#![allow(unused_variables)]
#![feature(btree_set_entry)]
fn main() {
    use std::collections::btree_set::{Entry, BTreeSet};
    let mut set = BTreeSet::from(["a", "b"]);
    match set.entry("a") {
        Entry::Vacant(_) => unreachable!(),
        Entry::Occupied(_) => {}
    }
}