It actually isn't :).

The Messenger component is usable without the Serializer one, it's a soft dependency only. See #26649 which fixed an error when the serializer wasn't installed, should be all good now.

Though, we should work at making the error message clearer. I've already discussed this and I believe the best way is to have the serializer argument of the messenger.transport.serialize_message_with_type_in_headers as if-invalid="null" and throw an exception within the constructor of Symfony\Component\Messenger\Transport\Serialization\Serializer if $serializer is null. That way, we throw an exception if that default transport is used.

Thanks for the PR but as per our comments, I'm going to close this one.

ah, I see. I was getting this though:-

In CheckExceptionOnInvalidReferenceBehaviorPass.php line 32:

  [Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException]   
  The service "messenger.adapter.amqp.factory" has a dependency on a non-exis  
  tent service "messenger.transport.default_encoder".

It will be more explicit with #26908 :)