Function non_domination_ranks 

pub fn non_domination_ranks(
    points: &[Point<'_>],
    directions: &[Direction],
) -> Vec<usize>

The non-domination rank of every point: 0 for the Pareto front, 1 for what is left once it is removed, and so on.

A flattened fast_non_dominated_sort, indexed like points. A point that somehow reaches no front (impossible for the relation defined here, since it is a strict partial order) keeps usize::MAX, which sorts last.