Yes, there are a number of optimizations that require UB, or something like it. For example, say you have this valid C or Rust code:

    if (my_signed_int + 1 > my_signed_int) {
            foo();
    }

Since signed integer overflow is UB in C, C compilers can optimize out the conditional, but since Rust specifies that overflow may wrap, the compiler can’t assume the condition is true.