Maybe this isn't what you're suggesting, but it's already possible to make an interface that prevents callers from doing math on indices in Rust — just return a struct that has a private member for the index. The caller can pass the value back at which point you can unwrap it and do index arithmetic.
More than that, in theory an opaque handle would also do things like statically prevent a handle taken from one array from being used to access a different array. I feel like this should be possible in Rust with type-level shenanigans (e.g. GhostCell).
this is possible in rust, albeit with a lot of shenanigans. See this article where someone made a GC in rust where the external references are bound to a specific GC via a unique lifetime: https://kyju.org/blog/tokioconf-2026/
