futures-cpupool support with boxed future

[raviqqe]

The code below cannot be compiled. That seems to be because Box<Future> is not Send. While #[async(boxed)] is not necessary and #[async] works well instead here, it's better to support some kind of boxed generator which is Send as they would be common in complex applications (e.g. when writing recursive functions.)

This issue may be related to #15 because the problem is caused by trait objects. I guess something like Box<G> where G: Generator fixes it. Or some way to support Box<Future + Send>?

#![feature(proc_macro, generators)]

extern crate futures_await as futures;
extern crate futures_cpupool;

use futures::prelude::*;
use futures_cpupool::*;

#[async(boxed)]
fn foo() -> Result<(), ()> {
    Ok(())
}

fn main() {
    CpuPool::new_num_cpus().spawn(foo()).wait();
}

Build log:

> cargo build
   Compiling rust-async-await-examples v0.1.0 (file:///home/raviqqe/src/github.com/raviqqe/rust-async-await-examples)
error[E0277]: the trait bound `futures::Future<Error=(), Item=()>: std::marker::Send` is not satisfied
  --> src/main.rs:15:29
   |
15 |     CpuPool::new_num_cpus().spawn(foo()).wait();
   |                             ^^^^^ `futures::Future<Error=(), Item=()>` cannot be sent between threads safely
   |
   = help: the trait `std::marker::Send` is not implemented for `futures::Future<Error=(), Item=()>`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<futures::Future<Error=(), Item=()>>`
   = note: required because it appears within the type `std::boxed::Box<futures::Future<Error=(), Item=()>>`

error: aborting due to previous error

error: Could not compile `rust-async-await-examples`.

To learn more, run the command again with --verbose.

[raviqqe]

I made an experimental branch just with Box<Future + Send + Sync> and it works well with my project. However, they should be optional.

[alexcrichton]

Thanks! Want to add it upstream? I imagine we could do something like #[async(boxed_send)] or something like that.