I’m wondering how this interacts with the “young objects mostly die” assumption of a generational garbage collector. It seems like using an arena for the young generation might work better for some programs, while an ECS-like scheme works better for other programs.

Thank you for asking! I've not implemented and thus haven't proved this in action yet, but my thinking is that this interacts very well indeed: Each heap vector can designate an index that marks the beginning of the young generation. We don't need separate old and new spaces, instead promotion is just the act of moving the young generation beginning index up.

Side note: I have a corollary on the "most objects die young" that is very much at the heart of Nova: Most objects live together. If they are created at the same time, then they're likely also used together. Hence why I don't swap items around in the heap vectors, or use a free list for allocation: It would mess up the temporal order of items in the vectors, leading to less chances at useful cache line sharing.