cpp-coverage: bazel coverage builds filtered-out test targets (breaks consumers with Miri/Rust tests) #164

Problem

cpp-coverage.yml runs bazel coverage [flags] -- <targets> without --build_tests_only. Consumers typically pass --test_lang_filters=-rust,-miri via extra-bazel-flags, but test language filters only control which tests run — not what gets built. With wildcard targets (//score/...), Bazel still builds every test target, including filtered-out ones.

Observed breakage

eclipse-score/lifecycle#369 (bump to score_toolchains_rust 0.9.2): the C++ Coverage job fails with

error[E0463]: can't find crate for `profiler_builtins`

Chain: the job builds lifecycle's Miri test targets (execution filtered, build not) → the rules_rust fork's Miri support compiles their dependency subgraph against the Miri sysroot → since score_toolchains_rust 0.9.2, bazel coverage instruments Rust compiles (--codegen=instrument-coverage), which requires the profiler_builtins crate from the sysroot → the Miri sysroot artifact ships without the profiler runtime → compile error aborts the whole invocation, killing the in-flight C++ tests (NO STATUS).

Fix

Add --build_tests_only to the bazel coverage invocation in cpp-coverage.yml. With it, test filters apply to building as well, so filtered-out (Rust/Miri) tests are neither run nor built. This matches how the communication and persistency coverage setups invoke bazel coverage, and it is semantically right for a coverage job: only test targets and their dependencies matter for coverage.

Verified locally on lifecycle with the 0.9.2 bump: without the flag → E0463 build abort (16/19 tests, job red); with the flag → exit 0, 19/19 tests pass.

Related follow-ups (tracked elsewhere)

• ferrocene_toolchain_builder: ship libprofiler_builtins in the miri-sysroot tarballs
• rules_rust fork: suppress -Cinstrument-coverage for miri_enabled compiles (instrumenting interpreted-mode builds is meaningless)