[leod]

This is true for some scenarios, like invalidating iterators through deletion (detected at compile-time by the borrow checker), but other scenarios still require runtime checks, right? Consider e.g. array out of bound accesses -- are you aware of approaches that move bounds checks to compile-time? It seems to me that this would be a painstaking process that would require programmers to annotate their code in many places to enable compile-time verification.

[steveklabnik]

It just depends. Yes, Rust has no UB in safe Rust (modulo bugs). Sometimes, that means compile-time checks. Sometimes, that means runtime checks. It depends on the specific thing.

The compiler _will_ attempt to prove that bounds checks aren't needed and eliminate them; see https://godbolt.org/z/7QPfhR vs https://godbolt.org/z/Vx39fv for example. In the first, there's an array and the compiler knows that it has a length of 3, so an index of zero needs no checks. In the second, we don't know how long the slice is, so we have to do the check.