core/lib/Drupal/Core/Render/Renderer.php     | 23 ++++++----------
 core/modules/big_pipe/src/Render/BigPipe.php | 39 +++++++++++++++-------------
 2 files changed, 29 insertions(+), 33 deletions(-)

diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index a624c0f..14d9713 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -665,25 +665,18 @@ protected function replacePlaceholders(array &$elements) {
     // to this one special case, with this hard-coded solution.
     // @see \Drupal\Core\Render\Element\StatusMessages
     // @see https://www.drupal.org/node/2712935#comment-11368923
-    $possible_message_placeholders = [
-      '<drupal-render-placeholder callback="Drupal\Core\Render\Element\StatusMessages::renderMessages" arguments="0" token="a8c34b5e"></drupal-render-placeholder>',
-      '<drupal-render-placeholder callback="Drupal\Core\Render\Element\StatusMessages::renderMessages" arguments="0=status" token="a26f536e"></drupal-render-placeholder>',
-      '<drupal-render-placeholder callback="Drupal\Core\Render\Element\StatusMessages::renderMessages" arguments="0=warning" token="67320bbd"></drupal-render-placeholder>',
-      '<drupal-render-placeholder callback="Drupal\Core\Render\Element\StatusMessages::renderMessages" arguments="0=error" token="35b549cb"></drupal-render-placeholder>',
-    ];
-
-    // We hardcoded the placeholders above to avoid doing this on every request,
-    // but we ensure those hardcoded strings are accurate via assertions.
-    assert('$possible_message_placeholders[0] === (string) $this->placeholderGenerator->createPlaceholder(["#lazy_builder" => ["Drupal\\Core\\Render\\Element\\StatusMessages::renderMessages", [NULL]],      "#create_placeholder" => TRUE])["#markup"]');
-    assert('$possible_message_placeholders[1] === (string) $this->placeholderGenerator->createPlaceholder(["#lazy_builder" => ["Drupal\\Core\\Render\\Element\\StatusMessages::renderMessages", ["status"]],  "#create_placeholder" => TRUE])["#markup"]');
-    assert('$possible_message_placeholders[2] === (string) $this->placeholderGenerator->createPlaceholder(["#lazy_builder" => ["Drupal\\Core\\Render\\Element\\StatusMessages::renderMessages", ["warning"]], "#create_placeholder" => TRUE])["#markup"]');
-    assert('$possible_message_placeholders[3] === (string) $this->placeholderGenerator->createPlaceholder(["#lazy_builder" => ["Drupal\\Core\\Render\\Element\\StatusMessages::renderMessages", ["error"]],   "#create_placeholder" => TRUE])["#markup"]');
+    $message_placeholders = [];
+    foreach ($elements['#attached']['placeholders'] as $placeholder => $placeholder_element) {
+      if (isset($placeholder_element['#lazy_builder']) && $placeholder_element['#lazy_builder'][0] === 'Drupal\Core\Render\Element\StatusMessages::renderMessages') {
+        $message_placeholders[] = $placeholder;
+      }
+    }
 
     // Render placeholders in the appropriate order.
     $placeholders = array_keys($elements['#attached']['placeholders']);
     $ordered_placeholders = array_merge(
-      array_diff($placeholders, $possible_message_placeholders),
-      array_intersect($placeholders, $possible_message_placeholders)
+      array_diff($placeholders, $message_placeholders),
+      array_intersect($placeholders, $message_placeholders)
     );
     foreach ($ordered_placeholders as $placeholder) {
       $elements = $this->renderPlaceholder($placeholder, $elements);
diff --git a/core/modules/big_pipe/src/Render/BigPipe.php b/core/modules/big_pipe/src/Render/BigPipe.php
index 00ec4a2..a965068 100644
--- a/core/modules/big_pipe/src/Render/BigPipe.php
+++ b/core/modules/big_pipe/src/Render/BigPipe.php
@@ -128,7 +128,7 @@ public function sendContent($content, array $attachments) {
 
     list($pre_body, $post_body) = explode('</body>', $content, 2);
     $this->sendPreBody($pre_body, $nojs_placeholders, $cumulative_assets);
-    $this->sendPlaceholders($placeholders, $this->getPlaceholderOrder($pre_body), $cumulative_assets);
+    $this->sendPlaceholders($placeholders, $this->getPlaceholderOrder($pre_body, $placeholders), $cumulative_assets);
     $this->sendPostBody($post_body);
 
     // Close the session again.
@@ -528,6 +528,9 @@ protected function renderPlaceholder($placeholder, array $placeholder_render_arr
    *
    * @param string $html
    *   HTML markup.
+   * @param array $placeholders
+   *   Associative array; the BigPipe placeholders. Keys are the BigPipe
+   *   placeholder IDs.
    *
    * @return array
    *   Indexed array; the order in which the BigPipe placeholders must be sent.
@@ -535,17 +538,17 @@ protected function renderPlaceholder($placeholder, array $placeholder_render_arr
    *   placeholders are kept: if the same placeholder occurs multiple times, we
    *   only keep the first occurrence.
    */
-  protected function getPlaceholderOrder($html) {
+  protected function getPlaceholderOrder($html, $placeholders) {
     $fragments = explode('<div data-big-pipe-placeholder-id="', $html);
     array_shift($fragments);
-    $placeholders = [];
+    $placeholder_ids = [];
 
     foreach ($fragments as $fragment) {
       $t = explode('"></div>', $fragment, 2);
-      $placeholder = $t[0];
-      $placeholders[] = $placeholder;
+      $placeholder_id = $t[0];
+      $placeholder_ids[] = $placeholder_id;
     }
-    $placeholders = array_unique($placeholders);
+    $placeholder_ids = array_unique($placeholder_ids);
 
     // The 'status messages' placeholder needs to be special cased, because it
     // depends on global state that can be modified when other placeholders are
@@ -560,20 +563,20 @@ protected function getPlaceholderOrder($html) {
     // @see \Drupal\Core\Render\Element\StatusMessages
     // @see \Drupal\Core\Render\Renderer::replacePlaceholders()
     // @see https://www.drupal.org/node/2712935#comment-11368923
-    $possible_message_placeholders = [
-      'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]&amp;token=a8c34b5e',
-      'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]=status&amp;token=a26f536e',
-      'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]=warning&amp;token=67320bbd',
-      'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]=error&amp;token=35b549cb',
-    ];
+    $message_placeholder_ids = [];
+    foreach ($placeholders as $placeholder_id => $placeholder_element) {
+      if (isset($placeholder_element['#lazy_builder']) && $placeholder_element['#lazy_builder'][0] === 'Drupal\Core\Render\Element\StatusMessages::renderMessages') {
+        $message_placeholder_ids[] = $placeholder_id;
+      }
+    }
 
-    // Return placeholder IDs in DOM order, but with the 'status messages' at
-    // the end, if it exists.
-    $ordered_placeholders = array_merge(
-      array_diff($placeholders, $possible_message_placeholders),
-      array_intersect($placeholders, $possible_message_placeholders)
+    // Return placeholder IDs in DOM order, but with the 'status messages'
+    // placeholders at the end, if they are present.
+    $ordered_placeholder_ids = array_merge(
+      array_diff($placeholder_ids, $message_placeholder_ids),
+      array_intersect($placeholder_ids, $message_placeholder_ids)
     );
-    return $ordered_placeholders;
+    return $ordered_placeholder_ids;
   }
 
 }