Function static_sorted_indices 

pub fn static_sorted_indices<T: Ord, const N: usize>(
    keys: &[T; N],
) -> [usize; N]

Returns the indices of keys in sorted order. These indices are used to reorder some other array according to the sorted order of the keys, e.g. by static_apply_reordering.

“Static” in the name refers to the fact that it takes and returns an array of statically known size, avoiding Vec allocations.