Mastering Rust - Second Edition

Cell<T>

Consider this program, where we have a requirement to mutate bag with two mutable references to it:

// without_cell.rs use std::cell::Cell; #[derive(Debug)]struct Bag {     item: Box<u32>} fn main() {     let mut bag = Cell::new(Bag { item: Box::new(1) });     let hand1 = &mut bag;    let hand2 = &mut bag;    *hand1 = Cell::new(Bag {item: Box::new(2)});    *hand2 = Cell::new(Bag {item: Box::new(2)});}

But, of course, this does not compile due to the borrow checking rules:

We can make this work by encapsulating the bag value inside a Cell. Our code is updated as follows:

// cell.rs use std::cell::Cell; #[derive(Debug)]struct Bag {     item: Box<u32>}