struct Level {
    entities: Vec<Entity>,
    current_entity: usize,
    foo: i32,
}

struct Entity;

enum Action {
    Walk,
}

impl Level {
    fn update(&mut self) {
        while self.current_entity < self.entities.len() {
            let action = self.entities[self.current_entity].get_action(self);

            action.perform(self);
            //logic...
            self.current_entity += 1;
        }
        self.current_entity = 0;
    }
}

impl Entity {
    fn get_action(&mut self, _level: &Level) -> Action {
        // logic...
        Action::Walk
    }
}

impl Action {
    fn perform(&self, level: &mut Level) -> bool {
        level.foo += 1; // mutate level
        //logic...
        true
    }
}

fn main() {
    let mut level = Level { entities: vec![Entity], current_entity: 0, foo: 0 };
    level.update();
}