Add a Boolean library_evolution attribute to swift_library

[brentleyjones]

Historically, library evolution has been supported by a global flag that Apple framework rules transition on in order to build all dependencies with library evolution enabled. In retrospect, this is wrong; we shouldn't be automatically applying library evolution to the whole subgraph in this way. Instead, the owner of the framework should explicitly specify library evolution on the client-facing modules that make up their SDK.

The goal here is to ensure that framework owners carefully audit their public deps vs. implementation-only private_deps so that the .swiftinterface doesn't attempt to import anything that it shouldn't, especially with Swift 5.7 validating emitted interfaces after compilation by default.

This change only adds the new attribute; the flag is still honored while we have clients and rules_apple depending on it.

PiperOrigin-RevId: 486750877
(cherry picked from commit b9175e6)

Cherry-pick notes: With this change the private swiftinterface is enabled by ctx.attr.library_evolution as well. It can be disabled with features = ["-swift.emit_private_swiftinterface”].

[tumata]

Hi,

I've recently started using Bazel to automate the generation of XCFrameworks for distributing our internal frameworks. While reviewing the documentation on the master branch, I encountered the error: no such attribute 'library_evolution' in 'swift_library' rule when attempting to use the library_evolution parameter.

Now finding this PR from only 4 days ago, it that explains my issue!

From your PR description, it seems that library evolution is enabled by default in the current version 2.1.1. Could you please confirm if that's the case?

Thank you for your help!

[luispadron]

@tumata this commit hasn't made it into a release yet. You can enable it via --features="swift.enable_library_evolution" on the current releases

[tumata]

Thanks for your help @luispadron. Where do I add this feature you're mentioning?

Should I add it to my bazel build command?

bazel build //:my_xcframework --features='swift.enable_library_evolution'

Please forgive my ignorance, I'm very fresh with Bazel :)

[luispadron]

Yes, it's a flag, you can read more on it: https://bazel.build/reference/command-line-reference#flag--features

You can also add it to a .bazelrc file so it's always applied, or use the features attribute for specific targets