Setup mutation testing

.github/workflows/build.yml

@@ -8,6 +8,10 @@ on:
     branches:
       - "2.0.x"
 
+concurrency:
+  group: build-${{ github.head_ref || github.run_id }} # will be canceled on subsequent pushes in pull requests but not branches
+  cancel-in-progress: true
+
 jobs:
   lint:
     name: "Lint"
@@ -159,3 +163,67 @@ jobs:
 
       - name: "PHPStan"
         run: "make phpstan"
+
+  mutation-testing:
+    name: "Mutation Testing"
+    runs-on: "ubuntu-latest"
+    needs: ["tests", "static-analysis"]
+
+    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: "${{ matrix.php-version }}"
+          extensions: mbstring
+
+      - name: "Install dependencies"
+        run: "composer install --no-interaction --no-progress"
+
+      - name: "Install build-infection dependencies"
+        working-directory: "build-infection"
+        run: "composer install --no-interaction --no-progress"
+
+      - name: "Configure infection"
+        run: |
+          php build-infection/bin/infection-config.php \
+          > infection.json5
+          cat infection.json5 | jq
+
+      - name: "Cache Result cache"
+        uses: actions/cache@v4
+        with:
+          path: ./tmp
+          key: "result-cache-v1-${{ matrix.php-version }}-${{ github.run_id }}"
+          restore-keys: |
+            result-cache-v1-${{ matrix.php-version }}-
+
+      - name: "Run infection"
+        run: |
+          git fetch --depth=1 origin $GITHUB_BASE_REF
+          infection \
+            --git-diff-base=origin/$GITHUB_BASE_REF \
+            --git-diff-lines \
+            --ignore-msi-with-no-mutations \
+            --min-msi=100 \
+            --min-covered-msi=100 \
+            --log-verbosity=all \
+            --debug \
+            --logger-text=php://stdout

phpstan.neon

@@ -5,5 +5,7 @@ includes:
 	- vendor/phpstan/phpstan-phpunit/extension.neon
 
 parameters:
+	resultCachePath: tmp/resultCache.php
+
 	excludePaths:
 		- tests/*/data/*

phpunit.xml

@@ -10,7 +10,8 @@
          beStrictAboutTodoAnnotatedTests="true"
          failOnRisky="true"
          failOnWarning="true"
-         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xml"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         executionOrder="random"
 >
     <coverage>
         <include>