`#[cfg(...)]` in the select macro

[ogoffart]

It would be nice if it was possible to guard some arms with #[cfg] in the select macro.

For example: (based on the example of https://docs.rs/crossbeam/latest/crossbeam/macro.select.html#examples , but adding a channel2 feature )

use crossbeam_channel::{select, unbounded};

let (s1, r1) = unbounded();
#[cfg(feature = "channel2")]
let (s2, r2) = unbounded();
s1.send(10).unwrap();

// Since both operations are initially ready, a random one will be executed.
select! {
    recv(r1) -> msg => assert_eq!(msg, Ok(10)),
    #[cfg(feature = "channel2")]
    send(s2, 20) -> res => {
        assert_eq!(res, Ok(()));
        assert_eq!(r2.recv(), Ok(20));
    }
}

I know that the workaround is to use Select directly, but I'd thought this would be convenient and natural.