Smart Pointers

Heap allocation, shared ownership, interior mutability, and borrowed data abstractions.

Box<T>	Heap allocation, trait objects, recursive types
Rc<T> / Arc<T>	Shared ownership — single-threaded vs multi-threaded
Cell<T> / RefCell<T>	Interior mutability — no borrow checker cost at compile time
Cow<'a, B>	Clone-on-write — avoid cloning borrowed data
Pin<P>	Prevent value from being moved — required for self-referential futures