Rust needs a safe abstraction over uninitialized memory

[programmerjake]

@CAD97

Formally, ang-ndet picks the execution without UB if there exists one. Modifying it to cause UB if the programmer "expected" behavior would cause UB, though, shouldn't invalidate any reasoning about it, at least non-formal reasoning.

I would argue that asm!-based freeze can't have angelic nondeterminism because it can produce outputs that later cause UB, e.g. if the output is MaybeUninit<&u8>, it could produce null, which would be UB to assume_init(). angelic nondeterminism specifically chooses whichever values won't produce UB, but here the asm! and assume_init can easily produce UB.