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...