#![allow(unused_variables)]
#![feature(map_try_insert)]
fn main() {
    use std::collections::BTreeMap;
    let mut map = BTreeMap::new();
    assert_eq!(map.try_insert(37, "a").unwrap(), & "a");
    let err = map.try_insert(37, "b").unwrap_err();
    assert_eq!(err.entry.key(), & 37);
    assert_eq!(err.entry.get(), & "a");
    assert_eq!(err.value, "b");
}