Add mutation testing

.gitattributes

@@ -1,4 +1,5 @@
 .*               export-ignore
+infection.json   export-ignore
 Makefile         export-ignore
 phpstan.neon     export-ignore
 phpunit.xml.dist export-ignore

.github/settings.yml

@@ -21,6 +21,7 @@ branches:
           - "Tests (7.4, lowest)"
           - "Tests (7.4, highest)"
           - "Code Coverage (7.4)"
+          - "Mutation Tests (7.4)"
         strict: true
 
       required_pull_request_reviews: null

.github/workflows/continuous-integration.yml

@@ -182,3 +182,38 @@ jobs:
         env:
           CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
         run: bash <(curl -s https://codecov.io/bash)
+
+  mutation-tests:
+    name: "Mutation Tests"
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        php-version:
+          - 7.4
+
+    steps:
+      - name: "Checkout"
+        uses: actions/checkout@v1
+
+      - name: "Install PHP with extensions"
+        uses: shivammathur/setup-php@v1
+        with:
+          coverage: pcov
+          extensions: "mbstring"
+          php-version: ${{ matrix.php-version }}
+
+      - name: "Cache dependencies installed with composer"
+        uses: actions/cache@v1
+        with:
+          path: ~/.composer/cache
+          key: php${{ matrix.php-version }}-composer-
+          restore-keys: |
+            php${{ matrix.php-version }}-composer-
+
+      - name: "Install locked dependencies with composer"
+        run: composer install --no-interaction --no-progress --no-suggest
+
+      - name: "Run mutation tests with pcov and infection/infection"
+        run: vendor/bin/infection --ignore-msi-with-no-mutations --min-covered-msi=75 --min-msi=65

infection.json

@@ -0,0 +1,11 @@
+{
+  "timeout": 10,
+  "source": {
+    "directories": [
+      "src"
+    ]
+  },
+  "logs": {
+    "text": "build/infection/infection-log.txt"
+  }
+}

vendor-bin/test/composer.json

@@ -1,5 +1,6 @@
 {
     "require": {
+        "infection/infection": "^0.15",
         "matthiasnoback/symfony-dependency-injection-test": "^4.0",
         "phpunit/phpunit": "^8.2",
         "symfony/phpunit-bridge": "^4.1"