Add the = assignment operator

src/ExpressionParser/Infix/AssignmentExpressionParser.php

@@ -0,0 +1,55 @@
+<?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\ExpressionParser\Infix;
+
+use Twig\Error\SyntaxError;
+use Twig\ExpressionParser\InfixAssociativity;
+use Twig\Node\Expression\AbstractExpression;
+use Twig\Node\Expression\Binary\AbstractBinary;
+use Twig\Node\Expression\Binary\SetBinary;
+use Twig\Node\Expression\Variable\ContextVariable;
+use Twig\Parser;
+use Twig\Token;
+
+/**
+ * @internal
+ */
+class AssignmentExpressionParser extends BinaryOperatorExpressionParser
+{
+    public function __construct(
+        string $name,
+    ) {
+        parent::__construct(SetBinary::class, $name, 0, InfixAssociativity::Right);
+    }
+
+    /**
+     * @return AbstractBinary
+     */
+    public function parse(Parser $parser, AbstractExpression $left, Token $token): AbstractExpression
+    {
+        if (!$left instanceof ContextVariable) {
+            throw new SyntaxError(\sprintf('Cannot assign to "%s", only variables can be assigned.', $left::class), $token->getLine(), $parser->getStream()->getSourceContext());
+        }
+        $right = $parser->parseExpression(InfixAssociativity::Left === $this->getAssociativity() ? $this->getPrecedence() + 1 : $this->getPrecedence());
+        $right = match ($this->getName()) {
+            '=' => $right,
+            default => throw new \LogicException(\sprintf('Unknown operator: %s.', $this->getName())),
+        };
+
+        return new SetBinary($left, $right, $token->getLine());
+    }
+
+    public function getDescription(): string
+    {
+        return 'Assignment operator';
+    }
+}