Allow returning Vec<T>

[rookboom]

It would really be great to be able to return a vector of structs or tuples:

For example. I have the following type:

#[wasm_bindgen]
struct Range {
    offset: u32,
    length: u32
}

that I want to return in my function

#[wasm_bindgen]
pub fn token_ranges(text: &str) -> Vec<Range> 

I am getting this error:

   Compiling picl_wasm_runtime v0.1.0 (file:///A:/Repos/picl_native_runtime/bindings/typescript)
error[E0277]: the trait bound `std::boxed::Box<[Range]>: wasm_bindgen::convert::WasmBoundary` is not satisfied
  --> src\lib.rs:15:1
   |
15 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::convert::WasmBoundary` is not implemented for `std::boxed::Box<[Range]>`
   |
   = help: the following implementations were found:
             <std::boxed::Box<[u16]> as wasm_bindgen::convert::WasmBoundary>
             <std::boxed::Box<[i16]> as wasm_bindgen::convert::WasmBoundary>
             <std::boxed::Box<[f32]> as wasm_bindgen::convert::WasmBoundary>
             <std::boxed::Box<[i32]> as wasm_bindgen::convert::WasmBoundary>
           and 5 others
   = note: required because of the requirements on the impl of `wasm_bindgen::convert::WasmBoundary` for `std::vec::Vec<Range>`

My workaround is to flatten my data and return a Vec and then splice my token ranges on the JS side. This is unfortunate...

How can I add a WasmBoundary trait for a custom type?
Is there a better way?

[alexcrichton]

Thanks for the report!

Right now there's not a great way to do this in wasm-bindgen itself in terms of the conversion here has to be also accompanied with a conversion on the JS side which may be somewhat tricky. It should certainly be possible though given enough support!

[Hywan]

Same issue here :-).

What kind of support do you need?

[alexcrichton]

@rookboom @Hywan do y'all basically need Vec<T> where T has #[wasm_bindgen] on it?

[Hywan]

@alexcrichton Exactly, that's precisely my usecase.

[rookboom]

Yes, that would be fantastic.

[alexcrichton]

Ok this is then I think pretty similar to #104. We somehow need a way to communicate this to the CLI tool but currently the strategy for doing that is quite limited.

[gabrielcarneiro97]

I'm with a similar issue here, but with a complex struct.

code:

#[derive(Clone)]
#[wasm_bindgen]
pub struct Coords {
    x: usize,
    y: usize
}

#[wasm_bindgen]
pub struct Cell {
    state: State,
    position: Coords,
    neighboors: Vec<Coords>,
    neighboors_alive: i32
}

#[wasm_bindgen]
impl Cell {
    pub fn new(state: State, position: Coords, neighboors: Vec<Coords>) -> Cell {
        Cell {
            state,
            position,
            neighboors,
            neighboors_alive: 0
        }
    }
}

error:

error[E0277]: the trait bound `std::boxed::Box<[Coords]>: wasm_bindgen::convert::FromWasmAbi` is not satisfied
  --> src\lib.rs:36:1
   |
36 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::convert::FromWasmAbi` is not implemented for `std::boxed::Box<[Coords]>`
   |
   = help: the following implementations were found:
             <std::boxed::Box<[u16]> as wasm_bindgen::convert::FromWasmAbi>
             <std::boxed::Box<[wasm_bindgen::JsValue]> as wasm_bindgen::convert::FromWasmAbi>
             <std::boxed::Box<[u8]> as wasm_bindgen::convert::FromWasmAbi>
             <std::boxed::Box<[f32]> as wasm_bindgen::convert::FromWasmAbi>
           and 5 others
   = note: required because of the requirements on the impl of `wasm_bindgen::convert::FromWasmAbi` for `std::vec::Vec<Coords>`

[David-OConnor]

Gabriel and rook - have y'all found a workaround? Solving or working around this would add much flexibility to WASM in Rust.

Eventually, being able to use HashMaps, or structs from other packages (Like ndarrays) would be nice, but having some type of collection that maps to JS arrays would be wonderful; not sure if I can continue my project without this.

[rookboom]

My workaround is to pass JSON over the wasm boundary... Not ideal but works for now.