Add support for split_coverage_post_processing

[UebelAndre]

This change introduces experimental_use_coverage_metadata_files (#2082) which is required to support --experimental_split_coverage_postprocessing'

Changes:

• Implemented coverage collection logic in Rust.
• Added a flag --@rules_rust//rust/settings:experimental_use_coverage_metadata_files to toggle the changes necessary for supporting --experimental_split_coverage_postprocessing.
• Added regression testing in CI to test --experimental_split_coverage_postprocessing.

[UebelAndre]

bazelbuild/bazel#18634 is required as rules_rust currently only supports llvm based coverage and utilizes the same collect_cc_coverage attr hook.

[UebelAndre]

Seeing some odd timeouts on some tests: https://buildkite.com/bazel/rules-rust-rustlang/builds/8718#0188ad81-d635-4fc5-be1a-cb4c1222f476

@illicitonion wondering if you have any ideas as to what might be causing this?

[illicitonion]

I'm not sure, sorry! Hopefully if you can fetch some logs from the timing out tests there's something useful in there...

[UebelAndre]

I figured out the issue and am now waiting for bazelbuild/bazel#18634

[UebelAndre]

@illicitonion or @krasimirgg friendly ping 😅

[krasimirgg]

@UebelAndre I'm looking into this, also reading up and going over the dependencies.
From reading #2000 (comment), I gather this is necessary to support experimental_split_coverage_postprocessing, but from reading
bazelbuild/bazel#18634 (comment), I gather that this isn't necessary?

So is this just a "nice simplification" towards experimental_split_coverage_postprocessing support?

[UebelAndre]

@UebelAndre I'm looking into this, also reading up and going over the dependencies. From reading #2000 (comment), I gather this is necessary to support experimental_split_coverage_postprocessing, but from reading bazelbuild/bazel#18634 (comment), I gather that this isn't necessary?

So is this just a "nice simplification" towards experimental_split_coverage_postprocessing support?

@krasimirgg It turned out it was necessary. If you look at https://github.com/bazelbuild/bazel/blob/6.3.0/src/main/starlark/builtins_bzl/common/cc/cc_binary.bzl#L204 the fix is using internal functionality from cc_common to add additional files to the coverage action. My assumption was that whatever rules_cc did to fix this issue would be usable by rules_rust but once I had time to get back to this problem I found that to be false. So, it's a great coincidence that coverage_common.instrumented_files_info.metadata_files was made public so pure starlark rules can access the test binary in coverage actions. If there's a better way to implement this though I'm happy to change directions. I only care that coverage works with this flag.

[krasimirgg]

Thanks for clarifying! Looks good!