renderer.diff

diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index 48b3bcd..825a0ff 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Render;
 
+use Drupal\Component\Serialization\Json;
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Utility\UrlHelper;
@@ -395,6 +396,9 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     $elements['#cache']['max-age'] = isset($elements['#cache']['max-age']) ? $elements['#cache']['max-age'] : Cache::PERMANENT;
     $elements['#attached'] = isset($elements['#attached']) ? $elements['#attached'] : array();
 
+    // Save cache state before render.
+    $elements['#debug']['#cache_start'] = $elements['#cache'];
+
     // Allow #pre_render to abort rendering.
     if (!empty($elements['#printed'])) {
       // The #printed element contains all the bubbleable rendering metadata for
@@ -519,7 +523,8 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     $prefix = isset($elements['#prefix']) ? $this->xssFilterAdminIfUnsafe($elements['#prefix']) : '';
     $suffix = isset($elements['#suffix']) ? $this->xssFilterAdminIfUnsafe($elements['#suffix']) : '';
 
-    $elements['#markup'] = SafeString::create($prefix . $elements['#children'] . $suffix);
+    // Safe string created latter for debug.
+    $elements['#markup'] = $prefix . $elements['#children'] . $suffix;
 
     // We've rendered this element (and its subtree!), now update the context.
     $context->update($elements);
@@ -553,8 +558,23 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     // Rendering is finished, all necessary info collected!
     $context->bubble();
 
+    // Add debug output.
+    $interesting_keys = ['keys', 'contexts', 'tags', 'max-age'];
+    $prefix = '';
+    if (array_intersect(array_keys($elements['#debug']['#cache_start']), $interesting_keys)) {
+      $prefix .= '<!--' . Json::encode($elements['#debug']['#cache_start']) .  '-->';
+    }
+    if (array_intersect(array_keys($elements['#cache']), $interesting_keys)) {
+      $prefix .= '<!--' . Json::encode($elements['#cache']) .  '-->';
+    }
+    if ($prefix) {
+      $prefix = '<!--RENDERER_START-->' . $prefix;
+      $suffix = '<!--RENDERER_END-->';
+      $elements['#markup'] = $prefix . $elements['#markup'] . $suffix;
+    }
+
     $elements['#printed'] = TRUE;
-    return $elements['#markup'];
+    return SafeString::create($elements['#markup']);
   }
 
   /**