Misleading error returning impl Future

[NickeZ]

I'm trying to create async methods on my struct. I looked at #1858 but that issue is closed without an explanation why the code below doesn't work. Is there any workaround?

use std::future::Future;
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub struct A {}

#[wasm_bindgen]
impl A {
    #[wasm_bindgen(constructor)]
    pub fn new() -> Self {
        A {}
    }

    pub fn test(&mut self) -> impl Future<Output = Result<JsValue, JsValue>> {
        Ok(JsValue::from(true))
    }
}

error[E0667]: `impl Trait` is not allowed in path parameters
 --> src/lib.rs:7:1
  |
7 | #[wasm_bindgen]
  | ^^^^^^^^^^^^^^^

error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
 --> src/lib.rs:7:1
  |
7 | #[wasm_bindgen]
  | ^^^^^^^^^^^^^^^

error[E0277]: the trait bound `impl std::future::Future: wasm_bindgen::convert::traits::IntoWasmAbi` is not satisfied
 --> src/lib.rs:7:1
  |
7 | #[wasm_bindgen]
  | ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::convert::traits::IntoWasmAbi` is not implemented for `impl std::future::Future`
  |
  = note: required because of the requirements on the impl of `wasm_bindgen::convert::traits::ReturnWasmAbi` for `impl std::future::Future`

error: aborting due to 3 previous errors

[Pauan]

It'd be nice if we could support impl Future, but it seems rather tricky.

@alexcrichton does this seem doable, or would it be too tricky/hacky to support impl Future (even if it's special-cased)?

[NickeZ]

Otherwise maybe a nicer error message that points to js_sys::Promise?

[alexcrichton]

Definitely agreed that we should try to improve the error here! It may be somewhat difficult to do so, but @Pauan is also correct in that you'll need to return a Promise instead of an impl Trait