Reference Cycles

Start with the idea

Two Rc handles pointing at each other never reach a count of zero, so neither is freed. Weak<T> is the answer: it does not count towards ownership and must be upgraded before use, which turns a parent pointer into something that can legitimately be gone.

Reference counting releases an allocation only when its strong count reaches zero. If node A owns an Rc to B and B owns an Rc back to A, each keeps the other alive after all outside handles are dropped. Rust prevents memory unsafety here, but the allocations leak. A graph needs a deliberate ownership direction, not strong links in every direction.

A common tree design lets a parent own children through strong Rc handles while each child keeps a Weak reference back to its parent. A weak reference does not keep the parent alive. upgrade() returns an Option<Rc<T>>, because the parent may have been dropped.