>Incorrect with respect to an assumption furnished where? In the definition of the `NonZeroU8` type. Or in a more practical terms, in LLVM, when we generate LL

>Incorrect with respect to an assumption furnished where? In the definition of the `NonZeroU8` type. Or in a more practical terms, in LLVM, when we generate LLVM IR we communicate this property to LLVM and it in turn uses it to apply optimizations to our code. >Also, does this not require using the `unsafe` keyword? Yes, it requires `unsafe` and the point is that writing 0 to `NonZeroU8` is UB since it breaks the locality principle critical for correctness of optimizations. Applying just one incorrect (because of the broken assumption) optimization together with numerous other (correct) optimizations can easily lead to very surprising results, which are practically impossible to predict and debug. This is why it's considered such anathema to have UB in code, since having UB in one place may completely break code somewhere far away.