Comment by account42

> For example std::unique_ptr<T> looks like it's Box<T> but it's not, it's actually Option<Box<T>>, even newer types often do this but they might be more embarrassed about it.

This would be the case even with destructive moves unless you also add some way for std::optional<std::unique_ptr<T>> to be no larger than a pointer by letting std::optional take advantage of the fact that a non-null std::unique_ptr has a bit representation that leaves room for sentinel values.

Also, at the language level, C++ moves are sort-destructive as the moved from object only has to guarantee to be able to run the destructor. E.g. you could still have a std::nonull_ptr where move sets the internal pointer to zero but calling anything except the destructor on such an instance throws / calls std::terminate() / is UB. It's only the stdlib types that make additional guarantees - because in most cases it can be done without additional cost.