rust - 特质默认方法中的多次 self 借用

Question

我在借阅检查器上遇到了问题。我有一个特征(Physics)，它具有getters(例如velocity)和setters(例如velocity_mut)。它还具有使用getter和setter的默认方法accelerate和apply_force。为什么多次借贷不好，如何解决呢？这是一条错误消息:

       13:18    rustc           error       cannot borrow `*self` as immutable because it is also borrowed as mutable
                                            immutable borrow occurs here
       12:11    rustc           hint        mutable borrow occurs here
       12:11    rustc           hint        argument requires that `*self` is borrowed for `'static`

use gdnative::prelude::*;

// Running into issues with the borrow checker

trait Physics {
    fn apply_force(&mut self, force: &mut Vector2, truncate: bool) {
        let max_force = self.max_force();

        if force.square_length() > max_force * max_force && truncate {
            *force = force.normalize() * max_force;
        }
        let a = self.acceleration_mut();// Line 12
        *a += *force / self.mass();//Line 13
        self.accelerate(truncate);
    }
    fn accelerate(&mut self, truncate: bool) {
        let v = self.velocity_mut();
        *v += self.acceleration();

        let max_speed = self.max_speed();

        if v.square_length() > max_speed * max_speed && truncate {
            *v = v.normalize() * max_speed;
        }
    }
    fn velocity(&self) -> Vector2;
    fn velocity_mut(&mut self) -> &mut Vector2;
    fn acceleration(&self) -> Vector2;
    fn acceleration_mut(&mut self) -> &mut Vector2; 
    fn max_speed(&self) -> f32;
    fn max_speed_mut(&mut self) -> &mut f32;
    fn max_force(&self) -> f32;
    fn max_force_mut(&mut self) -> &mut f32;
    fn mass(&self) -> f32;
    fn mass_mut(&mut self) -> &mut f32;
}

struct Actor {
    velocity: Vector2,
    acceleration: Vector2,
    max_speed: f32,
    max_force: f32,
    mass: f32
}

impl Physics for Actor {
    fn velocity(&self) -> Vector2 {
        self.velocity
    }
    fn velocity_mut(&mut self) -> &mut Vector2 {
        &mut self.velocity
    }
    fn acceleration(&self) -> Vector2 {
        self.acceleration    
    }
    fn acceleration_mut(&mut self) -> &mut Vector2 {
        &mut self.acceleration
    }
    fn mass(&self) -> f32 {
        self.mass    
    }
    fn mass_mut(&mut self) -> &mut f32 {
        &mut self.mass    
    }
    fn max_speed(&self) -> f32 {
        self.max_speed    
    }
    fn max_speed_mut(&mut self) -> &mut f32 {
        &mut self.max_speed    
    }
    fn max_force(&self) -> f32 {
        self.max_force    
    }
    fn max_force_mut(&mut self) -> &mut f32 {
        &mut self.max_force
    }
}

fn main() {

}

Answer 1

在还有可变借项的情况下您不能一成不变地借用的原因是，否则，您可能会使用可变借用来修改由不可变借用表示的基础数据。而且由于(显然)不应该更改不可变的借贷，因此借贷检查器无法承担风险(即使您“知道”可变的借贷也不会影响不可变的借贷)。
在这种情况下，我相信您可以通过在调用self.mass()之前将self.acceleration_mut()分配给本地来解决此问题

let mass = self.mass();
let a = self.acceleration_mut();
*a += *force / mass;

由于mass只是一个f32，它在返回时被复制，因此将在下一行之前进行不可变的借用。