根据Cerad的评论，这是完整的答案。

以下只是与问答相关的代码部分。这些不是完整的文件。

此外，这仅适用于不使用保护来验证用户的Symfony ^5.2 。

/* config/packages/security.yaml */

security:
    enable_authenticator_manager: true
    firewalls:
        main:
            custom_authenticators:
                - AppSecuritySecurityAuthenticator

/* src/Security/SecurityAuthenticator.php */

use SymfonyComponentSecurityHttpAuthenticatorAbstractLoginFormAuthenticator;

/* automatically generated with the make:auth command,
     the important part is to undestand that this is not a Guard implement 
     for the Authenticator class */
class SecurityAuthenticator extends AbstractLoginFormAuthenticator
{
  
}

/* src/Controller/RegistrationController.php */

use AppEntityUser;
use AppFormRegistrationFormType;
use AppSecuritySecurityAuthenticator;

use SymfonyBundleFrameworkBundleControllerAbstractController;
use SymfonyComponentHttpFoundationRequest;
use SymfonyComponentHttpFoundationResponse;
use SymfonyComponentRoutingAnnotationRoute;
use SymfonyComponentSecurityCoreEncoderUserPasswordEncoderInterface;
use SymfonyComponentSecurityHttpAuthenticationUserAuthenticatorInterface;

class RegistrationController extends AbstractController
{

    /**
     * @Route("/register", name="app_register")
     */
    public function register(
        Request $request, 
        UserPasswordEncoderInterface $passwordEncoder, 
        UserAuthenticatorInterface $authenticator, 
        SecurityAuthenticator $formAuthenticator): Response
    {
      /* Automatically generated by make:registration-form, but some changes are
         needed, like the auto-wiring of the UserAuthenticatorInterface and 
         SecurityAuthenticator */
        $user = new User();
        $form = $this->createForm(RegistrationFormType::class, $user);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            // encode the plain password
            $user->setPassword($passwordEncoder->encodePassword($user, $form->get('password')->getData()));

            $entityManager = $this->getDoctrine()->getManager();
            $entityManager->persist($user);
            $entityManager->flush();

            // substitute the previous line (redirect response) with this one.
            return $authenticator->authenticateUser(
                $user, 
                $formAuthenticator, 
                $request); 
        }

        return $this->render('registration/register.html.twig', [
            'registrationForm' => $form->createView(),
        ]);
    }
}

• Good answer. I was wondering how to get the user authenticator for the current firewall. Never occurred to me to just typehint against it. The user authenticator is actually a security bundle class which determines the current firewall based on the master request. Good stuff to know.