Returning `Result<Vec<u8>, JsError>` from an `async` function doesn't work

[nasso]

Describe the Bug

The following compiles:

#[wasm_bindgen]
pub fn foo() -> Result<Vec<u8>, js_sys::Error> {
  unimplemented!()
}

While the following does not:

#[wasm_bindgen]
pub async fn foo() -> Result<Vec<u8>, js_sys::Error> {
  unimplemented!()
}

Steps to Reproduce

1. Copy and paste the second snippet above
2. cargo check
3. See error

Expected Behavior

Whatever works without async works the same with async.

Actual Behavior

Sometimes, it does not 😔.

Additional Context

Not the first instance of this issue (see #3059).

[Teebor-Choka]

This issue is unpleasant. Any chance of extending to functionality for async to match the sync behavior?

[daxpedda]

I didn't look into it, but I don't think this requires any design work, so PRs are welcome.

[Teebor-Choka]

I created a custom type to wrap around the Vec. Was asking about future prospects.

[daxpedda]

I'm not aware of any current maintainers interested in working on something like this.

[benma]

Fyi I ran into the same and would welcome a fix. For String it works both with async fn and regular fn, but not with Vec.

[Steven19Lee]

Hi @daxpedda I would like to work on this bug if no one is working on it! Thanks ~

[daxpedda]

Go right ahead 😃!

[Steven19Lee]

Hi @daxpedda this currently solution I can think about is like

impl<T> From<Vec<T>> for JsValue {
    #[inline]
    fn from(s: Vec<T>) -> JsValue {
        JsValue::from(s.as_ptr() as usize)
    }
}

but this would first convert the raw pointer into f64 ,

    /// Creates a new JS value which is a number.
    ///
    /// This function creates a JS value representing a number (a heap
    /// allocated number) and returns a handle to the JS version of it.
    #[inline]
    pub fn from_f64(n: f64) -> JsValue {
        unsafe { JsValue::_new(__wbindgen_number_new(n)) }
    }

then pass it to the JS function which adds it to heap

module.exports.__wbindgen_number_new = function(arg0) {
    const ret = arg0;
    return addHeapObject(ret);
};
function addHeapObject(obj) {
    if (heap_next === heap.length) heap.push(heap.length + 1);
    const idx = heap_next;
    heap_next = heap[idx];

    heap[idx] = obj;
    return idx;
}

I do not know if the behavior is correct or not, since it just adds the address of vec to the JS heap, but in wasm-bindgen guide, it seems to be ok to convert type into a number, because it just abi ? Any help would be very appreciate !

---

It seems like we need to convert vec into JS Array ?
reference

[Liamolucko]

Hi @daxpedda I would like to work on this bug if no one is working on it! Thanks ~

Oh, I'm actually already partway through implementing this. I'd forgotten that this issue existed, I was working on it because I was fixing the issue @cschramm pointed out in #3554 (comment) that #3554 didn't cover async functions. But I should probably put more effort into making sure I've self-assigned myself to anything I'm working on.

[daxpedda]

Sorry @Solo-steven, poor communication on our part here ...

[Steven19Lee]

It is OK, and thanks @Liamolucko for giving me the link to related PR so that I can learn from it! thanks