(Atomic) Reference Counted structs. They count their own references, there is no external mechanism tracking all of the reference counts at runtime. Modern rust does not include a garbage collector. You might be confusing a garbage collector with the general concept of memory management, which is a feature of (AFAIK) every high level language. Many C projects also have reference counted structs, but a reference count and a call to malloc/free doesn't quite qualify as garbage collection to most developers.

By the name i'd say Arc is reference counting, which isn't exactly garbage collection...