Add \Drupal\Core\Path\PathInterface

This looks perfect. People would argue that we need a change record.

+1

Next best step would be to rename both the class + interface to some more appropriate and less confusing name (in a separate issue).

PathHandler, PathManager, PathRepository, or similar. — Anything that doesn't sound like the domain object itself (a "Path") :-)

1. The AliasManager needs to be updated to typehint on PathInterface
2. path.api.php should point to \Drupal\Core\Path\PathInterface::method() instead of \Drupal\Core\Path\Path::method()

+++ b/core/lib/Drupal/Core/Path/PathInterface.php
@@ -0,0 +1,101 @@
+  public function save($source, $alias, $langcode = Language::LANGCODE_NOT_SPECIFIED, $pid = NULL);

There is no use Drupal\Core\Language\Language.

The patch in #7 appears not to be affected by #2209145: Move all path alias SQL queries to a single storage controller.

Documentation clean-up.

Great!

+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/destination/UrlAlias.php
@@ -38,7 +38,7 @@ class UrlAlias extends DestinationBase implements ContainerFactoryPluginInterfac
-   * @param \Drupal\Core\Path\Path $path
+   * @param \Drupal\Core\Path\PathInterface $path
    *   The path crud service.
    */
   public function __construct(array $configuration, $plugin_id, array $plugin_definition, MigrationInterface $migration, Path $path) {

Need to update the tyephint too

Back to RTBC

Committed 1a51606 and pushed to 8.x. Thanks!

Can I request small followup in the same issue?

I have not looked at the code in this patch closely, but while rerolling something against this, I spotted the following comment...

    *
    * This function may be used build the data for a menu tree only, for example
    * to further massage the data manually before further processing happens.
-   * menu_tree_check_access() needs to be invoked afterwards.
+   * $this->checkAccess() needs to be invoked afterwards.
    *
    * @param string $menu_name
    *   The name of the menu.

Well-spotted! However, that code is not related to this issue, so please open another one under the documentation component, so jhodgdon, the documentation maintainer, will find it and commit it to core.