Wasm-bindgen: Support converting js_sys typed arrays to Vec

[aschampion]

Currently there's no way to convert TypedArrays to Vec<T>s in rust, e.g., js_sys::Uint8Array to Vec<u8>, even though one can pass (via an implicit copy) a Uint8Array across the wasm boundary to rust functions expecting Vec<u8> or &[u8]. See this comment for motivation.

[alexcrichton]

One thing I've been pondering recently is if it's possible to, in a generic fashion, convert anything that implements IntoWasmAbi into a JsValue. This would fit the bill perfectly for this issue as well as solve a number of other issues I've wanted to handle in the past. We currently do this ad-hoc in two locations:

• You can extract a JsValue from a Closure<T>
• #[wasm_bindgen] structs can also be converted

It'd be great to somehow unify the two of these and provide this for all types (like &[u8] which can be converted into a JsValue). I'll noodle on this a bit...

[alexcrichton]

I believe I've implemented this in https://github.com/rustwasm/wasm-bindgen/pull/918

[alexcrichton]

Ok I've made another attempt to solve this in https://github.com/rustwasm/wasm-bindgen/pull/1147!

[dakom]

would it make sense to implement a to_vec() on top of copy_to()?

i.e. to allocate a new local Vec, copy_to() it, and return that?