Pass --locked to the cargo-mutants install fallback (#364)

[leynos]

Summary

cargo binstall falls back to a source build when no prebuilt binary matches
the runner. The Install cargo-mutants step ran binstall without --locked,
so that fallback resolved cargo-mutants' dependency tree to the newest
permitted versions and pulled in a transitive crate whose MSRV exceeded the
pinned nightly toolchain — cargo-platform 0.3.3 requiring rustc 1.91 broke a
scheduled run against nightly-2025-06-26, failing the install before any
mutant ran.

This adds --locked to both binstall invocations. cargo-binstall forwards
--locked to the source-build fallback, which then resolves against
cargo-mutants' committed Cargo.lock and stays reproducible on the pinned
toolchain. This mirrors the existing cargo-nextest install in the
generate-coverage action, which already passes --locked.

Design note

Two options were considered: (a) add --locked, or (b) disable the compile
strategy (--disable-strategies compile) and fail fast when no prebuilt binary
exists. Option (a) is chosen: it keeps the resilient source-build fallback
working (so a runner without a matching prebuilt binary still installs) while
removing the MSRV-drift failure mode, and it matches the established pattern in
this repo. Failing fast would trade a latent fragility for a hard outage
whenever a prebuilt binary is briefly unavailable.

Closes #364.

Review walkthrough

• .github/workflows/mutation-cargo.yml — cargo binstall --no-confirm --locked
  in both branches of Install cargo-mutants, with an explanatory comment.
• workflow_scripts/tests/test_mutation_workflow_shape.py — new invariant
  test_cargo_mutants_install_is_locked.

Validation

• pytest workflow_scripts/tests/test_mutation_workflow_shape.py — 5 passed.
• ruff format --check / ruff check — clean.

Note for maintainers

The axinite sweep noted a cargo-nextest fallback missing --locked. In
shared-actions the nextest install (generate-coverage/scripts/install_cargo_nextest.py)
already passes --locked, so any remaining gap is in a caller repository, not
here.

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⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: aa42c71e-6987-4948-a298-7c382a711d5e

📥 Commits

Reviewing files that changed from the base of the PR and between 074f7d8 and da40201.

📒 Files selected for processing (2)

• .github/workflows/mutation-cargo.yml
• workflow_scripts/tests/test_mutation_workflow_shape.py

🔗 Linked repositories identified

CodeRabbit considers these linked repositories for cross-repo context during reviews:

• leynos/polythene (auto-detected)

---

Summary

• Added --locked to both cargo binstall cargo-mutants commands, ensuring source-build fallbacks use the committed lockfile and remain compatible with the pinned nightly toolchain.
• Added explanatory workflow comments and a shape test enforcing the locked installation behaviour.
• Addresses issue #364 while retaining the source-build fallback.

Validation

• 5 workflow-shape tests passed.
• Ruff formatting and lint checks passed.

Walkthrough

Update the mutation workflow to pass --locked through both cargo binstall paths, document the fallback behaviour, and add a test that enforces the flag for every matching installation step.

Changes

Mutation tool installation

Layer / File(s)	Summary
Lock installation and validate workflow .github/workflows/mutation-cargo.yml, workflow_scripts/tests/test_mutation_workflow_shape.py	Pass --locked to both cargo-mutants installation paths and assert that every matching cargo binstall invocation includes the flag.

Possibly related issues

• leynos/axinite#288 — Addresses similar cargo-binstall source-build fallback behaviour requiring locked dependency resolution.

Possibly related PRs

• leynos/corbusier#141 — Updates GitHub Actions cargo binstall commands to use --locked and adds a locked installation fallback.

Suggested labels: Issue

Poem

Lock the crates, the night stays bright,
No drifting versions sneak from sight.
Binstall builds with anchors tight,
Tests guard the workflow’s flight.
Rusty gears now turn just right.

🚥 Pre-merge checks | ✅ 19 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Developer Documentation	⚠️ Warning	The workflow now depends on a locked cargo-binstall source-build fallback, but docs/developers-guide.md and the execplan never mention this tooling requirement.	Document the cargo-mutants install contract in docs/developers-guide.md and record the --locked decision in the mutation-workflows execplan or an ADR.

✅ Passed checks (19 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title matches the change and includes the linked issue reference (#364).
Description check	✅ Passed	The description is clearly about locking the cargo-mutants install fallback and matches the workflow and test changes.
Linked Issues check	✅ Passed	The PR passes --locked to cargo-binstall in both paths, preserving source-build fallback and addressing the MSRV drift in #364.
Out of Scope Changes check	✅ Passed	The changes stay within scope: workflow hardening and a shape test for the same install behaviour.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Testing (Overall)	✅ Passed	PASS: the new workflow-shape test parses mutation-cargo.yml and asserts each cargo-binstall invocation contains --locked, so it fails on the intended regression.
User-Facing Documentation	✅ Passed	Keep this as an internal workflow fix; no user-facing API or usage changed, and the existing mutation workflow guide already covers the reusable workflow.
Module-Level Documentation	✅ Passed	Keep the touched Python module docstring: it states the workflow shape-test purpose and ties it to .github/workflows.
Testing (Unit And Behavioural)	✅ Passed	The new shape test parses the workflow YAML and asserts both cargo-binstall branches include --locked, so it verifies the workflow boundary and a real invariant.
Testing (Property / Proof)	✅ Passed	Treat this as a fixed workflow-shape invariant: the test asserts two concrete cargo-binstall calls in one YAML file, not a range of inputs or transitions.
Testing (Compile-Time / Ui)	✅ Passed	Treat the rule as not applicable: only workflow YAML and a Python shape test changed; no Rust/TypeScript compile-time or UI surface is introduced.
Unit Architecture	✅ Passed	The change keeps the side-effecting install in the workflow command step and adds a read-only shape test, so boundaries and dependencies remain clear.
Domain Architecture	✅ Passed	PASS: the patch stays in CI workflow and workflow-shape tests; it adds no domain models, repositories, or transport/persistence logic.
Observability	✅ Passed	Treat this as PASS: the PR only hardens an install step and adds a shape test; it adds no new operational path needing extra observability.
Security And Privacy	✅ Passed	The patch only adds --locked to an existing install command and a shape test; it introduces no secrets, permission changes, or unsafe input handling.
Performance And Resource Use	✅ Passed	PASS: The new test does one bounded parse of a single workflow file and scans small job/step lists; the workflow tweak only adds --locked, with no hot-path work.
Concurrency And State	✅ Passed	No shared state, async work, locks, ordering, or cancellation paths were introduced; the PR only adds a workflow flag and a shape test.
Architectural Complexity And Maintainability	✅ Passed	PASS: The patch is a small workflow flag change plus one focused shape test; it adds no new layers, registries, or dependencies, and reuses the existing workflow-test module.
Rust Compiler Lint Integrity	✅ Passed	PASS: the PR touches only a YAML workflow and a Python shape test, so it introduces no Rust lint suppressions or clone churn.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/mutation-cargo-binstall-locked

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