Run it anywhere

The GitHub Action is the shortest path. Everywhere else — your laptop, GitLab, Gitea, Forgejo, Jenkins — use the Docker image, which carries the Rust toolchain and cargo-mutants, or the static binary.

What it needs

• Linux. cargo-mutants cannot test mutants on Windows (a temp-path length limit). macOS works, more slowly. On Windows, use the image.
• Your Rust toolchain and dependencies — each mutant is compiled and tested the way your CI builds the project.
• cargo-mutants on PATH. The Action and the image bring it; with the bare binary, cargo install cargo-mutants.
• Full history for the base ref, so there is a merge-base to diff against: fetch-depth: 0, GIT_DEPTH: "0", or a git fetch of the target branch.

Docker image

mancube/mergestro-gate — Alpine with the Rust toolchain, cargo-mutants and git, for linux/amd64 and linux/arm64, about 280 MB compressed. It runs as a non-root user; mount the repository at /work.

$ git fetch origin main
$ docker run --rm -v "$PWD:/work" mancube/mergestro-gate:0.6.1 \
    --repo /work --base origin/main --advisory

Prebuilt binary

A static musl binary for Linux x86-64, with a SHA-256 checksum on every release.

$ curl -sSL -o slop-gate.tar.gz \
    https://github.com/lucheeseng827/mergestro-gate/releases/download/v0.6.1/slop-gate-x86_64-unknown-linux-musl.tar.gz
$ tar xzf slop-gate.tar.gz && sudo mv slop-gate /usr/local/bin/
$ cargo install cargo-mutants --locked
$ slop-gate --repo . --base origin/main --advisory

GitLab CI

Run the image on merge-request pipelines and write the report as Markdown. The gate job's exit code is the verdict.

.gitlab-ci.yml — the gate job

behavioral-gate:
  image:
    name: mancube/mergestro-gate:0.6
    entrypoint: [""]          # let GitLab run the script in a shell
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
  variables:
    GIT_DEPTH: "0"
  script:
    - git fetch origin "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME"
    - |
      set +e
      slop-gate --repo . --base "origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" \
        --max-survivors 0 --format markdown > mergestro-report.md
      GATE_RC=$?
      set -e
      cat mergestro-report.md
      exit $GATE_RC
  artifacts:
    when: always
    paths: [mergestro-report.md]

The full example adds a second job that posts the report as a merge-request note and updates it in place on the next push.

Gitea, Forgejo and Jenkins

Same image, same exit codes. Working pipelines, including the pull-request comment:

• Gitea / Forgejo Actions
• Jenkins

Predict the cost first

Wall-clock time is roughly the number of mutants times your suite's run time. estimate enumerates the mutants a diff produces, after the per-function cap, without building or testing anything:

$ slop-gate estimate --repo . --base origin/main    # add --json for CI

If the number is large, bound the run with a time budget or split it across jobs.

Next: What's new in 0.6 →