+    | _ -> false in
   match (from, fn) with
   | Some from, _ | None, Lfunction { arity = from } -> (
-      if from = to_ then fn
+      if from = to_ || is_async_fn then fn
       else if to_ = 0 then
         match fn with
-        | Lfunction { params = [ param ]; body; attr = {async} } ->
-            Lam.function_ ~arity:0 ~attr:{Lambda.default_function_attribute with async}
+        | Lfunction { params = [ param ]; body } ->
+            Lam.function_ ~arity:0 ~attr:Lambda.default_function_attribute
               ~params:[]
               ~body:(Lam.let_ Alias param Lam.unit body)
             (* could be only introduced by

jscomp/test/async_inline.js

@@ -1,6 +1,7 @@
 'use strict';
 
 var Curry = require("../../lib/js/curry.js");
+var React = require("react");
 
 async function willBeInlined(param) {
   return 3;
@@ -72,6 +73,12 @@ async function nested2(param) {
   };
 }
 
+function onSubmit(param) {
+  return React.useCallback(async function (_a, b) {
+              return await b;
+            });
+}
+
 var tci = 3;
 
 exports.willBeInlined = willBeInlined;
@@ -90,4 +97,5 @@ exports.tui = tui;
 exports.tuia = tuia;
 exports.nested1 = nested1;
 exports.nested2 = nested2;
+exports.onSubmit = onSubmit;
 /* inlined Not a pure module */

jscomp/test/async_inline.res

@@ -50,3 +50,13 @@ let tuia = uncurriedIdAsync(. 3)
 let nested1 = () => async (y) => await y
 
 let nested2 = async () => async (y) => await y
+
+type callback<'input, 'output> = 'input => 'output
+
+@module("react")
+external useCallback: (@uncurry ('input => 'output)) => callback<'input, 'output> = "useCallback"
+
+let onSubmit = () =>
+  useCallback(async (_a, b) => {
+    await b
+  })