Function select_either 

pub async fn select_either<'a, F1, F2>(
    future1: F1,
    future2: F2,
) -> Either<F1::Output, F2::Output>
where
    F1: Future + 'a,
    F2: Future + 'a,

Waits for either of two futures to complete.

This function polls both futures concurrently and returns the result of the first one that finishes. The future that did not complete is dropped.