core/lib/Drupal/Core/Render/Element/StatusMessages.php |  4 +---
 core/lib/Drupal/Core/Render/Renderer.php               | 11 +++++++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/core/lib/Drupal/Core/Render/Element/StatusMessages.php b/core/lib/Drupal/Core/Render/Element/StatusMessages.php
index c2fa305..b845c91 100644
--- a/core/lib/Drupal/Core/Render/Element/StatusMessages.php
+++ b/core/lib/Drupal/Core/Render/Element/StatusMessages.php
@@ -46,8 +46,6 @@ public function getInfo() {
    *   The updated renderable array containing the placeholder.
    */
   public static function generatePlaceholder(array $element) {
-    $plugin_id = 'status_messages';
-
     $callback = get_class() . '::renderMessages';
     $context = [
       'display' => $element['#display'],
@@ -57,7 +55,7 @@ public static function generatePlaceholder(array $element) {
         $callback => $context,
       ],
       '#create_placeholder' => TRUE,
-      // Make it uncacheable.
+      // Messsages are not cacheable.
       '#cache' => [
         'max-age' => 0,
       ],
diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index 184f06a..f2e19c9 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -126,6 +126,7 @@ public function renderPlaceholder($placeholder, array $elements) {
 
     // Replace the placeholder with its rendered markup, and merge its
     // bubbleable metadata with the main element's.
+    // @todo this cannot happen here, because BigPipe cannot do this.
     $elements['#markup'] = str_replace($placeholder, $markup, $elements['#markup']);
     $elements = $this->mergeBubbleableMetadata($elements, $placeholder_elements);
 
@@ -555,6 +556,16 @@ protected function createPlaceholder(array $element) {
       '#cache' => TRUE,
     ]);
 
+ /*   // @todo This at least makes development of this patch easier, but I'm not
+    //   sure whether we can keep this; it'd too easily prevent us from doing
+    //   "auto-placeholdering"?
+    // @todo Convert to an assertion once https://www.drupal.org/node/2408013
+    //   lands; having more properties is harmless but also useless.
+    $unsupported_keys = array_diff(array_keys($elements), ['#pre_render_cache', '#cache', '#create_placeholder']);
+    if (count($unsupported_keys)) {
+      throw new \Exception(implode($unsupported_keys));
+    }*/
+
     // Generate placeholder markup.
     $attributes = new Attribute();
     $i = 0;