#![allow(unused_variables)]
#![feature(btree_entry_insert)]
fn main() {
    use std::collections::BTreeMap;
    let mut map: BTreeMap<&str, String> = BTreeMap::new();
    let entry = map.entry("poneyland").insert_entry("hoho".to_string());
    assert_eq!(entry.key(), & "poneyland");
}