Fix issue with @this and async functions (#6105)

* Fix issue with `@this` and async functions

* Asked a "friend" to improve the changelog.

Author: cristianoc

CHANGELOG.md

@@ -67,6 +67,7 @@ These are only breaking changes for unformatted code.
 - Fix printing of uncurried application when the lhs is a function definition https://github.com/rescript-lang/rescript-compiler/pull/6084
 - Fix parsing uncurried type starting with path https://github.com/rescript-lang/rescript-compiler/pull/6089
 - Fix bigInt comparison https://github.com/rescript-lang/rescript-compiler/pull/6097
+- Fixed a bug where the async attribute was not preserved when using the `@this` decorator in ReScript functions. This fix allows proper handling of async functions with the `@this` decorator. Issue: https://github.com/rescript-lang/rescript-compiler/issues/6100
 
 #### :nail_care: Polish
 

jscomp/frontend/ast_uncurry_gen.ml

@@ -34,18 +34,18 @@ let to_method_callback loc (self : Bs_ast_mapper.mapper) label
   Bs_syntaxerr.optional_err loc label;
   let rec aux acc (body : Parsetree.expression) =
     match Ast_attributes.process_attributes_rev body.pexp_attributes with
-    | Nothing, _ -> (
+    | Nothing, attrs -> (
         match body.pexp_desc with
         | Pexp_fun (arg_label, _, arg, body) ->
             Bs_syntaxerr.optional_err loc arg_label;
-            aux ((arg_label, self.pat self arg) :: acc) body
+            aux ((arg_label, self.pat self arg, attrs) :: acc) body
         | _ -> (self.expr self body, acc))
     | _, _ -> (self.expr self body, acc)
   in
-  let result, rev_extra_args = aux [ (label, self_pat) ] body in
+  let result, rev_extra_args = aux [ (label, self_pat, []) ] body in
   let body =
-    Ext_list.fold_left rev_extra_args result (fun e (label, p) ->
-        Ast_helper.Exp.fun_ ~loc label None p e)
+    Ext_list.fold_left rev_extra_args result (fun e (label, p, attrs) ->
+        Ast_helper.Exp.fun_ ~loc ~attrs label None p e)
   in
   let arity = List.length rev_extra_args in
   let arity_s = string_of_int arity in

jscomp/test/UncurriedExternals.js

@@ -71,6 +71,13 @@ var StandardNotation = {
   set: StandardNotation_set
 };
 
+function methodWithAsync(param) {
+  var $$this = this ;
+  return (async function (arg) {
+              return $$this + arg | 0;
+            })(param);
+}
+
 function dd$1(param) {
   throw {
         RE_EXN_ID: "Not_found",
@@ -120,11 +127,19 @@ var match$1 = React.useState(function (param) {
       return 3;
     });
 
+function methodWithAsyncU() {
+  var $$this = this ;
+  return async function (arg) {
+    return $$this + arg | 0;
+  };
+}
+
 var get = match$1[0];
 
 var set = match$1[1];
 
 exports.StandardNotation = StandardNotation;
+exports.methodWithAsync = methodWithAsync;
 exports.dd = dd$1;
 exports.h = h$1;
 exports.M = M$1;
@@ -138,4 +153,5 @@ exports.tsiC = tsiC$1;
 exports.tsiU = tsiU$1;
 exports.get = get;
 exports.set = set;
+exports.methodWithAsyncU = methodWithAsyncU;
 /* h Not a pure module */

jscomp/test/UncurriedExternals.res

@@ -39,6 +39,8 @@ module StandardNotation = {
   let (get, set) = useState(() => 3)
 }
 
+let methodWithAsync = @this this => async arg => this + arg
+
 @@uncurried.swap
 
 external raise: exn => 'a = "%raise"
@@ -78,3 +80,5 @@ let tsiU = c => setIncrementU(c, @this (me, amount) => Js.log(. me))
 @module("react")
 external useState: (@uncurry (unit => 'state)) => ('state, ('state => 'state) => unit) = "useState"
 let (get, set) = useState(() => 3)
+
+let methodWithAsyncU = @this this => async arg => this + arg