Implement a threadsafe version of wasm-bindgen-futures

[alexcrichton]

Currently the wasm-bindgen-futures crate provides a number of unsafe implementations to implement a runtime to execute futures within. These unsafe blocks, however, are predicated on the application being single threaded. The crate isn't safe in a threaded environment!

We should implement a threadsafe version of wasm-bindgen-futures, gated on the target_feature = "atomics" feature of WebAssembly!

[alexcrichton]

For those interested in getting their feet wet with the multithreading story of Rust and WebAssembly, this is definitely a meaty project to take on!

[ibaryshnikov]

So the task is to replace Cell and RefCell inside a Package struct? Would you mind if I try it?

[fitzgen]

@ibaryshnikov all yours!

I think that should do the trick, ignoring potential dead locks. I don't remember if there is unsafe code that needs to be updated too...

[alexcrichton]

To solve this issue the unsafe code from crates/futures/src/lib.rs basically needs to be removed, notably these two unsafe impl blocks. Those are safe in a single-threaded scenario, but with multithreaded wasm those are legitimately unsafe.