WP_REST_Menus_Controller::get_menu_locations( int $menu_id )

	protected function get_menu_locations( $menu_id ) {
		$locations      = get_nav_menu_locations();
		$menu_locations = array();

		foreach ( $locations as $location => $assigned_menu_id ) {
			if ( $menu_id === $assigned_menu_id ) {
				$menu_locations[] = $location;
			}
		}

		return $menu_locations;
	}