In addition to enabling interior mutability, the existing _Cell type in the standard library guarantees address stability until the value (or rather, an owning binding of that value) is consumed. Having stable addresses is important for certain C interoperability scenarios, where a called C function (temporarily) escapes a passed pointer out of its body; for instance, if a C function registers an event callback with the pointer as context argument.
Will the pitched Cell and ConstCell types guarantee address stability like _Cell does today? That is, can the pointer passed to the body of their withUnsafe{…}Pointer methods be safely escaped and used until the original owning binding of the cell is consumed? In that case, would it make sense to directly expose the pointer as a property, similar to _Cell, since escaping a pointer from a with-style closure is normally an unsafe anti-pattern?