PHP Attributes can be used for Form route definition and discovery

Using attributes for route definition and discovery, first introduced in PHP Attributes can be used for route definition and discovery, has been extended to routes to forms. Any classes implementing Drupal\Core\Form\FormInterface in a module's Form namespace (for example, Drupal\example\Form) and having the Symfony\Component\Routing\Attribute\Route attribute will be picked up as route definitions. The attribute must be applied to the class; any Route attributes applied to a form object's methods will be ignored.

Note that entity form routes should still be defined in a route provider specified by an entity type definition. The Route attribute discovery replaces andor/or supplements the existing .routing.yml based declarations.

Example using .yml file

system.site_information_settings:
  path: '/admin/config/system/site-information'
  defaults:
    _form: '\Drupal\system\Form\SiteInformationForm'
    _title: 'Basic site settings'
  requirements:
    _permission: 'administer site configuration'

Example using a route attribute

<?php

namespace Drupal\system\Form;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Config\TypedConfigManagerInterface;
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Form\RedundantEditableConfigNamesTrait;
use Drupal\Core\Path\PathValidatorInterface;
use Drupal\Core\Routing\RequestContext;
use Drupal\Core\StringTranslation\TranslatableMarkup;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\Routing\Attribute\Route;

/**
 * Configure site information settings for this site.
 *
 * @internal
 */
#[Route(
  path: '/admin/config/system/site-information',
  name: 'system.site_information_settings',
  requirements: [
    '_permission' => 'administer site configuration',
  ],
  defaults: ['_title' => new TranslatableMarkup('Basic site settings')],
)]
class SiteInformationForm extends ConfigFormBase {