In other words, what is the Drupal 8 equivalent of hook_menu_alter()?

Drupal 8 still uses hook_menu(), but for what I can see, the information returned by the hook is different from what the hook returned in Drupal 7. For example, the definition given in user_menu() for user is the following.

  $items['user'] = array(     'title' => 'User account',     'title callback' => 'user_menu_title',     'weight' => -10,     'route_name' => 'user_page',     'menu_name' => 'account',   ); 

The route_name property links to an entry in the user.routing.yml file.

user_page:   pattern: '/user'   defaults:     _content: 'DrupaluserControllerUserController::userPage'   requirements:     _access: 'TRUE' 

This is different from what done with Symphony, and it confuses me about how a module can alter the route defined from another user.

The only function that is still invoking hook_menu_alter() is menu_router_build(), but that function still contains code that needs to be updated, since it is still using the now deprecated drupal_alter().

  // Alter the menu as defined in modules, keys are like user/%user.   drupal_alter('menu', $callbacks);   foreach ($callbacks as $path => $router_item) {     // If the menu item is a default local task and incorrectly references a     // route, remove it.     // @todo This may be removed later depending on the outcome of     // http://drupal.org/node/1889790     if (isset($router_item['type']) && $router_item['type'] == MENU_DEFAULT_LOCAL_TASK) {       unset($callbacks[$path]['route_name']);     }     // If the menu item references a route, normalize the route information     // into the old structure. Note that routes are keyed by name, not path,     // so the path of the route takes precedence.     if (isset($router_item['route_name'])) {       $router_item['page callback'] = 'USES_ROUTE';       $router_item['access callback'] = TRUE;       $new_path = _menu_router_translate_route($router_item['route_name']);        unset($callbacks[$path]);       $callbacks[$new_path] = $router_item;     }   }