Sniff for the rule "The use of final keyword is prohibited"

Author: lenaorobei

Magento/Sniffs/PHP/FinalImplementationSniff.php

@@ -0,0 +1,36 @@
+<?php
+/**
+ * Copyright © Magento. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Sniffs\PHP;
+
+use PHP_CodeSniffer\Sniffs\Sniff;
+use PHP_CodeSniffer\Files\File;
+
+/**
+ * Magento is a highly extensible and customizable platform. The use of final classes and methods is prohibited.
+ */
+class FinalImplementationSniff implements Sniff
+{
+    /**
+     * @inheritdoc
+     */
+    public function register()
+    {
+        return [T_FINAL];
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        $phpcsFile->addError(
+            // phpcs:ignore Generic.Files.LineLength.TooLong
+            'Final keyword is prohibited in Magento. It decreases extensibility and is not compatible with plugins and proxies.',
+            $stackPtr,
+            'FoundFinal'
+        );
+    }
+}

Magento/Tests/PHP/FinalImplementationUnitTest.inc

@@ -0,0 +1,13 @@
+<?php
+
+final class ImABadExample {}
+
+class ImAlsoABadExample {
+    final public function badFinalFunction(){}
+}
+
+class ImAGoodExample {}
+
+class ImAlsoAGoodExample {
+    private function emptyFunction(){}
+}

Magento/Tests/PHP/FinalImplementationUnitTest.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * Copyright © Magento. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Tests\PHP;
+
+use PHP_CodeSniffer\Tests\Standards\AbstractSniffUnitTest;
+
+/**
+ * Class FinalImplementationUnitTest
+ */
+class FinalImplementationUnitTest extends AbstractSniffUnitTest
+{
+    /**
+     * @inheritdoc
+     */
+    public function getErrorList()
+    {
+        return [
+            3 => 1,
+            6 => 1,
+        ];
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function getWarningList()
+    {
+        return [];
+    }
+}

Magento/ruleset.xml

@@ -104,6 +104,9 @@
     <rule ref="Magento.PHP.DiscouragedFunction">
         <severity>8</severity>
     </rule>
+    <rule ref="Magento.PHP.FinalImplementation">
+        <severity>10</severity>
+    </rule>
     <rule ref="Magento.PHP.Goto">
         <severity>10</severity>
     </rule>