Macro join 

macro_rules! join {
    ($f1:expr, $f2:expr $(,)?) => { ... };
    ($f1:expr, $f2:expr, $f3:expr $(,)?) => { ... };
    ($f1:expr, $f2:expr, $f3:expr, $f4:expr $(,)?) => { ... };
}

Polls futures concurrently, returning a tuple of all outputs when every future has completed.