Constructive description here

vendor/psy/psysh/src/Psy/CodeCleaner/AssignThisVariablePass.php

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+<?php
+
+/*
+ * This file is part of Psy Shell.
+ *
+ * (c) 2012-2017 Justin Hileman
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Psy\CodeCleaner;
+
+use PhpParser\Node;
+use PhpParser\Node\Expr\Assign;
+use PhpParser\Node\Expr\Variable;
+use Psy\Exception\FatalErrorException;
+
+/**
+ * Validate that the user input does not assign the `$this` variable.
+ *
+ * @author Martin Hasoň <martin.hason@gmail.com>
+ */
+class AssignThisVariablePass extends CodeCleanerPass
+{
+    /**
+     * Validate that the user input does not assign the `$this` variable.
+     *
+     * @throws RuntimeException if the user assign the `$this` variable
+     *
+     * @param Node $node
+     */
+    public function enterNode(Node $node)
+    {
+        if ($node instanceof Assign && $node->var instanceof Variable && $node->var->name === 'this') {
+            throw new FatalErrorException('Cannot re-assign $this');
+        }
+    }
+}