This is pretty alarming. I don't think Apple can do a full rewrite of Foundation, unless its opt-in only for Swift apps that have been ported to the new Swift-

This is pretty alarming. I don't think Apple can do a full rewrite of Foundation, unless its opt-in only for Swift apps that have been ported to the new Swift-only codebase. If they are trying to opt in non-Swift apps, well, I'm glad I don't use OSX nor have to support it. The article linked has few details on how they plan to do this, or how they plan on limiting the blast range.