"Given the importance of getting the core ABI and the related fundamentals correct, we are going to defer the declaration of ABI stability out of Swift 4 while still focusing the majority of effort to get to the point where the ABI can be declared stable."
Do I read that correctly? Swift 4 will have a ABI under the hood but they aren't going to come out and formally lock it down until after Swift 4?
Every compiler supporting separate compilation has an ABI -- the question is whether the ABI changes between releases or not.