core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php  | 8 ++++++--
 core/lib/Drupal/Core/Render/RenderCache.php                       | 4 ++--
 core/modules/smart_cache/smart_cache.module                       | 2 --
 .../smart_cache/src/EventSubscriber/SmartCacheSubscriber.php      | 2 +-
 4 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php b/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
index 660b0b6..301f8c2 100644
--- a/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
+++ b/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
@@ -134,10 +134,10 @@ public function processAttachments(AttachmentsInterface $response) {
   }
 
   /**
-   * Renders placeholders (#attached[placeholders]).
+   * Renders placeholders (#attached['placeholders']).
    *
    * @param \Drupal\Core\Render\HtmlResponse $response
-   *   The HTML response whose placeholders to replace.
+   *   The HTML response whose placeholders are being replaced.
    *
    * @see \Drupal\Core\Render\Renderer::replacePlaceholders()
    * @see \Drupal\Core\Render\Renderer::renderPlaceholder()
@@ -148,6 +148,10 @@ protected function renderPlaceholders(HtmlResponse $response) {
       '#markup' => SafeString::create($response->getContent()),
       '#attached' => $response->getAttachments(),
     ];
+    // RendererInterface::renderRoot() renders the $build render array, it
+    // updates $build by reference. We don't care about the return value (which
+    // is just $build['#markup']), but about the resulting render array.
+    // @todo Simplify this when https://www.drupal.org/node/2495001 lands.
     $this->renderer->renderRoot($build);
 
     // Update the Response object now that the placeholders have been rendered.
diff --git a/core/lib/Drupal/Core/Render/RenderCache.php b/core/lib/Drupal/Core/Render/RenderCache.php
index 6143c3c..37be19d 100644
--- a/core/lib/Drupal/Core/Render/RenderCache.php
+++ b/core/lib/Drupal/Core/Render/RenderCache.php
@@ -17,8 +17,8 @@
 /**
  * Wraps the caching logic for the render caching system.
  *
- * @todo Refactor this out into a separate service that is capable of cache
- *   redirects in a non-render array-specific way.
+ * @todo Refactor this out into a generic service capable of cache redirects,
+ *   let RenderCache use that. https://www.drupal.org/node/2551419
  */
 class RenderCache implements RenderCacheInterface {
 
diff --git a/core/modules/smart_cache/smart_cache.module b/core/modules/smart_cache/smart_cache.module
index cbd0f44..4c0a9e6 100644
--- a/core/modules/smart_cache/smart_cache.module
+++ b/core/modules/smart_cache/smart_cache.module
@@ -5,8 +5,6 @@
  * Caches HTML responses, request and response policies allowing.
  */
 
-use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\PageCache\RequestPolicyInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 
 /**
diff --git a/core/modules/smart_cache/src/EventSubscriber/SmartCacheSubscriber.php b/core/modules/smart_cache/src/EventSubscriber/SmartCacheSubscriber.php
index 4edc76f..2308f96 100644
--- a/core/modules/smart_cache/src/EventSubscriber/SmartCacheSubscriber.php
+++ b/core/modules/smart_cache/src/EventSubscriber/SmartCacheSubscriber.php
@@ -19,7 +19,7 @@
 use Symfony\Component\HttpKernel\KernelEvents;
 
 /**
- * Returns cached responses, as early and avoiding as much work as possible.
+ * Returns cached responses as early and avoiding as much work as possible.
  *
  * SmartCache is able to cache so much because it exploits cache contexts: the
  * cache contexts that are present capture the variations of every component of