async in extern "C" block is accepted but does nothing

[rodrigorc]

Describe the Bug

In version 0.2.55, to use a JS async function in Rust I would write:

#[wasm_bindgen]
extern "C" {
    fn js_test() -> js_sys::Promise;
}
async fn js_test_() -> Result<JsValue, JsValue> {
    wasm_bindgen_futures::JsFuture::from(js_test()).await
}

Because writing async directly inside the extern "C" block was a parse error.

But now in 0.2.62 the following compiles fine:

#[wasm_bindgen]
extern "C" {
    async fn js_test() -> Result<JsValue, JsValue>;
}

Although the js_test does not return a Future, but a plain old Result. The async seems to be totally ignored.

The documentation does not mention my use case, so maybe it is not supported.

Expected Behavior

I would expect one of these, in order of preference:

1. The JS Promise is automatically converted into a Rust Future with a hidden call to wasm_bindgen_futures::JsFuture::from(), and the return Ok type is automatically converted if different of JsValue.
2. The compiler complaints that this usage of async is not supported, (yet?).

Actual Behavior

The code compiles but the async keyword is ignored.

[alexcrichton]

Thanks for the report! It's definitely a bug that this doesn't work right now (or at least isn't an error). That being said I think it'd be super slick if this worked natively and returned an impl Future<...>, that'd be an awesome feature to support!

[rodrigorc]

I'd be awesome indeed, and all the pieces are already there, so it should not be too complicated, I think.

I can try and implement it myself, if you are willing to do the review.

[alexcrichton]

Sounds good to me!

[rodrigorc]

Closing it, as #2196 fixes it.