Macro queue_sync 

macro_rules! queue_sync {
    ($($e:expr),*) => { ... };
}

Creates a Queue that implements Sync, containing the given arguments:

let q = Queue::new_sync()
    .enqueue(1)
    .enqueue(2)
    .enqueue(3);

assert_eq!(queue_sync![1, 2, 3], q);

fn is_sync() -> impl Sync {
    queue_sync![0, 1, 3]
}