Comment by pornel

You can implement this in Rust.

It's an equivalent of Rc<Cell<(Option<Box<T>>, Option<Box<T>>)>>, but with the Rc replaced by a custom shared type that avoids keeping refcount by having max 2 owners.

You're going to need UnsafeCell to implement the exact solution, which needs a few lines of code that is as safe as the C++ version.