Add RUST_COVERAGE_IGNORE_REGEX for configurable coverage filtering

[tamasvajk]

Summary

Add RUST_COVERAGE_IGNORE_REGEX environment variable for configurable llvm-cov export filtering.

Problem

The collect_coverage.rs script hardcodes two -ignore-filename-regex patterns (.*external/.+ and /tmp/.+) passed to llvm-cov export. Projects often need additional filters — for example, to exclude vendored C/C++ code or generated files — but there's no way to add them without patching the script.

Fix

Read an optional RUST_COVERAGE_IGNORE_REGEX environment variable: a comma-separated list of additional regex patterns passed as -ignore-filename-regex arguments to llvm-cov. The existing hardcoded patterns are preserved as defaults.

Users can set this via --test_env in .bazelrc:

coverage --test_env=RUST_COVERAGE_IGNORE_REGEX=.*vendor/.*,^/rustc/

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Note: This PR was largely AI-generated using Claude Code, with human review and guidance throughout.

[UebelAndre]

Thanks! Is there any prior-art for this in other rules? I'm wondering if a custom environment variable is really needed

[tamasvajk]

Thanks! Is there any prior-art for this in other rules? I'm wondering if a custom environment variable is really needed

I couldn't find direct prior art in other Bazel rule sets — C++ coverage in Bazel uses collect_coverage.sh with its own hardcoded logic, and rules_go doesn't use llvm-cov.

The env var approach was chosen because the filtering happens at runtime (in the collect_coverage binary, not during analysis), so --test_env is the standard Bazel mechanism for passing runtime configuration to tests without invalidating the analysis cache.

An alternative would be to add a build setting like --@rules_rust//rust/settings:coverage_ignore_regex and plumb it through to the test action's environment. That would be more discoverable and consistent with how rules_rust exposes other configuration, though it would require more plumbing and invalidate the analysis cache when changed. Happy to rework it that way if you'd prefer.