How do I pass a reference to mutable data in Rust?

Question

I want to create a mutable struct on the stack and mutate it from helper functions.

#[derive(Debug)]
struct Game {
    score: u32,
}

fn addPoint(game: &mut Game) {
    game.score += 1;
}

fn main() {
    let mut game = Game { score: 0 };

    println!("Initial game: {:?}", game);

    // This works:
    game.score += 1;

    // This gives a compile error:
    addPoint(&game);

    println!("Final game:   {:?}", game);
}

Trying to compile this gives:

error[E0308]: mismatched types
  --> src/main.rs:19:14
   |
19 |     addPoint(&game);
   |              ^^^^^ types differ in mutability
   |
   = note: expected type `&mut Game`
              found type `&Game`

What am I doing wrong?

Answer 1

The reference needs to be marked as mutable too:

addPoint(&mut game);