Mastering Rust - Second Edition

RefCell<T>

If you need Cell-like features for non-Copy types, there is the RefCell type. It uses a read/write pattern similar to how borrowing works, but moves the checks to runtime, which is convenient but not zero-cost. RefCell hands out references to the value, instead of returning things by value as is the case with the Cell type. Here's a sample program that

// refcell_basics.rs use std::cell::RefCell; #[derive(Debug)]struct Bag {     item: Box<u32>} fn main() {     let bag = RefCell::new(Bag { item: Box::new(1) });     let hand1 = &bag;    let hand2 = &bag;    *hand1.borrow_mut() = Bag { item: Box::new(2)};     *hand2.borrow_mut() = Bag { item: Box::new(3)};    let borrowed = hand1.borrow();    println!("{:?}", borrowed);}

As you can see, we can borrow