core/modules/forum/forum.module                           |  4 ++--
 .../src/EventSubscriber/ThemeTestSubscriber.php           | 15 +++++++++++++--
 .../tests/modules/theme_test/theme_test.services.yml      |  2 +-
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index eb25bf8..8ce12ba 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -61,12 +61,12 @@ function forum_help($route_name, RouteMatchInterface $route_match) {
       );
       $container = array(
         '#theme' => 'container',
-        '#children' => drupal_render($more_help_link),
+        '#children' => $more_help_link,
         '#attributes' => array(
           'class' => array('more-link'),
         ),
       );
-      $output .= drupal_render($container);
+      $output .= \Drupal::service('renderer')->renderPlain($container);
       return $output;
 
     case 'forum.add_container':
diff --git a/core/modules/system/tests/modules/theme_test/src/EventSubscriber/ThemeTestSubscriber.php b/core/modules/system/tests/modules/theme_test/src/EventSubscriber/ThemeTestSubscriber.php
index d8f609a..86a0128 100644
--- a/core/modules/system/tests/modules/theme_test/src/EventSubscriber/ThemeTestSubscriber.php
+++ b/core/modules/system/tests/modules/theme_test/src/EventSubscriber/ThemeTestSubscriber.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\theme_test\EventSubscriber;
 
+use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\Url;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Symfony\Component\HttpKernel\KernelEvents;
@@ -33,12 +34,22 @@ class ThemeTestSubscriber implements EventSubscriberInterface {
   protected $currentRouteMatch;
 
   /**
+   * The renderer.
+   *
+   * @var \Drupal\Core\Render\RendererInterface
+   */
+  protected $renderer;
+
+  /**
    * Constructs a new ThemeTestSubscriber.
    *
    * @param \Drupal\Core\Routing\RouteMatchInterface $current_route_match
+   * @param \Drupal\Core\Render\RendererInterface $renderer
+   *   The renderer.
    */
-  public function __construct(RouteMatchInterface $current_route_match) {
+  public function __construct(RouteMatchInterface $current_route_match, RendererInterface $renderer) {
     $this->currentRouteMatch = $current_route_match;
+    $this->renderer = $renderer;
   }
 
   /**
@@ -62,7 +73,7 @@ public function onRequest(GetResponseEvent $event) {
         '#url' => Url::fromRoute('user.page'),
         '#attributes' => array('title' => 'Themed output generated in a KernelEvents::REQUEST listener'),
       );
-      $GLOBALS['theme_test_output'] = drupal_render($more_link);
+      $GLOBALS['theme_test_output'] = $this->renderer->renderPlain($more_link);
     }
   }
 
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.services.yml b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
index add8b4c..482c219 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.services.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
@@ -1,7 +1,7 @@
 services:
   theme_test.subscriber:
     class: Drupal\theme_test\EventSubscriber\ThemeTestSubscriber
-    arguments: [@current_route_match]
+    arguments: [@current_route_match, @renderer]
     tags:
       - { name: event_subscriber }