Who moved my value?

When we talked about ownership in Rust, we’ve excluded values who’s size are known at compile time, but what happens to a value that is stored on the Stack:

let mut a: i32 = 2;

// Here the value is copied
let copy_of_a: i32 = a;

a = 3;

// 🖨️ Amount copied: 2 🖨️
println!("Amount copied: {}", copy_of_a);

The 32-bits integer (8 bytes) is copied when we assign it to the variable copy_of_a. The same would have happened if we passed it to a function as a parameter.

Memory layout

This is basically the reason why useEffect in React has a dependency array, the function is memoized and the value is enclosed/copied inside the scope!

Point struct

What if we wanted to create a Point type that behaves like a value?

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

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

    // Here the ownership will be transferred, aka a move
    let point_is_moved = point;

    point.x = 5; // 💥 Compilation Error 💥
}

Compiler’s output

✦ ▶ cargo -q run
error[E0382]: assign to part of moved value: `point`
 --> src/main.rs:9:5
  |
4 |     let mut point = Point { x: 0, y: 0 };
  |         --------- move occurs because `point` has type `Point`,
                      which does not implement the `Copy` trait
...
7 |     let point_is_moved = point;
  |                          ----- value moved here
8 |
9 |     point.x = 5;
  |     ^^^^^^^^^^^ value partially assigned here after move

Clone

If we look at other types in rust we’ll notice that some of them have a Clone method (String), we could implement that for our type:

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

    // we clone the value
    let point_is_moved = point.clone();

    point.x = 5;

    // 🖨️ Point copied: { x: 0, y: 0 } 🖨️
    println!("Point copied: {:?}", point_is_moved);
}

#[derive(Debug)]
pub struct Point {
    pub x: i32,
    pub y: i32,
}

impl Clone for Point {
    fn clone(&self) -> Self {
        Point {
            x: self.x,
            y: self.y,
        }
    }
}

Implementing the Clone trait is a very explicit way of create a copy, which may or may not be expensive.

We can also derive from the Clone trait 💪:

#[derive(Debug, Clone)]
pub struct Point {
    pub x: i32,
    pub y: i32,
}

Copy

But what we said we really wanted to mimic the copy behavior of for instance i32? That is the beauty of the Rust language ❤️,, there is actually a supertrait of Clone which can be derived from :

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

    // we copy the value like `i32`
    let point_is_moved = point;

    point.x = 5;

    // 🖨️ Point copied: { x: 0, y: 0 } 🖨️
    println!("Point copied: {:?}", point_is_moved);
}

#[derive(Debug, Copy, Clone)]
pub struct Point {
    pub x: i32,
    pub y: i32,
}

To be able to derive from the Copy trait, all of the underlying values must implement Copy! This is the main difference with Clone, Copy is inexpensive.