Function sort_with_vec_by_key 

pub fn sort_with_vec_by_key<T, F: FnMut(&T) -> K, K: Ord>(
    v: &mut [T],
    scratch_buf: &mut Vec<T>,
    f: F,
)

Like sort_by_key, except this function allocates its scratch space with scratch_buf.reserve(_).

This allows you to re-use the same allocation many times.