Stop the escaping safe analysis from retaining every analyzed node

tests/NodeVisitor/SafeAnalysisTest.php

@@ -0,0 +1,51 @@
+<?php
+
+/*
+ * This file is part of Twig.
+ *
+ * (c) Fabien Potencier
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Twig\Tests\NodeVisitor;
+
+use PHPUnit\Framework\TestCase;
+use Twig\Environment;
+use Twig\Loader\ArrayLoader;
+use Twig\Node\Node;
+use Twig\Source;
+
+class SafeAnalysisTest extends TestCase
+{
+    public function testCompilingDoesNotRetainTheAst()
+    {
+        $env = new Environment(new ArrayLoader(), ['cache' => false, 'autoescape' => 'html']);
+        $source = new Source('{{ "a"|upper }}{{ foo.bar }}{{ [1, 2]|join("-") }}{{ b ? "y" : "n" }}', 'index');
+
+        $ast = $env->parse($env->tokenize($source));
+
+        $refs = [];
+        $collect = static function (Node $node) use (&$collect, &$refs) {
+            $refs[] = \WeakReference::create($node);
+            foreach ($node as $child) {
+                $collect($child);
+            }
+        };
+        $collect($ast);
+        $this->assertNotEmpty($refs);
+
+        unset($ast, $collect);
+        gc_collect_cycles();
+
+        $retained = 0;
+        foreach ($refs as $ref) {
+            if (null !== $ref->get()) {
+                ++$retained;
+            }
+        }
+
+        $this->assertSame(0, $retained, \sprintf('%d of %d AST nodes are still referenced after compilation.', $retained, \count($refs)));
+    }
+}