Setup mutation testing (#4480)

Author: staabm

.github/workflows/tests.yml

@@ -197,3 +197,81 @@ jobs:
 
       - name: "Tests"
         run: "make tests"
+
+  mutation-testing:
+    name: "Mutation Testing"
+    runs-on: "ubuntu-latest"
+    needs: ["tests"]
+    if: github.event_name == 'pull_request'
+
+    strategy:
+      fail-fast: false
+      matrix:
+        php-version:
+          - "8.2"
+          - "8.3"
+          - "8.4"
+        operating-system: [ubuntu-latest]
+
+    steps:
+      - name: "Checkout"
+        uses: actions/checkout@v5
+
+      - name: "Checkout build-infection"
+        uses: actions/checkout@v5
+        with:
+          repository: "phpstan/build-infection"
+          path: "build-infection"
+          ref: "1.x"
+
+      - uses: ./build-infection/.github/actions/setup-php
+        with:
+          php-version: "${{ matrix.php-version }}"
+          extensions: ds,mbstring
+
+      - name: "Install dependencies"
+        run: "composer install --no-interaction --no-progress"
+
+      - name: "Install build-infection dependencies"
+        working-directory: "build-infection"
+        run: "composer install --no-interaction --no-progress"
+
+      - name: "Configure infection"
+        run: |
+          php build-infection/bin/infection-config.php \
+            --source-directory='build/PHPStan/Build' \
+          > infection.json5
+          cat infection.json5 | jq
+
+      - name: "Determine default branch"
+        id: default-branch
+        run: |
+          echo "name=$(git remote show origin | sed -n '/HEAD branch/s/.*: //p')" >> $GITHUB_OUTPUT
+
+      - name: "Restore result cache"
+        uses: actions/cache/restore@v4
+        with:
+          path: ./tmp
+          key: "result-cache-v1-${{ matrix.php-version }}-${{ github.run_id }}"
+          restore-keys: |
+            result-cache-v1-${{ matrix.php-version }}-
+
+      - name: "Run infection"
+        run: |
+          git fetch --depth=1 origin ${{ steps.default-branch.outputs.name }}
+          infection \
+            --git-diff-base=origin/${{ steps.default-branch.outputs.name }} \
+            --git-diff-lines \
+            --ignore-msi-with-no-mutations \
+            --min-msi=100 \
+            --min-covered-msi=100 \
+            --log-verbosity=all \
+            --debug \
+            --logger-text=php://stdout
+
+      - name: "Save result cache"
+        uses: actions/cache/save@v4
+        if: ${{ !cancelled() }}
+        with:
+          path: ./tmp
+          key: "result-cache-v1-${{ matrix.php-version }}-${{ github.run_id }}"