This is quite the... unique way of defining "undefined behavior". Rust is currently implementation-defined. That doesn't even remotely mean its behavior is undefined in theory. And it certainly doesn't mean that "all of Rust" is UB in any practical, real-world sense. In practice, your average Rust program is more well-defined than nearly every C codebase, in terms of what invariants the compiler is allowed to assume about the code for the sake of performing transformations (which is what UB actually means, rather than "is there an international standard").