Role of UB / uninitialized memory

[le-jzr]

@RalfJung

And this is why @nikomatsakis brought up this union type, which is supposed to express "either T or Undef":

union Uninit<T> { init: T, uninit: () }

I might be missing something, but if you can't tell statically whether or not something is initialized, how does a union like that help? If you read init from uninitialized memory, it's still UB, and you have no way of knowing whether it is.