Stream combinator for different underlying stream types

eitherstream.rs

/// Either is not implemented for Streams in futures 0.1.x, so we use our own.
enum EitherStream<A, B> {
    A(A),
    B(B),
}

impl<A, B> Stream for EitherStream<A, B>
where
    A: Stream,
    B: Stream<Item = A::Item, Error = A::Error>,
{
    type Item = A::Item;
    type Error = A::Error;

    fn poll(&mut self) -> Poll<Option<A::Item>, A::Error> {
        match *self {
            EitherStream::A(ref mut a) => a.poll(),
            EitherStream::B(ref mut b) => b.poll(),
        }
    }
}