rust - Downpass self: cannot borrow `self` as mutable more than once at a time

Question

I want to downpass a mutable object, so that the entity responsible for working and managing certain aspects of the application can change parts of the world... for example make a bunny appear if a particular item fades away.

pub struct World {
    world_items: WorldItemManager
}

impl World {
    pub fn new() -> World {
        World {
            world_items: WorldItemManager{},
        }
    }

    pub fn process(&mut self) {
        //
        // Uncomment to make it fail:
        //
        //self.world_items.process(&mut self);
    }
}

pub struct WorldItemManager {
    // items, events, anything that it should own
}

impl WorldItemManager {
    pub fn process(&mut self, _world: &mut World) {
        // If the sun stands at the exact spot, it makes something in the _world appear.
        // Also: yes, if I don't take a _world argument it compiles. But.. that's the point.
    }
}

struct Game {
    world: World
}

impl Game {
    pub fn new() -> Game {
        Game {
            world: World::new(),
        }
    }

    pub fn run(&mut self) {
        loop {
            self.world.process();
        }
    }
}

fn main() {
    let mut game = Game::new();
    game.run();
}

I know that I can fix this by..

• not passing the World at all
• making the World not mutable in the manager
• putting all the logic of the manager directly in world

I'm not looking for those, as I just want to find out how I can have a normal object graph where I can downpass what needs changing without making everything so cumbersome. It should be simple, but I can't get it to work:

error[E0499]: cannot borrow `self.world_items` as mutable more than once at a time
  --> src/main.rs:16:9
   |
16 |         self.world_items.process(&mut self);
   |         ^^^^^^^^^^^^^^^^^-------^---------^
   |         |                |       |
   |         |                |       first mutable borrow occurs here
   |         |                first borrow later used by call
   |         second mutable borrow occurs here

How can I downpass myself to a child of myself?

question from:https://stackoverflow.com/questions/65646749/downpass-self-cannot-borrow-self-as-mutable-more-than-once-at-a-time

Answer 1

Waitting for answers