code: protected function checkConstantExpression(Twig_NodeInterface $node)
      {
          if (!($node instanceof Twig_Node_Expression_Constant || $node instanceof Twig_Node_Expression_Array
              || $node instanceof Twig_Node_Expression_Unary_Neg || $node instanceof Twig_Node_Expression_Unary_Pos
          )) {
              return false;
          }
  
          foreach ($node as $n) {
              if (!$this->checkConstantExpression($n)) {
                  return false;
              }
          }
  
          return true;
      }
docs: checks that the node only contains "constant" elements