mutation-testing.yml

# Mutation testing — pre-release validation only.
#
# This workflow runs ONLY on PRs from develop → main, serving as a pre-release
# quality gate. It is NOT part of the default CI pipeline.
#
# Mutation testing validates test quality: cargo-mutants modifies source code
# (operator mutations, boundary changes, etc.) and checks if existing tests
# catch the mutation. "Missed mutants" = tests that pass on buggy code.
#
# Usage:
#   - Runs automatically on develop → main PRs
#   - Can be triggered manually via workflow_dispatch
#   - Results are informational — they don't block merge by default
#
# To fix missed mutants, see the pattern in:
#   crates/uteke-core/src/salience_recency.rs (mutation-killing test comments)
#   Reference: nginjen project (78% missed-mutant reduction)

name: Mutation Testing

on:
  pull_request:
    branches: [main]
  workflow_dispatch:

# Cancel in-progress runs for the same PR
concurrency:
  group: mutation-testing-${{ github.ref }}
  cancel-in-progress: true

jobs:
  mutation-test:
    name: Cargo Mutants
    runs-on: ubuntu-latest
    # Only run on PRs from develop (pre-release validation)
    if: |
      github.event_name == 'workflow_dispatch' ||
      (github.event_name == 'pull_request' && startsWith(github.head_ref, 'develop'))
    steps:
      - uses: actions/checkout@v7

      - uses: dtolnay/rust-toolchain@stable

      - name: Install cargo-mutants
        run: cargo install cargo-mutants --locked

      - name: Run cargo-mutants on core logic
        working-directory: crates/uteke-core
        run: cargo mutants --timeout 120 -j 2
        continue-on-error: true

      - name: Upload mutant results
        if: always()
        uses: actions/upload-artifact@v4
        with:
          name: mutant-results
          path: crates/uteke-core/mutants.out/
          retention-days: 30