Doing this properly means putting `GcData`'s fields (and/or `ObjFun`/`ObjClosure`/etc's fields) in plain `Cell`s, and using that for all mutation without ever forming any `&mut` references.
If I remember correctly, there were some panics cause by RefCell borrows. But those got sorted out once I handled borrowing in the correct order.
What I would expect a production language implementation to do is build its own table that integrates its solution for interior mutability into its design. There are several other things (e.g. iterators, concurrent GC) that will also contribute to the need for a custom table, so (again for a production language) this is not too crazy.