Macro join 

join!() { /* proc-macro */ }

Use to combine sync values. It produces tuple of values or single value in case of 1 branch.

Example:

extern crate join;

use join::join;

let filtered: Vec<_> = join! { vec![1,2,3].into_iter() ?> |v| v % 2 == 0 =>[] };
assert_eq!(filtered, vec![2]);