Option<enum> returns undefined

[attente]

Hello! I have a reproducer for this bug here: https://github.com/attente/wasm-bindgen-option

Basically:

use wasm_bindgen::prelude::*;

// doesn't work :(
#[wasm_bindgen]
pub fn foo() -> Option<Foo> {
    Some(Foo::Foo)
}

// works!
#[wasm_bindgen]
pub fn bar() -> Foo {
    Foo::Bar
}

// works!
#[wasm_bindgen]
pub fn baz() -> Option<i32> {
    Some(3)
}

pub enum Foo {
    Foo = 1,
    Bar = 2,
}

impl wasm_bindgen::describe::WasmDescribe for Foo {
    fn describe() {
        wasm_bindgen::describe::inform(wasm_bindgen::describe::I32)
    }
}

impl wasm_bindgen::convert::IntoWasmAbi for Foo {
    type Abi = i32;

    fn into_abi(self, _extra: &mut dyn wasm_bindgen::convert::Stack) -> Self::Abi {
        self as Self::Abi
    }
}

impl wasm_bindgen::convert::OptionIntoWasmAbi for Foo {
    fn none() -> Self::Abi {
        0
    }
}

Returning an enum that implementswasm_bindgen::convert::OptionIntoWasmAbi works. (bar())
Returning an Option<i32> works. (baz())
But returning Option<enum: wasm_bindgen::convert::OptionIntoWasmAbi> doesn't work. (foo())

The output I got from the reproducer is:

Some(Foo::Foo) = undefined
Foo::Bar = 2
3 = 3

[alexcrichton]

Thanks for the report! Currently WasmDescribe is an internal trait, however, not intended to be implemented externally (but we have to expose for implementation reasons for now). I suspect there's probably subtelties there, but the exact pieces escape me. Would adding #[wasm_bindgen] directly on the enum work for your use case?

[attente]

Adding #[wasm_bindgen] on the enum causes the compiler to complain that there are multiple implementations of wasm_bindgen::convert::traits::IntoWasmAbi and wasm_bindgen::describe::WasmDescribe for type Foo. Fair enough, but after removing my own definitions, wasm-bindgen then complains:

error: failed to generate bindings for Rust export `foo`
	caused by: unsupported optional return type for calling Rust function from JS: Enum

[alexcrichton]

Oh dear that sounds like a bug! That may just be an unimplemented code path in wasm-bindgen's JS generating for now

[alexcrichton]

Ok option support should be added in #1214