Implement FromWasmAbi for Result<i32|u32, JsValue>

[ibaryshnikov]

It looks like there's no FromWasmAbi implementation for Result<u32, JsValue> and Result<i32, JsValue>
The feature is required by #1379, since there are return types like Result<i32, JsValue> in Atomics
https://github.com/rustwasm/wasm-bindgen/blob/master/crates/js-sys/src/lib.rs#L528

[alexcrichton]

It's intended that Result can be returned from functions but for FromWasmAbi that typically means it's an argument I think? Are you trying to take a Result as an argument? Or is there a different situation where the requirement for Result: FromWasmAbi is coming up?

[ibaryshnikov]

Here is an example

pub mod Atomics {
    use super::*;

    #[wasm_bindgen]
    extern "C" {
        #[wasm_bindgen(js_namespace = Atomics, catch)]
        pub fn add(typed_array: &JsValue, index: u32, value: i32) -> Result<JsValue, JsValue>;

When I run wasm-pack test --headless --safari in crates/futures folder, it fails with an error

error[E0277]: the trait bound `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>: wasm_bindgen::convert::traits::FromWasmAbi` is not satisfied
   --> crates/js-sys/src/lib.rs:522:5
    |
522 |     #[wasm_bindgen]
    |     ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::convert::traits::FromWasmAbi` is not implemented for `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>`

The error points here https://github.com/rustwasm/wasm-bindgen/blob/master/crates/js-sys/src/lib.rs#L519
Originally the return type was Result<i32, JsValue>, but changing i32 to JsValue didn't help. I wonder, what could cause the error

[alexcrichton]

Sorry I think I still may not be quite following, could you perhaps gist a standalone example that exhibits the error?

[ibaryshnikov]

Found it! I was confused by error message.

use wasm_bindgen::prelude::*;

pub mod Namespace {
    use super::*;

    #[wasm_bindgen]
    extern "C" {
        #[wasm_bindgen(js_namespace = Namespace)]
        pub fn example() -> Result<JsValue, JsValue>;
    }
}

If we try to run wasm-pack build, the error will be

error[E0277]: the trait bound `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>: wasm_bindgen::convert::traits::FromWasmAbi` is not satisfied
 --> src/lib.rs:6:5
  |
6 |     #[wasm_bindgen]
  |     ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::convert::traits::FromWasmAbi` is not implemented for `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>

The mistake in code is missing catch here:

#[wasm_bindgen(js_namespace = Namespace)]
pub fn example() -> Result<JsValue, JsValue>;

and the following code compiles:

#[wasm_bindgen(js_namespace = Namespace, catch)]
pub fn example() -> Result<JsValue, JsValue>;

[alexcrichton]

Hm I'm unfortunately still having difficulty reproducing this and I don't know why, @ibaryshnikov can you perhaps send a PR adding a test to crates/macro/ui-tests/*.rs which produces the desired error?

[ibaryshnikov]

@alexcrichton I tried to add the code to crates/macro/ui-tests/ but it doesn't trigger an error
I created a repo with minimal example, you can checkout it and run wasm-pack build to see the error
https://github.com/ibaryshnikov/fromwasmabi