Generate TypeScript return types for async functions

[trevyn]

Threads through an asyncness bool and an "inner" return value type descriptor for functions, then generates Promise<...> return types for async functions.

Fixes #2394

[trevyn]

Per the docs, returning Result only allows Result<T, JsValue>:

The Result type can be returned from functions exported to JS as well as closures in Rust. Only Result<T, JsValue> is supported where T can be converted to JS.

These tests are now failing, whereas before the non-JsValue error types were being hidden by the return type elision for async functions:

#[wasm_bindgen]
pub async fn async_return_7() -> Result<AsyncCustomReturn, u32> {
    Ok(AsyncCustomReturn { val: 7 })
}

#[wasm_bindgen]
pub async fn async_return_8() -> Result<AsyncCustomReturn, AsyncCustomReturn> {
    Ok(AsyncCustomReturn { val: 8 })
}

#[wasm_bindgen]
pub async fn async_throw() -> Result<(), js_sys::Error> {
    Err(js_sys::Error::new("async message"))
}

the trait `wasm_bindgen::describe::WasmDescribe` is not implemented for
`Result<AsyncCustomReturn, u32>`

help: the following implementations were found:
<Result<T, wasm_bindgen::JsValue> as wasm_bindgen::describe::WasmDescribe>

And indeed, they don't work when removing the async.

[alexcrichton]

Thanks for the PR! The implementation here looks pretty reasonable to me. I think it's fine to update those tests as well, yeah.

Can you add some specific tests for this new feature though? Ideally there'd be reference tests to see the output, tests in the main tests suite to exercise that things work, and tests in the typescript area to ensure that everything typechecks as expected.

[trevyn]

@alexcrichton Thanks for the review!

I added a test in the typescript-tests area.

I don't see any existing reference tests at all for .d.ts output; am I missing something, or shall I add reference tests for .d.ts to the cli/tests/reference section? (Right now they just test the .js and .wat output)

Also, the only intended output change is to the .d.ts files, so there shouldn't be any new behavior outside of TypeScript, and existing async fn tests should still apply. Not sure if there was something specific you wanted me to add in the main tests suite?

[alexcrichton]

Nah this looks good to me, thanks!