mutarust

Catch weak tests in Rust before they calcify: small source changes that should fail the suite and do not. Mutation testing answers whether the tests would catch a real bug of that shape — not only whether lines executed.

mutarust is a local CLI. It mutates Rust production source, runs your Cargo tests against each mutant, and reports kills, escapes, and scores. Rust 1.85+.

Quick start

cargo install mutarust
mutarust .

That mutates the crate and prints each mutant status on stdout. Exit 0 is clean against any configured gates, 4 means a score gate failed, and other non-zero codes mean the tool or the test run failed.

Common next steps:

mutarust --coverage .
mutarust --coverage --min-msi 75 --min-covered-msi 80 .
mutarust --quiet --coverage --logger-github .

Full command guide: docs/cli.md. Configuration: docs/config.md. Mutators: docs/mutators.md. Reports: docs/json-outputs.md.

Install

cargo install mutarust
mutarust --version

From a local checkout:

cargo install --path .

Tune the gate

Start without score floors while you learn the escape set. Add coverage so uncovered lines do not drag the total score, then pin floors the suite can hold:

mutarust --coverage --min-msi 75 --min-covered-msi 80 .

Put the same policy in mutarust.yml when thresholds need to live in the repo. See docs/config.md.

On a legacy crate with accepted survivors, record a baseline and fail only on new escapes:

mutarust --update-baseline .
mutarust --baseline mutarust-baseline.json --fail-on-escaped .

Suppress one intentional exception

Blacklist a known false positive by checksum, or skip source lines with config patterns. Each mutant result shows its Blacklist checksum: value. See docs/cli.md (--blacklist) and docs/config.md (ignore_source_lines, disable_mutators).

Drop it into CI

# GitHub Actions
- uses: dtolnay/rust-toolchain@stable
  with:
    components: llvm-tools-preview
- run: cargo install cargo-llvm-cov mutarust --locked
- run: mutarust --coverage --min-msi 75 --min-covered-msi 80 --logger-github .

# GitLab Code Quality
script: mutarust --coverage --logger-gitlab --min-msi 75 .
artifacts:
  reports:
    codequality: mutarust-gitlab.json

Full workflows and baseline adoption: docs/ci.md.

Maintainers

Install details: docs/install.md. Release process: docs/release.md. Domain glossary: docs/glossary.md. Mutago parity table: docs/parity.md. Custom mutators: docs/custom-mutators.md.

Optional local hooks live in githooks/. Enable with git config core.hooksPath githooks.

License

MIT. See LICENSE.