Note that `unsafe` code blocks (or having some unsafe primitives) fundamentally results in some kind of UB in the language as a whole, and you totally can create tons of problems from it, since the rest of the language has invariants it can't itself violate, but the unsafe code can (potentially much easier than C, if there are more invariants) - that is the "UB", and the invariants that can be violated are what the compiler optimizes based on.

Lest we forget, any language with a C FFI capability must have some notion of UB, because the FFI effectively includes the C code in its own semantics (unless fully sandboxed, which may be too expensive to be done).