"The primary goal of Swift 5.0 is for the language to achieve ABI stability."
"Apple’s Chris Lattner, original creator of the Swift language, has recently announced on the Swift Evolution mailing list that ABI stability, one of the goals originally planned for Swift 3, will be postponed."
Well, one "surprise" is that a new thing called "module stability" was invented. The Swift ABI isn't going to be the ABI-thing you are used to from C (or COM) or JVM or, well ABIs ;-)
And sure, they'll give you an ABI. But whether that is going to be a _stable_ ABI is to be seen. I have some doubts given the complexity of the language, but we'll see how it turns out.
"Apple’s Chris Lattner, original creator of the Swift language, has recently announced on the Swift Evolution mailing list that ABI stability, one of the goals originally planned for Swift 3, will be postponed."
https://www.infoq.com/news/2016/05/swift-3-no-stable-abi