SpecialAttributesRouteSubscriber.php

<?php

namespace Drupal\Core\EventSubscriber;

use Drupal\Core\Routing\RouteBuildEvent;
use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Cmf\Component\Routing\RouteObjectInterface;
use Symfony\Component\Routing\RouteCollection;

/**
 * Provides a route subscriber which checks for invalid pattern variables.
 */
class SpecialAttributesRouteSubscriber extends RouteSubscriberBase {
    
    /**
     * {@inheritdoc}
     */
    protected function alterRoutes(RouteCollection $collection) {
        $special_variables = [
            'system_path',
            '_legacy',
            '_raw_variables',
            RouteObjectInterface::ROUTE_OBJECT,
            RouteObjectInterface::ROUTE_NAME,
            '_content',
            '_controller',
            '_form',
        ];
        foreach ($collection->all() as $name => $route) {
            if ($not_allowed_variables = array_intersect($route->compile()
                ->getVariables(), $special_variables)) {
                $reserved = implode(', ', $not_allowed_variables);
                trigger_error(sprintf('Route %s uses reserved variable names: %s', $name, $reserved), E_USER_WARNING);
            }
        }
    }
    
    /**
     * Delegates the route altering to self::alterRoutes().
     *
     * @param \Drupal\Core\Routing\RouteBuildEvent $event
     *   The route build event.
     */
    public function onAlterRoutes(RouteBuildEvent $event) {
        $collection = $event->getRouteCollection();
        $this->alterRoutes($collection);
    }

}