Anticipating pushback: yes, you can disallow "pointer arithmetic" on handles, and store fingerprints in the "slots" to ensure they still contain the handle's identity to detect user-after-free, but congrats, you've implemented sparse sets, which there are dozen's of C++ implementations of with the same safety guarantees, so it's unclear what rust is bringing in that case (e.g. https://github.com/skypjack/entt/blob/master/src/entt/entity...)
That C++ already has many implementations of sparse sets seems to be a point in favor of sparse sets rather than a point against Rust, especially given that C++ doesn't need them the same way Rust does.
Well, there's several implementations because it's a bit of a leaky abstraction, and implementation details matter/vary across use-cases, which is consistent w/ my experience of rust having heavy fragmentation in applying the array-index pattern for "weak pointers."
If only devs had any other reason to use Rust.....
The topic is "Arenas in Rust" not "Arenas in Rust or anything other tradeoff."
Indeed, the topic is "in Rust", so you'd do arenas in Rust due to those memory safety benefits outside of arenas. And since arenas aren't as bad due determinism etc, it's still a net benefit: safer outside, safer inside