Symfony認証でパスワード無しでログインする方法

security:
    providers:
        user_db_provider:
            entity:
                class: AppBundle:User\User
                property: mail
    firewalls:
        admin:
            pattern: ^/admin/
            # ... 省略
        user:
            context: user    # <- これがログインするコンテクスト
            pattern: ^/user/
            anonymous: ~
            guard:
                authenticators:
                    - app.tanto_authenticator
    access_control:
        - { path: ^/user/login, roles: IS_AUTHENTICATED_ANONYMOUSLY }
        - { path: ^/user/, roles: ROLE_USER}

    /**
     * @Config\Route("/admin/user/{user_id}/login", name="admin-user-login")
     * @param int     $user_id
     * @return Response
     */
    public function loginAction($user_id)
    {
        $user = $this->getDoctrine()->getManager()
             ->getRepository(User::class)->find($user_id);
        if (!$user) {
            return $this->redirectToUserList('ユーザーを読み込めません');
        }
        $this->loginToContext($user, 'user');
        return $this->redirectToRoute('user-top'); // ユーザー画面にリダイレクト
    }
    /**
     * @param UserInterface $user
     * @param string        $context
     * @param array         $roles
     */
    private function loginToContext(UserInterface $user, $context, $roles = [])
    {
        $roles   = $roles ?: $user->getRoles();
        $token   = new UsernamePasswordToken($user, null, $context, $roles);
        $session = $this->get('session');
        $session->set('_security_'.$context, serialize($token));
        $session->save();
    }

    /**
     * @param UserInterface $user
     * @param string        $context
     * @param array         $roles
     */
    private function updateContext(UserInterface $user, $context, $roles = [])
    {
        $roles   = $roles ?: $user->getRoles();
        $token   = new UsernamePasswordToken($user, null, $context, $roles);
        $storage = $this->get('security.token_storage');
        $storage->getToken()->setAuthenticated(false);
        $storage->setToken($token);
    }

security:
    firewalls:
        user:    # コンテキストがないので、この名前が使われる。
            pattern: ^/user/
            anonymous: ~
            guard:
                authenticators:
                    - app.tanto_authenticator

    $storage = $this->get('security.token_storage');
    $context = $storage->getToken()->getProviderKey();