Evolving Swift On Apple Platforms After ABI Stability

With the release of Swift 5.0, Swift is now ABI stable and is delivered as a core component of macOS, iOS, tvOS, and watchOS. ABI stability has been a goal for Swift since its inception, and brings with it many benefits for developers and users of these platforms.

→ swift.org/blog/abi-stability-and-apple/

Swift 5.0 achieves ABI stability on Apple platforms, allowing apps to use the OS-provided Swift runtime instead of bundling it, which reduces download sizes and enables better performance through deeper integration.

This milestone means future Swift features requiring new runtime support may necessitate updated OS versions, though source compatibility with Swift 4 modes persists without imposing OS requirements for non-runtime-dependent changes.

While apps built with Swift 5 can run on older OS's by embedding the runtime, bundling newer runtimes for forward compatibility is not supported, and backward deployment of features will be considered case-by-case.