Perma-unstable marker trait. Indicates that the type has a well-behaved Deref
(and, if applicable, DerefMut) implementation. This is relied on for soundness
of deref patterns.
Indicates that a struct can be used as a method receiver, without the
arbitrary_self_types feature. This is implemented by stdlib pointer types like Box<T>,
Rc<T>, &T, and Pin<P>.