Later on, it adds generational references and constraint references to relax the restrictions. These are both more flexible than SlotMap because they don't requ

Later on, it adds generational references and constraint references to relax the restrictions. These are both more flexible than SlotMap because they don't require a new parameter to be passed in from the callers (and callers' callers etc), which can cause problems when an indirect caller's signature can't change (trait method override, public API, drop, etc.)