Rust by Example

13.3.3 Aliasing

Data can be immutably borrowed any number of times, but while immutably borrowed, the original data can't be mutably borrowed. On the other hand, only one mutable borrow is allowed at a time. The original data can be borrowed again only after the mutable reference goes out of scope.

struct Point { x: i32, y: i32, z: i32 }

fn main() {
    let mut point = Point { x: 0, y: 0, z: 0 };

    {
        let borrowed_point = &point;
        let another_borrow = &point;

        // Data can be accessed via the references and the original owner
        println!("Point has coordinates: ({}, {}, {})",
                 borrowed_point.x, another_borrow.y, point.z);

        // Error! Can't borrow point as mutable because it's currently
        // borrowed as immutable.
        //let mutable_borrow = &mut point;
        // TODO ^ Try uncommenting this line

        // Immutable references go out of scope
    }

    {
        let mutable_borrow = &mut point;

        // Change data via mutable reference
        mutable_borrow.x = 5;
        mutable_borrow.y = 2;
        mutable_borrow.z = 1;

        // Error! Can't borrow `point` as immutable because it's currently
        // borrowed as mutable.
        //let y = &point.y;
        // TODO ^ Try uncommenting this line

        // Error! Can't print because `println!` takes an immutable reference.
        //println!("Point Z coordinate is {}", point.z);
        // TODO ^ Try uncommenting this line

        // Ok! Mutable references can be passed as immutable to `println!`
        println!("Point has coordinates: ({}, {}, {})",
                 mutable_borrow.x, mutable_borrow.y, mutable_borrow.z);

        // Mutable reference goes out of scope
    }

    // Immutable references to point are allowed again
    let borrowed_point = &point;
    println!("Point now has coordinates: ({}, {}, {})",
             borrowed_point.x, borrowed_point.y, borrowed_point.z);
}