Bazel platforms support

[keith]

This is a tracking issue for supporting bazel's platforms for rules_apple. When relevant issues come up it would be useful to link them here for context.

[keith]

Related:

• Platforms aren't propagated in transitions #1650
• iOS platform constraints locations bazel#15041

[jiawen]

We should author a "state of affairs" document for building Apple apps, covering macOS, iOS, and tvOS. Especially relevant to my use case are how multiarch binaries and libraries interact with platforms, and the sharp edges the remain when it comes to objc_library vs cc_library and swift_library. Ideally, this document would include what we imagine the ideal state should be wrt platforms and a roadmap for getting there (e.g., using platform mappings).

[louwers]

Oops. I have been trying to debug why my platform constraint variables weren't propagated. Turns out there was a ios_application in the dependency tree. 😅

[keith]

In general platforms are supported at this point. You don't need a platform_mappings and with bazel 7.x + the new apple_support toolchain everything should work as expected. If you find otherwise please file an issue!

[jiawen]

This is fantastic - a major milestone!

Quick question / suggestion wrt apple_support_toolchain/README.md. When it says

To use the Apple CC toolchain, pull this repository into your build and add this to your .bazelrc:

Does this still apply with Bazel 7?

Additionally, when the instructions say to add

build:macos --apple_crosstool_top=@local_config_apple_cc//:toolchain to .bazelrc

does that mean in general, users should pass --config=macos on the command line (or pass --enable_platform_specific_config?

[keith]

Thanks for the comment, I submitted a PR to update the docs here bazelbuild/apple_support#290

That is no longer required for bazel 7+. The build:macos setup is meant to be used with --enable_platform_specific_config

[jiawen]

Amazing work as always, thanks!