I wouldn't say it's crazy, or even unprecedented. Consider the reference-counted `Rc` smart pointer type, which allows a given type to act as though it were cloneable (when used inside of `Rc`) or not cloneable (when used without `Rc`).

The hypothetical opt-in moveability proposal is difficult to compare directly to `Pin` because they take such different approaches to the problem, but they would both address the problem (at least, as far as I have considered it).