Safely passing a mutable reference to Python?

I have a feeling I'm not alone in wanting to pass a mutable Rust reference to Python code. In my case, I want Python code to draw into a Piet RenderContext. There is basically no reasonable way to do this without a mutable reference, for example by copying data.

Having read through the code and some blogs on the issue, I believe I may have a workable answer. Probably easiest to post my code:

https://gist.github.com/raphlinus/479df97a7ba715b494b87af9071e9b87

The short explanation is that the PyCell holds a RefCell to an optional mutable reference. There's also a guard object which clears that reference when the guard is dropped. There is a small amount of unsafe, but it's only to adjust lifetimes; I'm not mucking with raw pointers at all (as I had done in a previous iteration). In particular, I'm relying on RefCell for mutability guarantees, so I'm not particularly worried about that.

I post this issue mostly to ask for review that the approach is sound. It's also possible there's some easier way to do this I'm missing. If it is a good approach, likely it should be documented, as it seems like something a lot of people want to do.

Many thanks for your consideration!

Start by reviewing the linked gist and the issue's description of PyCell, RefCell, the optional mutable reference, and the guard object. Check whether the proposed lifetime adjustment and mutability guarantees are sound, and determine whether an easier approach exists. Done means reaching a decision on the approach and documenting it if accepted.