diff -u b/core/lib/Drupal/Core/Url.php b/core/lib/Drupal/Core/Url.php
--- b/core/lib/Drupal/Core/Url.php
+++ b/core/lib/Drupal/Core/Url.php
@@ -199,7 +199,13 @@
    * @param string $user_input
    *   User input for a link or path. The first character must be one of the
    *   following characters:
-   *   - '/': A base-relative path for the current site.
+   *   - '/': A path within the current site. This path might be to a Drupal
+   *     route (e.g., '/admin'), to a file (e.g., '/README.txt'), or to
+   *     something processed by a non-Drupal script (e.g.,
+   *     '/not/a/drupal/page'). If the path matches a Drupal route, then the
+   *     URL generation will include Drupal's path processors (e.g.,
+   *     language-prefixing and aliasing). Otherwise, the URL generation will
+   *     just append the passed-in path to Drupal's base path.
    *   - '?': A query string for the current page or resource.
    *   - '#': A fragment (jump-link) on the current page or resource.
    *   This helps reduce ambiguity for user-entered links and paths, and
@@ -221,11 +227,16 @@
    *   characters: '/', '?', or '#'.
    */
   public static function fromUserInput($user_input, $options = []) {
+    // Ensuring one of these initial characters also enforces that what is
+    // passed is a relative URI reference rather than an absolute URI,
+    // because these are URI reserved characters that a scheme name may not
+    // start with.
     if ((strpos($user_input, '/') !== 0) && (strpos($user_input, '#') !== 0) && (strpos($user_input, '?') !== 0)) {
       throw new \InvalidArgumentException(String::format("The user-entered string @user_input must begin with a '/', '?', or '#'.", ['@user_input' => $user_input]));
     }
-    // Appending the scheme below ensures that the user input is treated as a
-    // relative (not absolute) reference.
+
+    // fromUri() requires an absolute URI, so prepend the appropriate scheme
+    // name.
     return static::fromUri('user-path:' . $user_input, $options);
   }
 
diff -u b/core/modules/path/src/Controller/PathController.php b/core/modules/path/src/Controller/PathController.php
--- b/core/modules/path/src/Controller/PathController.php
+++ b/core/modules/path/src/Controller/PathController.php
@@ -86,7 +86,8 @@
     $destination = drupal_get_destination();
     foreach ($this->aliasStorage->getAliasesForAdminListing($header, $keys) as $data) {
       $row = array();
-      // @todo Should Path module store leading slashes? Needs followup.
+      // @todo Should Path module store leading slashes? See
+      //   https://www.drupal.org/node/2430593.
       $row['data']['alias'] = $this->l(Unicode::truncate($data->alias, 50, FALSE, TRUE), Url::fromUserInput('/' . $data->source, array(
         'attributes' => array('title' => $data->alias),
       )));
diff -u b/core/modules/views/src/Plugin/views/row/RssFields.php b/core/modules/views/src/Plugin/views/row/RssFields.php
--- b/core/modules/views/src/Plugin/views/row/RssFields.php
+++ b/core/modules/views/src/Plugin/views/row/RssFields.php
@@ -160,8 +160,8 @@
     $item_guid = $this->getField($row_index, $this->options['guid_field_options']['guid_field']);
     if ($this->options['guid_field_options']['guid_field_is_permalink']) {
       $guid_is_permalink_string = 'true';
-      // @todo Views should expect and store a leading /. See:
-      //   https://www.drupal.org/node/2423913
+      // @todo Enforce GUIDs as system-generated rather than user input? See
+      //   https://www.drupal.org/node/2430589.
       $item_guid = Url::fromUserInput('/' . $item_guid)->setAbsolute()->toString();
     }
     $item->elements[] = array(
diff -u b/core/modules/views/views.theme.inc b/core/modules/views/views.theme.inc
--- b/core/modules/views/views.theme.inc
+++ b/core/modules/views/views.theme.inc
@@ -880,8 +880,8 @@
     // Compare the link to the default home page; if it's the default home page,
     // just use $base_url.
     $url_string = $url->setOptions($url_options)->toString();
-    // @todo Views should expect and store a leading /. See:
-    //   https://www.drupal.org/node/2423913
+    // @todo Should page.front be stored with a leading slash? See
+    //   https://www.drupal.org/node/2430595.
     if ($url_string === Url::fromUserInput('/' . $config->get('page.front'))->toString()) {
       $url_string = Url::fromRoute('<front>')->setAbsolute()->toString();
     }