Code Coverage

Rules Rust supports collecting code coverage data using bazel coverage, leveraging LLVM's source-based coverage instrumentation.

Basic Usage

bazel coverage //my_project/...

This instruments all targets matching the default --instrumentation_filter and produces an LCOV report at bazel-out/_coverage/_coverage_report.dat.

Controlling Instrumentation

Rules Rust respects Bazel's standard --instrumentation_filter flag to control which targets get compiled with -Cinstrument-coverage. Only targets whose label matches the filter are instrumented, consistent with how coverage works for other languages (C++, Java).

Recommended .bazelrc Settings

For projects with vendored or third-party dependencies, restrict instrumentation to workspace targets to avoid unnecessary recompilation:

coverage --instrumentation_filter=^//

To further exclude specific directories:

coverage --instrumentation_filter=^//,-^//third_party

rust_test with a crate attribute

When a rust_test target uses the crate attribute, Rust compiles the library source code directly into the test binary. Rules Rust automatically checks whether the underlying crate should be instrumented, so library code compiled into the test binary produces coverage data without needing --instrument_test_targets.

Note: because the entire crate (including #[cfg(test)] code) is compiled as one unit, test-specific code in the crate will also be instrumented. This is a known inconsistency with the usual Bazel convention where test code is only instrumented when --instrument_test_targets is set.

--instrument_test_targets

By default, Bazel excludes test targets from instrumentation. If you want coverage of the test code itself (not just the libraries it exercises), add:

coverage --instrument_test_targets

Flags Summary