> For example, Rust has decided that the array index type, the object size type, and the pointer size type are all the same type, [...]

Not really, at least not anymore: https://doc.rust-lang.org/std/ptr/index.html#provenance

This is also how Rust will work on CHERI. You can run your pointer-fiddling code in Miri today to check that you're following the rules.