Drupal 8: how to create a sub-request

SubRequestController.php

<?php

/**
 * @file
 *   Contains Drupal\dmouse\Controller\SubRequestController.
 * Generated by drupal/console.
 */

namespace Drupal\dmouse\Controller;

use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpKernel\HttpKernelInterface;
use Symfony\Component\HttpFoundation\Request;


class SubRequestController extends ControllerBase implements ContainerInjectionInterface {

  /**
   * Symfony\Component\HttpKernel\HttpKernelInterface definition.
   *
   * @var Symfony\Component\HttpKernel\HttpKernelInterface
   */
  protected $http_kernel;

  public function __construct(HttpKernelInterface $http_kernel) {
    $this->httpKernel = $http_kernel;
  }

  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('http_kernel')
    );
  }

  /**
   * Index.
   *
   * @return string
   *   Return Hello string.
   */
  public function index() {
    $sub_request = Request::create('/user', 'GET');

    $subResponse = $this->httpKernel->handle($sub_request, HttpKernelInterface::SUB_REQUEST);
    $html = $subResponse->getContent();

    return $subResponse;
  }
}

Thanks for sharing this, it was very helpful in working out some issues I had with my own subrequest implementation.