Function join 

pub fn join<A, B, RA, RB>(op_a: A, op_b: B) -> (RA, RB)
where
    A: FnOnce() -> RA + Send,
    B: FnOnce() -> RB + Send,
    RA: Send,
    RB: Send,

Rayon join with local context.

This function captures the LocalContext of the calling thread and propagates it to the threads that run the operations.

See rayon::join for more details about join.