Function select 

pub async fn select<A, B>(
    fut_a: A,
    fut_b: B,
) -> Either<<A as Future>::Output, <B as Future>::Output> 
where
    A: Future,
    B: Future,

Waits for either one of two differently-typed futures to complete.

fut_a is polled first, so it wins when both futures are ready during the same poll. The other future is dropped.