<?php

use Drupal\Core\Template\TwigReferenceFunction;
use Drupal\Core\Template\TwigTemplate;
use Drupal\Core\Template\TwigNodeExpressionGetAttrReference;
use Drupal\Core\Template\TwigNodeExpressionNameReferenceEnd;
use Drupal\Core\Template\TwigNodeExpressionNameReference;
use Drupal\Core\Template\TwigReferenceNodeVisitor;
use Drupal\Core\Template\TwigReference;

function twig_change($val, &$context, $template = NULL) {
    // Unwrap reference if twig_reference key is set
    if (isset($val['#twig_reference']) && isset($template) && $val['#twig_reference'] === TRUE) {
        $val = &$template->unwrapReference($context, $val);
    }
    // Add some key to verify the value could be modified
    if (is_array($val)) {
        $val['#printed'] = TRUE;
    }
    var_dump($val);
    return '';
}
$twig = Drupal::service('twig');
//$loader = new Twig_Loader_String();
//$twig = new Twig_Environment($loader, array(
//  'debug' => TRUE,
//  'strict_variables' => FALSE,
//  'autoescape' => FALSE,
//  'cache' => false,
//  'base_template_class' => 'Drupal\Core\Template\TwigTemplate',
//));

$twig->addFunction('change', new TwigReferenceFunction('twig_change'));
$twig->addFunction('change_val', new Twig_Function_Function('twig_change'));

//$twig->addNodeVisitor(new TwigReferenceNodeVisitor());
//$twig->addExtension(new Twig_Extension_Debug());

$vars = array('x' => array('foo'), 'y' => array(
    '#markup' => 'bar',
    'bar1' => array(
        '#markup' => 'bar1',
    ),
    'bar2' => array(
        '#markup' => 'bar2',
    )

));
$name = 'test-twig-reference-node-visitor.html.twig';

//print $twig->parse($twig->tokenize($code));
//print $twig->loadTemplate($name)->render(array('vars' => $vars));
print twig_render_template($name, array('vars' => $vars));