[Prospective vision] Optional Strict Memory Safety for Swift

[xwu (Xiaodi Wu)]

@Karl

There is typically no way for me to create a Foo where x != 42. However, if I were to use unsafe APIs to load a value from raw memory, I could.

The creation of such a Foo is not undefined behaviour

Mmm, that is undefined behavior—it may not be "practical" UB when limited to only this code, but UB it is nonetheless.