StreamExt::collect does not preserve Send

[rmanoka]

Hi,

The StreamExt::collect in this crate (with unstable feature-flag) converts a Send stream into future that is not Send anymore. On the other hand, futures::StreamExt seems to preserve this auto trait. This is not a mission-critical issue, but thought I'll bring it to your attention anyway.

Test code to reproduce this, that yields a compilation error:

#[async_std::test]
async fn test_send() {
    fn test_send_trait<T: Send>(_: &T) {}

    let stream = futures::stream::pending::<()>();
    test_send_trait(&stream);

    use async_std::prelude::StreamExt;
    let fut = stream.collect::<Vec<_>>();

    // This line triggers a compilation error
    test_send_trait(&fut);
}

Tested on 1.4.0

[yoshuawuyts]

@rmanoka hi, thanks for raising this! Heh yeah that's no good; we should fix that.

[yoshuawuyts]

Okay I'm able to reproduce this. A concrete example of the issue above is:

use async_std::prelude::*;
use async_std::io;
use async_std::task;

#[test]
fn test_send() -> io::Result<()> {
    task::block_on(async {
        fn test_send_trait<T: Send>(_: &T) {}

        let stream = futures::stream::pending::<()>();
        test_send_trait(&stream);


        task::spawn(async move {
            let _output = stream.collect::<Vec<_>>().await; // compile error here
        });
        Ok(())
    })
}

A possible solution here is to add Send back to the trait bounds of the Future, which I believe we originally had. Trying that out now.