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Rc<T> keeps a count and frees when it reaches zero, but only hands out shared references. Pairing it with RefCell<T> lets you mutate through that shared handle, with the borrow rules checked at runtime instead — a violation panics rather than failing to compile. Neither is thread-safe; that is Arc and Mutex.
Rc<T> provides shared ownership within one thread. Each clone of an Rc is another strong handle to the same allocation, not a deep copy of T. The allocation is released when its last strong owner disappears. Because multiple owners can access the same value, Rc<T> normally provides shared read access. Wrapping the value in RefCell<T> permits mutation by checking the borrow rule at runtime.
In the sample, both shared and second refer to the same cell.
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