Problem with callback taking non-copyable argument

[ianhbell]

I have implemented a quadtree as linked set of nodes. I wrote a callback function that is supposed to be called on a node(which has unique_ptr<> for the children, and is therefore not copy-able). Thus I cannot use the callback function because it tries to copy the node before passing to the Python function (which makes sense), but it can't do that because the unique_ptr cannot be copied. I think we somehow need a similar semantics as nb::rv_policy::reference to handle this use-case, or an alternative solution is requested. In the end I had to implement the recursive walking of the tree on the Python-side, which was ok, but clunky.