rust-cpython: leverage RefCell::borrow() to guarantee there's no mutable ref...
rust-cpython: leverage RefCell::borrow() to guarantee there's no mutable ref
Since the underlying value can't be mutably borrowed by PyLeaked, we don't
have to manage yet another mutably-borrowed state. We can just rely on the
RefCell implementation.
Maybe we can add try_leak_immutable(), but this patch doesn't in order to
keep the patch series not too long.