class LoginType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $callback = function ($object, ExecutionContextInterface $context, $payload) {
            dump($context->getObject()->getName(), $context->getRoot()->getName());
        };

        $builder
            ->add('username', TextType::class, [
                'constraints' => new NotBlank(),
            ])
            ->add('form2', LoginType2::class, [
                'constraints' => [
                    new Callback($callback)
                ]
            ]);
        ;
    }
}

LoginType.php on line 19:
"login"
LoginType.php on line 19:
"login"

LoginType.php on line 19:
"form2"
LoginType.php on line 19:
"login"