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Suppose I initialize something in an unsafe block. I promise the compiler that it's properly initialized, but in reality it isn't. Importantly I never make use

Suppose I initialize something in an unsafe block. I promise the compiler that it's properly initialized, but in reality it isn't. Importantly I never make use of the garbage values in the unsafe block so no UB has occurred - yet. Later, the garage enters otherwise safe machinery and triggers UB. UB has now happened in safe rust as a result of my earlier contractual violation. You can extend this example to other scenarios where UB in unsafe begets further UB in safe later on.

tialaramex 6mo ago
Actually I think Rust says UB has occurred if you return a T which isn't properly initialized. For example if you unsafely MaybeUninit<u32>::assume_init() then Rust says that's UB even though all possible bit patterns for u32 are valid. This is because that's not going to emit any machine code at all, and yet it will cause LLVM to do very nasty things, for example code which either prints "Odd" or "Even" by examining the integer may now print neither because that's faster and this uninitialized integer isn't odd, yet it also isn't even so...