#![allow(unused_variables)]
#![feature(btree_set_entry)]
fn main() {
    use std::collections::BTreeSet;
    use std::collections::btree_set::Entry;
    let mut set = BTreeSet::new();
    if let Entry::Vacant(o) = set.entry("poneyland") {
        o.insert();
    }
    assert!(set.contains("poneyland"));
}