Another case of "async" missing in JS output

[cknitt]

While migrating a large project to use async/await, I came across the following problem:

type data

@module("SomeModule")
external fetchData: string => promise<data> = "fetchData"

let promiseCreators = ["1", "2"]->Js.Array2.map(id => async () => await fetchData(id))

compiles to

var promiseCreators = [
    "1",
    "2"
  ].map(function (id) {
      return function (param) {
        return await SomeModule.fetchData(id);
      };
    });

which is not valid because the inner function contains an await, but is not annotated with async.

This occurs in both 10.1.2 and current master.
https://rescript-lang.org/try?version=v10.1.2&code=C4TwDgpgBAJghsOAoJABAtgexgVwDYQAUARAMqboQCy2+ExAlEhAB7AQBOAdnHlAGYRgAYwAWAEQRwAXFADOwDgEsuAcygBeAHxQwHCkrkQAPPEQ6NUYoJESpxFAWC796QxADCHCAkwc5mlAA2sQAjMQANFYATMQAugC0WgBScgB0AIIcHHAg0WnocGCESjCaOnByIFzCUIQM5VBwAO5wSs42YpKIJTAMTEA

[cristianoc]

Simpler repro:

let foo = () => async y => await y

compiles to

function foo(param, y) {
  return await y;
}

[cristianoc]

@cknitt this should fix it: #5983

The PR should probably be ported to 10.1.
Another possible candidate fix for back-porting, though mostly harmless, is this #5982

[cknitt]

@cristianoc Thanks a lot! I can confirm that this solves the original issue that I experienced in our codebase.