Function futures::stream::select

pub fn select<St1, St2>(stream1: St1, stream2: St2) -> Select<St1, St2> where
    St1: Stream,
    St2: Stream<Item = <St1 as Stream>::Item>, 

This function will attempt to pull items from both streams. Each stream will be polled in a round-robin fashion, and whenever a stream is ready to yield an item that item is yielded.

After one of the two input stream completes, the remaining one will be polled exclusively. The returned stream completes when both input streams have completed.

Note that this function consumes both streams and returns a wrapped version of them.