Add mutation testing

.gitattributes

@@ -4,6 +4,7 @@
 /composer-require-checker.json export-ignore
 /box.json.dist export-ignore
 /Makefile export-ignore
+/infection.json.dist export-ignore
 /phpcs.xml.dist export-ignore
 /phpstan.neon.dist export-ignore
 /phpunit.xml.dist export-ignore

.github/workflows/integrate.yaml

@@ -147,6 +147,47 @@ jobs:
         timeout-minutes: 10
         run: "vendor/bin/phpunit"
 
+  mutation-tests:
+    name: "Mutation Tests"
+
+    runs-on: "ubuntu-latest"
+
+    strategy:
+      matrix:
+        php-version:
+          - "7.4"
+
+    steps:
+      - name: "Checkout"
+        uses: "actions/checkout@v2"
+
+      - name: "Install PHP"
+        uses: "shivammathur/setup-php@v2"
+        with:
+          coverage: "pcov"
+          php-version: "${{ matrix.php-version }}"
+          tools: composer:v1
+          extensions: sqlite
+
+      - name: "Get composer cache directory"
+        id: composercache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: "Cache dependencies"
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composercache.outputs.dir }}
+          key: ${{ runner.os }}-php-${{ matrix.php-version }}-${{ matrix.dependencies }}-composer-${{ hashFiles('**/composer.json') }}
+          restore-keys: ${{ runner.os }}-php-${{ matrix.php-version }}-${{ matrix.dependencies }}-composer-
+
+      - name: "Install dependencies"
+        run: "composer update --no-interaction --no-progress"
+
+      - name: "Infection"
+        run: "vendor/bin/infection -j4"
+        env:
+          INFECTION_BADGE_API_KEY: ${{ secrets.INFECTION_BADGE_API_KEY }}
+
   code-coverage:
     name: "Code Coverage"
 

composer.json

@@ -37,6 +37,7 @@
         "doctrine/coding-standard": "^8.1.0",
         "ekino/phpstan-banned-code": "^0.3.1",
         "ergebnis/phpstan-rules": "^0.15.1",
+        "infection/infection": "^0.17.2",
         "phpstan/phpstan": "^0.12.37",
         "phpstan/phpstan-deprecation-rules": "^0.12.5",
         "phpstan/phpstan-phpunit": "^0.12.16",

infection.json.dist

@@ -0,0 +1,19 @@
+{
+    "source": {
+        "directories": [
+            "src"
+        ]
+    },
+    "timeout": 10,
+    "logs": {
+        "text": "php:\/\/stderr",
+        "badge": {
+            "branch": "master"
+        }
+    },
+    "mutators": {
+        "@default": true
+    },
+    "minMsi": 79,
+    "minCoveredMsi": 88
+}