Function sort_with_vec_by 

pub fn sort_with_vec_by<T, F>(v: &mut [T], scratch_buf: &mut Vec<T>, compare: F)
where
    F: FnMut(&T, &T) -> Ordering,

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

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