Setup mutation testing

.github/workflows/ci.yaml

@@ -224,6 +224,37 @@ jobs:
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
 
+  mutation-test:
+    name: Mutation Testing
+
+    if: github.event_name == 'pull_request'
+
+    needs:
+      - end-to-end-tests
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install PHP with extensions
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 8.3
+          coverage: pcov
+          extensions: none, ctype, curl, date, dom, json, libxml, mbstring, phar, simplexml, soap, tokenizer, xml, xmlwriter, zlib
+          ini-values: assert.exception=1, zend.assertions=1, error_reporting=-1, log_errors_max_len=0, display_errors=On
+          tools: none
+
+      - name: Install dependencies with Composer
+        run: ./tools/composer install --no-ansi --no-interaction --no-progress
+
+      - name: Run mutation testing
+        run: |
+          git fetch origin $GITHUB_BASE_REF
+          php tools/infection.phar --threads=max --git-diff-lines --git-diff-base=origin/$GITHUB_BASE_REF --ignore-msi-with-no-mutations --only-covered          
+
   build-phar:
     name: Build PHAR
 

infection.json5.dist

@@ -0,0 +1,12 @@
+{
+    "$schema": "vendor/infection/infection/resources/schema.json",
+    "testFrameworkOptions": "--testsuite=unit",
+    "source": {
+        "directories": [
+            "src"
+        ]
+    },
+    "mutators": {
+        "@default": true,
+    }
+}

src/TextUI/Output/Printer/DefaultPrinter.php

@@ -70,8 +70,10 @@ public static function standardError(): self
     private function __construct(string $out)
     {
         $this->isPhpStream = str_starts_with($out, 'php://');
+        $x = 1;
 
         if (str_starts_with($out, 'socket://')) {
+            $x = 2;
             $tmp = explode(':', str_replace('socket://', '', $out));
 
             if (count($tmp) !== 2) {