rust-cpython: make PySharedRef::try_borrow_mut() return BorrowMutError As I said, it shouldn't be an error of Python layer, but is something like a coding error. Returning BorrowMutError makes more sense. There's a weird hack to propagate the borrow-by-leaked state to RefCell to obtain BorrowMutError. If we don't like it, maybe we can add our own BorrowMutError.