diff -u b/core/lib/Drupal/Component/Utility/ToStringTrait.php b/core/lib/Drupal/Component/Utility/ToStringTrait.php
--- b/core/lib/Drupal/Component/Utility/ToStringTrait.php
+++ b/core/lib/Drupal/Component/Utility/ToStringTrait.php
@@ -39,6 +39,9 @@
 
   /**
    * Renders the object as a string.
+   *
+   * @return string|object
+   *   The rendered string or an object implementing __toString().
    */
   abstract public function render();