async functions don't have proper type signatures in Typescript

[thomaseizinger]

Describe the Bug

As soon as a function is async, the return type in the Typescript definition is always any.

Steps to Reproduce

1. Annotate an async function with #[wasm_bindgen]:

#[wasm_bindgen]
pub fn greet() -> String {
    "Hello World!".to_owned()
}

#[wasm_bindgen]
pub async fn async_greet() -> String {
    "Hello World!".to_owned()
}

2. Run wasm-pack build

3. Observe the generated typescript types:

/* tslint:disable */
/* eslint-disable */
/**
* @returns {string}
*/
export function greet(): string;
/**
* @returns {any}
*/
export function async_greet(): any;

Expected Behavior

The type definition for async_greet should be:

export function async_greet(): Promise<string>;

Actual Behavior

Function has return type any.

[alexcrichton]

Thanks for the report! This is due to how async desugars internally in the wasm-bindgen macro, aand all promises are returned as any (unfortunately) rather than with their type parameters. This would be great to fix, however!

[tvolk131]

Also running into this problem - would love to see a fix! I'd be happy to work on it, but would need some direction since I'm unfamiliar with the codebase and somewhat new to writing macros.

[sdykae]

@alexcrichton why #[wasm_bindgen(typescript_type = "Promise<string>")] does not work?

[jeremyBanks]

@iuslkae Darn, I was just trying that now and landed here. :(

[sdykae]

@jeremyBanks maybe there is a way to override the output at own risk, cuz the tool they are using is working worng. Could be good if they document how to achieve this to make a pull req. Since the I'm just logging the responses to get their tipes and type each one on projects i work ;'D

[arnauorriols]

This not only affects Typescript types. JSDoc autogenerated documentation also outputs any instead of the proper type.

[trevyn]

@arnauorriols My PR linked above (which looks on track to be merged soon) also adds the JSDoc comments!