diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index e04631a..b02af34 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Theme;
 
+use Drupal\Core\Render\SafeString;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Routing\StackedRouteMatchInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
@@ -396,7 +397,7 @@ protected function theme($hook, $variables = array()) {
       $output = $render_function($template_file, $variables);
     }
 
-    return ($output instanceof \Twig_Markup) ? $output : (string) $output;
+    return ($output instanceof SafeString) ? $output : (string) $output;
   }
 
   /**
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 35cfe25..7f8e6eb 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -6,6 +6,7 @@
  */
 
 use Drupal\Component\Utility\SafeMarkup;
+use Drupal\Core\Render\SafeString;
 use Drupal\Core\Extension\Extension;
 
 /**
@@ -74,7 +75,7 @@ function twig_render_template($template_file, array $variables) {
   }
   if ($twig_service->isDebug()) {
     $output['debug_prefix'] .= "\n\n<!-- THEME DEBUG -->";
-    $output['debug_prefix'] .= "\n<!-- THEME HOOK: '" . SafeMarkup::checkPlain($variables['theme_hook_original']) . "' -->";
+    $output['debug_prefix'] .= "\n<!-- THEME HOOK: '" . htmlspecialchars($variables['theme_hook_original'], ENT_QUOTES, 'UTF-8') . "' -->";
     // If there are theme suggestions, reverse the array so more specific
     // suggestions are shown first.
     if (!empty($variables['theme_hook_suggestions'])) {
@@ -108,13 +109,13 @@ function twig_render_template($template_file, array $variables) {
         $prefix = ($template == $current_template) ? 'x' : '*';
         $suggestion = $prefix . ' ' . $template;
       }
-      $output['debug_info'] .= "\n<!-- FILE NAME SUGGESTIONS:\n   " . SafeMarkup::checkPlain(implode("\n   ", $suggestions)) . "\n-->";
+      $output['debug_info'] .= "\n<!-- FILE NAME SUGGESTIONS:\n   " . htmlspecialchars(implode("\n   ", $suggestions), ENT_QUOTES, 'UTF-8') . "\n-->";
     }
-    $output['debug_info']   .= "\n<!-- BEGIN OUTPUT from '" . SafeMarkup::checkPlain($template_file) . "' -->\n";
-    $output['debug_suffix'] .= "\n<!-- END OUTPUT from '" . SafeMarkup::checkPlain($template_file) . "' -->\n\n";
+    $output['debug_info']   .= "\n<!-- BEGIN OUTPUT from '" . htmlspecialchars($template_file, ENT_QUOTES, 'UTF-8') . "' -->\n";
+    $output['debug_suffix'] .= "\n<!-- END OUTPUT from '" . htmlspecialchars($template_file, ENT_QUOTES, 'UTF-8') . "' -->\n\n";
   }
   // This output has already been rendered and is therefore considered safe.
-  return new \Twig_Markup(implode('', $output), 'UTF-8');
+  return SafeString::create(implode('', $output));
 }
 
 /**