diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index 31c10a7..122bc6d 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;
@@ -387,7 +388,7 @@ public function render($hook, array $variables) {
       $output = $render_function($template_file, $variables);
     }
 
-    return (string) $output;
+    return ($output instanceof SafeString) ? $output : (string) $output;
   }
 
   /**
diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index a425f4f..0dd3ca1 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -812,7 +812,7 @@ protected function assertThemeOutput($callback, array $variables = array(), $exp
     /** @var \Drupal\Core\Render\RendererInterface $renderer */
     $renderer = \Drupal::service('renderer');
 
-    $output = $renderer->executeInRenderContext(new RenderContext(), function() use ($callback, $variables) {
+    $output = (string) $renderer->executeInRenderContext(new RenderContext(), function() use ($callback, $variables) {
       return \Drupal::theme()->render($callback, $variables);
     });
     $this->verbose(
diff --git a/core/modules/system/templates/dropbutton-wrapper.html.twig b/core/modules/system/templates/dropbutton-wrapper.html.twig
index ca0ff7e..d4c8d90 100644
--- a/core/modules/system/templates/dropbutton-wrapper.html.twig
+++ b/core/modules/system/templates/dropbutton-wrapper.html.twig
@@ -12,7 +12,7 @@
  * @ingroup themeable
  */
 #}
-{% if children %}
+{% if children|length %}
   {% spaceless %}
     <div class="dropbutton-wrapper">
       <div class="dropbutton-widget">
diff --git a/core/modules/views/src/Plugin/views/field/FieldPluginBase.php b/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
index c83b9c4..a62a862 100644
--- a/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
+++ b/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
@@ -1214,6 +1214,7 @@ public function isValueEmpty($value, $empty_zero, $no_skip_empty = TRUE) {
    */
   public function renderText($alter) {
     $value = $this->last_render;
+    $value_is_safe = SafeMarkup::isSafe($value);
 
     if (!empty($alter['alter_text']) && $alter['text'] !== '') {
       $tokens = $this->getRenderTokens($alter);
@@ -1239,6 +1240,9 @@ public function renderText($alter) {
     if ($alter['phase'] == static::RENDER_TEXT_PHASE_EMPTY && $no_rewrite_for_empty) {
       // If we got here then $alter contains the value of "No results text"
       // and so there is nothing left to do.
+      if ($value_is_safe) {
+        $value = SafeString::create($value);
+      }
       return $value;
     }
 
@@ -1275,6 +1279,11 @@ public function renderText($alter) {
     if (!empty($alter['nl2br'])) {
       $value = nl2br($value);
     }
+
+    // Preserve whether or not the string is safe.
+    if ($value_is_safe) {
+      $value = SafeString::create($value . $suffix);
+    }
     $this->last_render_text = $value;
 
     if (!empty($alter['make_link']) && (!empty($alter['path']) || !empty($alter['url']))) {
@@ -1284,7 +1293,13 @@ public function renderText($alter) {
       $value = $this->renderAsLink($alter, $value, $tokens);
     }
 
-    return $value . $suffix;
+    // Preserve whether or not the string is safe.
+    if ($value_is_safe) {
+      return SafeString::create($value . $suffix);
+    }
+    else {
+      return $value . $suffix;
+    }
   }
 
   /**
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index af5ebe7..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($variables['theme_hook_suggestions'])) {
@@ -108,12 +109,13 @@ function twig_render_template($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";
   }
-  return SafeMarkup::set(implode('', $output));
+  // This output has already been rendered and is therefore considered safe.
+  return SafeString::create(implode('', $output));
 }
 
 /**