Report coverage for binaries a rust_test runs as subprocesses

[robot-head]

A rust_test can list a rust_binary in data and spawn it through CARGO_BIN_EXE_<name>. This is the usual way to smoke-test a CLI. Under bazel coverage, lines that only that subprocess reaches are reported as not covered.

Cause

The binary is instrumented, and because it inherits LLVM_PROFILE_FILE it writes its .profraw into the test's COVERAGE_DIR. collect_rust_coverage merges that file with the test's own. It then runs llvm-cov export with only the test binary, and llvm-cov reads counters only for the objects it is given. So the subprocess's counters are merged and then discarded.

Change

• rust.bzl: rust_test lists each instrumented bin crate in data in RUST_COVERAGE_OBJECTS. The paths are resolved the same way as RUST_LLVM_COV: exec paths with experimental_use_coverage_metadata_files, runfiles paths without it.
• collect_rust_coverage.rs: passes each of those binaries to llvm-cov export as -object.
• rustc.bzl: data joins deps and crate in dependency_attributes, as it already is for cc_*, sh_* and py_* rules. This puts the binary's sources in the instrumented file set. It also puts the binary itself in the coverage metadata that --experimental_split_coverage_postprocessing stages, which the collector needs in order to open it.

RUST_COVERAGE_OBJECTS is only passed between the rule and the collector; it adds no attribute or setting.

Test

//test/coverage_subprocess has a binary whose lines only a subprocess reaches. Presubmit's coverage validation now requires greeter.rs to have lines hit.

	greeter.rs
main	LH:0 LF:0
this PR, split postprocessing (repo default)	LH:7 LF:7
this PR, --noexperimental_split_coverage_postprocessing --//rust/settings:experimental_use_coverage_metadata_files=false	LH:7 LF:7

bazel coverage --instrumentation_filter=^// --instrument_test_targets //test/... on linux-aarch64 (excluding the two multi-channel packages): 506 pass, 40 skipped.

This came up in a downstream Cargo workspace that builds through rules_rs. There, it restored coverage for CLI source files exercised only by CARGO_BIN_EXE smoke tests (e.g. one file went from 461/608 to 583/608 lines).

This change was written with AI assistance (Claude Code), per the AI tools policy in CONTRIBUTING.md. It stays a draft until it has been reviewed by hand.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.