What exactly is the use case for this? If the passed surface already has a role, this will be a protocol error, and a surface cannot be its own parent anyway, or else you get a protocol error. https://wayland.app/protocols/wayland#wl_subcompositor:request:get_subsurface

What exactly is the use case for this?

For me, I have existing code that takes a surface and stores it until I determine if it is going to be a xdg-toplevel or xdg-popup. Once I've determined what role it is going to be I pass the surface xdg-shell::create_window or Popup::from_surface respectively. I'd like to like to use similar logic to also create subsurfaces.

If the passed surface already has a role, this will be a protocol error,

I know that my surfaces won't have a role yet. I'd be happy to add a check for that, but I don't think we have enough information here to know.

and a surface cannot be its own parent anyway, or else you get a protocol error. https://wayland.app/protocols/wayland#wl_subcompositor:request:get_subsurface

I feel like I could just as easily use create_subsurface to generate a protocol error if I'd like to, I'm not sure that it's different here.

I know that my surfaces won't have a role yet. I'd be happy to add a check for that, but I don't think we have enough information here to know.

Ok, gotcha.

I think the function might need a better name because currently it's a bit hard to tell without checking the implementation if it is creating a subsurface that is a child of the passed surface, or if it is creating a subsurface from the passed surface. Maybe subsurface_from_surface?

It could possibly return an Option so that it doesn't have to unwrap on the parent surface.

Thanks, I wasn't too sure what a good name for it would be. Done.

Sorry for the lint error