[Prospective vision] Optional Strict Memory Safety for Swift

[xwu (Xiaodi Wu)]

@Karl

This:

It is undefined behaviour for lower not to be less than or equal to upper

Sure—or, if we go with this vision, @unsafe.

@Nevin

If we have a class [...]
Then calling that function will produce a range with broken invariants:

So, should we consider a notion of ValueSemantics to achieve some of our safety goals? Probably a separate conversation from this one.