Why is Swift ABI Stability a Big Deal?

Imagine writing some Swift code. You want to share it across multiple apps, so you build a framework. You compile it with the Swift 4.0 compiler.

Now imagine Apple announces Swift 4.1. Sweet! You decide to build a new app and compile it with the Swift 4.1 compiler.

Ah, but your apps needs some of that “shared feature” code from your framework.

Will it work? I’ll give you one guess. (the answer’s “nope”)

Prior to “ABI stability” being a feature of Swift, the only guarantee we had that two pieces of compiled code could talk to one another is if the two pieces of code were compiled with the same compiler.

A framework could be compiled with the Swift 4.0 compiler. You can build an app with the Swift 4.1 compiler. BUT. The two cannot communicate. Either the framework or your app needed to be recompiled using the same version of the compiler for it to work.

Now, “ABI stability” is a feature of Swift Proper. A framework could be compiled with the Swift 5.0 compiler. You can build an app with the Swift 5.1 compiler. The two can communicate!