diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -581,12 +581,6 @@
  * equivalent using other environment variables.
  *
  * @todo The above comment is incorrect: http://drupal.org/node/1547294.
- *
- * NB This function is intended for limited use, in cases before the $container
- * is available. Use $request->getRequestUri() whereever possible.
- *
- * @return string
- *   The request URI
  */
 function request_uri($omit_query_string = FALSE) {
   if (isset($_SERVER['REQUEST_URI'])) {
@@ -604,6 +598,12 @@
  * equivalent using other environment variables.
  *
  * @todo The above comment is incorrect: http://drupal.org/node/1547294.
+ *
+ * NB This function is intended for limited use, in cases before the $container
+ * is available. Use $request->getRequestUri() whereever possible.
+ *
+ * @return string
+ *   The request URI
  */
 function request_uri($omit_query_string = FALSE) {
   if (isset($_SERVER['REQUEST_URI'])) {
diff -u b/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
--- b/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -301,7 +301,7 @@
       $form['#build_id'] = 'form-' . Crypt::randomBytesBase64();
     }
 
-    // #action defaults to request_uri(), but in case of Ajax and other partial
+    // #action defaults to $request->getRequestUri(), but in case of Ajax and other partial
     // rebuilds, the form is submitted to an alternate URL, and the original
     // #action needs to be retained.
     if (isset($old_form['#action']) && !empty($rebuild_info['copy']['#action'])) {