Make Xcode version accessible to aspects

[blorente]

Description of the feature request:

For reasons outlined in the following sections, I want to fetch the Xcode version from an aspect. In normal build-land, a cc_* target would get the xcode version via the cc toolchain:

bazel cquery 'somepath(//src/lib:greeting_lib,  @local_config_xcode//:host_xcodes)'
//src/lib:greeting_lib (602a7de)
@bazel_tools//tools/cpp:current_cc_toolchain (602a7de)
@bazel_tools//tools/cpp:toolchain (602a7de)
@local_config_cc//:toolchain (602a7de)
@local_config_cc//:cc-compiler-tvos_sim_arm64 (602a7de)
@bazel_tools//tools/cpp:host_xcodes (602a7de)
@local_config_xcode//:host_xcodes (602a7de)

I have gotten the aspect to propagate up to the right target via the _xcode_config attribute, but I can't get it to pass the relevant information down.

This is the aspect: https://gist.github.com/blorente/95f74405142d5a2364909950e505e79d
And the output of running it on roughly any cc_* target: https://gist.github.com/blorente/296e5fd4bb9eec22299eaa0204dc13f3

As far as I can see, the fundamental pain point here is that the toolchains attribute of cc_toolchain_suite is a LABEL_DICT_UNARY, according to Bazel source code. This is not one of the "dependency attributes", according to Bazel docs. And so I can't really get the deps outputs from @local_config_cc//:toolchain to pass downstream (see this comment).

Therefore, I'd like a way to figure out which version of Xcode a particular cc toolchain is using from an aspect.

What underlying problem are you trying to solve with this feature?

I'm working on the CLion plugin. It uses an aspect to gather build configuration for each target, as well as about the toolchains configured (e.g. the CcToolchainInfo), which is then passed to the plugin to translate into the IDE's model of the world.

One such information we need is the CC compiler. Via CcToolchainInfo we're able to get the clang wrapper script, which we can pass to CLion as "the compiler to use to implement your smarts". This is correct behavior. However, that script has a baked-in assumption that it'll have DEVELOPER_DIR and SDKROOT set.

In Bazel, those are set roughly by querying @local_config_xcode//:host_xcodes (or equivalent) to get the xcode version, and then running xcode-locator -v <that version> to get the right DEVELOPER_DIR.

I want to replicate that behaviour in the compiler we give to CLion.

Which operating system are you running Bazel on?

macOS, arm

What is the output of bazel info release?

Tested with 6.0.0 and 5.4.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

I have googled and searched the GitHub issues, to no avail. Asked in the upstream slack with no success.

Any other information, logs, or outputs that you want to share?

No response

[github-actions]

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[blorente]

I think this is done in the PR linked.