Function collections::btree_set::cmp_opt

fn cmp_opt<T: Ord>(x: Option<&T>, y: Option<&T>, short: Ordering, long: Ordering) -> Ordering

Unstable (collections #27783)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

Compare x and y, but return short if x is None and long if y is None