> The biggest annoying magic I found with respect to Box (and other std containers like Rc) is that they’re the only ones capable of storing fat dyn pointers. You can’t construct a hybrid_rc::Rc<dyn Trait> like you can with Box/Rc.

It's perfectly possible to make a container capable of storing trait objects: just define the type parameter as <T: ?Sized>. The main issue is that unlike Box/Rc, the compiler won't give you an automatic coercion from MyRc<Type> to MyRc<dyn Trait>, so you have to write a method to explicitly perform that cast. It just isn't common for many existing third-party containers to support !Sized objects, since it takes tedious unsafe code to manipulate them in memory.