pub fn contains_key<Q: ?Sized>(&self, key: &Q) -> bool
    where
        K: Borrow<Q> + Ord,
        Q: Ord,
    {
        self.get(key).is_some()
    }
Tries to insert a key-value pair into the map, and returns
a mutable reference to the value in the entry.

If the map already had this key present, nothing is updated, and
an error containing the occupied entry and the value is returned.

# Examples

Basic usage:

```
#![feature(map_try_insert)]

use st
benchmarks/haystacks/code/rust-library.rs