@yoichitgy I would appreciate a review if you have time!

Looks good to me! Would we also need to add let synchronized: Bool to Assembler as well? Or would we do something like this:

myAssembler.resolver.synchronize()

I don't think it's needed on the Assembler because the documentation suggests not to synchronize until you've finished registering

/// Returns a synchronized view of the container for thread safety.
/// The returned container is ``Resolver`` type. Call this method after you finish all service registrations
/// to the original container.
///
/// - Returns: A synchronized container as ``Resolver``.

So I think what you've suggested with myAssembler.resolver.synchronize() would be the correct approach - unless there is a goal to synchronize assembly as well.