Add webidl `optional` support to web-sys

[richard-uk1]

Currently optional parameters are forced. I think they should be an Option<T>.

This depends on #14 to generate the correct glue code, provided the way options are represented in the end is compatible with these APIs.

Also, in the spec, optional parameters can have a default. This doesn't really affect the rust function, but it could be mentioned in some generated documentation, perhaps.

WebIDL snippet from the Console api (console.*):

[UseCounter]
void count(optional DOMString label = "default");

EDITS

• 2018-07-18 Added more detail.

[alexcrichton]

Unfortunately Option<T> is pretty hairy to support, so we may want to also brainstorm other ideas of how to not rely on #14. One option may be to export multiple bindings for optional methods, for example the above could become:

#[wasm_bindgen]
extern {
    fn count();
    #[wasm_bindgen(js_name = count)]
    fn count2(label: &str);
}

So perhaps new names could be manufactured for invocations with optional arguments specified?

[ohanar]

This also ties into dealing with function overloading, which we need to deal with for proper support of WebIDL (in particular, web-sys needs it for constructors). I think what @alexcrichton suggests is basically what rust-bindgen does right now for C++ (see also rust-lang/rust-bindgen#334).

[fitzgen]

What rust-bindgen does is pretty sucky, and I hope we can come up with something better. Even just a better mangling algorithm would be an improvement, eg

#[wasm_bindgen]
extern {
    fn count();
    #[wasm_bindgen(js_name = count)]
    fn count_label(label: &str); // concatenate each optional argument's name to the method name
}

[richard-uk1]

I can do this for now. I'm doing the console API, and maybe having a concrete implementation to see will aid bikeshedding.