Boxes

Another pointer type in Rust is called the boxed pointer Box<T>, which can be defined for a value of a generic type T. A box is a non-copyable value. This pointer type is used to allocate objects on the heap.

For example, here we allocate an Alien value on the heap with: 
// see code in Chapter 7/code/boxes1.rs    
let mut a1 = Box::new(Alien{ planet: "Mars".to_string(), n_tentacles: 4 }); 
println!("{}", a1.n_tentacles); // 4 

The mutable variable a1 is the only owner of this memory resource that may read from and write to it.

We can make a reference to the value pointed to by the box pointer, and if both the original box and this new reference are mutable, we can change the object through this reference:

let a2 = &mut a1; 
println!("{}", a2.planet ); // Mars 
a2.n_tentacles = 5; 

After such a borrow the usual ownership rules as above hold: a1 no longer has access, not even for reading:

// error: cannot borrow `a1.n_tentacles` as immutable because `a1` is also borrowed as mutable...