Umbrella objc_library in experimental_mixed_language_library produces malformed .swift.modulemap

[aaronsky]

Not entirely sure if this is rules_swift or rules_apple, but here we go.

I am trying to sort out workarounds to bazel#7944, in particular for propagating module maps from transitive dependencies of objc_librarys into their copts. While investigating this issue, I noticed that in the experimental_mixed_language_library macro from rules_apple, the objc_library that consumes an extended module map gets an additional module map created for it (I think by something in rules_swift). From a clean build of a module named PrintStream, the following module maps are created:

• PrintStream.internal.objc.modulemap
• PrintStream.internal.umbrella.modulemap
• PrintStream.internal.umbrella.swift.modulemap

This .swift.modulemap contains all manner of headers from a given CcCompilationContext, including things that are not headers but are provided as headers due to the absence of a cc_inputs-like parameter on objc_library, such as Clang configuration files, header maps, or other module maps, and as such Clang fails to parse them. Despite this, these module maps are still propagated as -fmodule-map-file to other library targets.

My suspicion is that, unless feature loading is totally broken in swift_library (i.e. something is failing to receive the swift.no_generated_module_map setting) the issue lies somewhere in how swift_clang_module_aspect is loading objc_library module info. But beyond that, I am uncertain how to move forward. @luispadron and I made a little progress investigating, but we could use more eyes.

[aaronsky]

I can work around this with the changes in #691, which allow me to suppress aspect behavior for my custom module maps.

[luispadron]

@aaronsky Is that something you think should be added to the experimental_mixed_language_library module maps?

[aaronsky]

Yes, there are a few related improvements that I need to push up to experimental_mixed_language_library. Without them, I have to continue using my highly-derivative home-cooked macro.