Twig_Environment::getExtension PHP Method

    public function getExtension($name)
    {
        if (!isset($this->extensions[$name])) {
            throw new Twig_Error_Runtime(sprintf('The "%s" extension is not enabled.', $name));
        }

        return $this->extensions[$name];
    }

 /**
  * @param EmailTemplate                  $value
  * @param Constraint|EmailTemplateSyntax $constraint
  */
 public function validate($value, Constraint $constraint)
 {
     // prepare templates to be validated
     $itemsToValidate = [['field' => 'subject', 'locale' => null, 'template' => $value->getSubject()], ['field' => 'content', 'locale' => null, 'template' => $value->getContent()]];
     $translations = $value->getTranslations();
     foreach ($translations as $trans) {
         if (in_array($trans->getField(), ['subject', 'content'])) {
             $itemsToValidate[] = ['field' => $trans->getField(), 'locale' => $trans->getLocale(), 'template' => $trans->getContent()];
         }
     }
     /** @var \Twig_Extension_Sandbox $sandbox */
     $sandbox = $this->twig->getExtension('sandbox');
     $sandbox->enableSandbox();
     // validate templates' syntax
     $errors = [];
     foreach ($itemsToValidate as &$item) {
         try {
             $this->twig->parse($this->twig->tokenize($item['template']));
         } catch (\Twig_Error_Syntax $e) {
             $errors[] = ['field' => $item['field'], 'locale' => $item['locale'], 'error' => $e->getMessage()];
         }
     }
     $sandbox->disableSandbox();
     // add violations for found errors
     if (!empty($errors)) {
         foreach ($errors as $error) {
             $this->context->addViolation($constraint->message, ['{{ field }}' => $this->getFieldLabel(ClassUtils::getClass($value), $error['field']), '{{ locale }}' => $this->getLocaleName($error['locale']), '{{ error }}' => $error['error']]);
         }
     }
 }