I don't mind requiring a minimum version of Swift for a given binary release. I would actually expect this. Will the compiler validate it and refuse to build if it doesn't understand the ABI rules used by a library the user is attempting to use?
This is what I was hoping to hear. From what you have written, it sounds like a library distributed as a binary would continue to work fine with future versions of the Swift compiler. If it is distributed with an app and everything upstream is rebuilt when the app is built then there isn't a need to declare ABI stability. Is this understanding correct?
What I am trying to understand is exactly where the line is between safe and dangerous (undefined behavior) uses of a binary library that does not use -enable-library-evolution. This line is still not 100% clear to me.