State of feature `swift.use_c_modules`

[dierksen]

We're looking into using explicit clang modules, but it looks like right now the swift stdlib isn't accessible when the feature is enabled.

I saw https://github.com/bazelbuild/rules_swift/pull/856 had gone in recently, so wanted to see if there was any plan for this in the near-term, or if the necessary toolchain modifications already exist and just didn't make it over to github.

Current state

Building an extremely simple hello world swift_binary with --features swift.use_c_modules results in

<unknown>:0: error: missing required module 'SwiftShims'

Happy to help build this out, but wanted to check and see if the work had already been started/done. 😃

[keith]

I believe this set of features is being used inside google, but not currently publicly. The biggest missing piece I know of currently is to generate the definitions for the system modules, at Lyft we started working on that a while ago but tabled it so we don't have anything to share there yet either. I think if you had those definitions and dependencies setup correctly the rules would work with them.

[dierksen]

OK, I'm looking at just adding a swift_c_module (with some tweaks) for SwiftShims since that seems to fix the "Hello world" case at least. Possibly a little tricky since the shims module is defined per-platform (i.e. /Applications/Xcode_13.4.1.app/Contents/Developer/Platforms/AppleTVOS.platform/Developer/SDKs/AppleTVOS.sdk/usr/lib/swift/shims but shouldn't be too hard to work with.

[keith]

Yea I think for the productionized version of this it would make sense to generate the BUILD file for the system frameworks as a repository rule