Macro try_join 

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

Polls futures concurrently, returning Ok(tuple) when all complete successfully, or the first Err encountered.