Add support for using async stream functions in traits

[taiki-e]

You can use async stream functions in traits by passing boxed or boxed_local as an argument.

#![feature(async_await, generators)]
use futures_async_stream::async_stream;

trait Foo {
    #[async_stream(boxed, item = u32)]
    async fn method(&mut self);
}

struct Bar(u32);

impl Foo for Bar {
    #[async_stream(boxed, item = u32)]
    async fn method(&mut self) {
        while self.0 < u32::max_value() {
            self.0 += 1;
            yield self.0;
        }
    }
}

A async stream function that received a boxed argument is converted to a function that returns Pin<Box<dyn Stream<Item = item> + Send + 'lifetime>>.
If you passed boxed_local instead of boxed, async stream function returns a non-threadsafe stream (Pin<Box<dyn Stream<Item = item> + 'lifetime>>).

#![feature(async_await, generators)]
use futures::stream::Stream;
use futures_async_stream::async_stream;
use std::pin::Pin;

// The trait itself can be defined without unstable features.
trait Foo {
    fn method(&mut self) -> Pin<Box<dyn Stream<Item = u32> + Send + '_>>;
}

struct Bar(u32);

impl Foo for Bar {
    #[async_stream(boxed, item = u32)]
    async fn method(&mut self) {
        while self.0 < u32::max_value() {
            self.0 += 1;
            yield self.0;
        }
    }
}

Closes #10

[taiki-e]

bors r+

[bors]

Build succeeded

• taiki-e.futures-async-stream