Rust 将可变的自引用传递给拥有对象的方法

下面是一个简单的模拟，场是一个矩形区域，两个球在其中弹跳.Field struct 有一个update方法，在每个球上调用update.在update法中，球需要根据其速度四处移动.但他们也需要对彼此以及领域的边界做出react

fn main() {
    let mut field = Field::new(Vector2d { x: 100, y: 100 });
    field.update();
}

#[derive(Copy, Clone)]
struct Vector2d {
    x: i32,
    y: i32,
}

struct Ball {
    radius: i32,
    position: Vector2d,
    velocity: Vector2d,
}

impl Ball {
    fn new(radius: i32, position: Vector2d, velocity: Vector2d) -> Ball {
        Ball {
            radius: radius,
            position: position,
            velocity: velocity,
        }
    }

    fn update(&mut self, field: &Field) {
        // check collisions with walls
        // and other objects
    }
}

struct Field {
    size: Vector2d,
    balls: [Ball; 2],
}

impl Field {
    fn new(size: Vector2d) -> Field {
        let position_1 = Vector2d {
            x: size.x / 3,
            y: size.y / 3,
        };
        let velocity_1 = Vector2d { x: 1, y: 1 };
        let position_2 = Vector2d {
            x: size.x * 2 / 3,
            y: size.y * 2 / 3,
        };
        let velocity_2 = Vector2d { x: -1, y: -1 };

        let ball_1 = Ball::new(1, position_1, velocity_1);
        let ball_2 = Ball::new(1, position_2, velocity_2);

        Field {
            size: size,
            balls: [ball_1, ball_2],
        }
    }

    fn update(&mut self) {
        // this does not compile
        self.balls[0].update(self);
        self.balls[1].update(self);
    }
}

如何将有关边界和另一个球的信息获取到Ball struct的更新函数中？Field::update中的这些行不可编译:

self.balls[0].update(self);
self.balls[1].update(self);

给出以下错误:

error[E0502]: cannot borrow `*self` as immutable because `self.balls[..]` is also borrowed as mutable
  --> src/main.rs:62:30
   |
62 |         self.balls[0].update(self);
   |         -------------        ^^^^- mutable borrow ends here
   |         |                    |
   |         |                    immutable borrow occurs here
   |         mutable borrow occurs here

我明白，但我不知道该怎么解决这个问题.