Indexing into an array or vec will always be checked in safe Rust, so if an overflow gives you a bad index you'll just get a panic (or an Option::None if you decided to access it that way); never a safety issue
It also helps that, in practice, when writing idiomatic Rust you'll almost always be accessing array/vec members using an iterator. Direct indexing is uncommon, which means indexing bugs specifically are very uncommon. Of course that's not the only bug that overflows can cause, but it's a big one.
Not every bug can be prevented by the language (at compile time or runtime). You have to make some choices about which things are just in the programmer's hands, and I think Rust has struck a reasonable balance. Especially since the user can always choose differently in this case, and just compile their code with `overflow-checks = true`