Back when Swift 5.0 was first announced with ABI stability on Apple platforms, we wrote a blog post outlining some of the reasons the Swift runtime cannot be embedded in apps anymore:
Can I choose to bundle a newer Swift runtime with my apps going forward to be able to use new runtime features without requiring a new OS?
This will not be possible for a number of reasons:
- The coexistence functionality that is used to maintain compatibility with pre-stable Swift runtimes depends on there being no more than two Swift runtimes active in a single process, and that all Swift code using the pre-stable runtime is self-contained as part of the app. If the same mechanism were used to allow a newer Swift runtime to be bundled to run alongside the OS Swift runtime, the new runtime would have no access to Swift libraries in the OS or ABI-stable third-party Swift libraries linked against the OS runtime.
- Outright replacing the OS runtime with a bundled runtime would circumvent the security of the system libraries, which are code-signed based on their using the OS version of the runtime.
- Furthermore, if the OS Swift runtime could be replaced, this would add a dimension to the matrix of configurations that the OS, Swift runtime, and third-party libraries and apps all have to be tested against. “DLL hell” situations like this make testing, qualifying, and delivering code more difficult and expensive.
- By being in the OS, the Swift runtime libraries can be tightly integrated with other components of the OS, particularly the Objective-C runtime and Foundation framework. The OS runtime libraries can also be incorporated into the dyld shared cache so that they have minimal memory and load time overhead compared to dylibs outside the shared cache. Eventually, it may be impossible for a runtime built outside the OS to fully replicate the behavior of the OS runtime, or doing so may come with significant performance costs when constrained to using stable API.
Hopefully that helps answer some of the questions about why back-deploying concurrency is not a mere matter of bundling a new runtime with old OSes.