特质默认方法中的多重自我借用

Question

我遇到了借用检查器的问题。我有一个特性(Physics(，它有getter(例如velocity(和setter(例如，velocity_mut(。它还有使用getter和setter的默认方法accelerate和apply_force。为什么多次借款不好？有什么方法可以解决这个问题？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

在存在可变借用的情况下不能进行不可变借用的原因是，否则，您可能会使用可变借用来修改由不可变借用表示的底层数据。由于不可变借款(显然(不应该改变，借款检查器不能承担风险(即使你"知道"可变借款不会影响不可变借款(。

在这种情况下，我相信您可以通过在调用self.acceleration_mut()之前将self.mass()分配给本地来解决这个问题

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

由于质量只是一个f32，它在返回时被复制，因此不可变的借位将在下一行之前完成。

Answer 2

由于"getters"；返回所拥有的值，另一种设计是使用局部变量来计算结果，并仅在最后分配(通过可变引用(：

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

避免使用局部变量(例如，如果您假设基础值是克隆的，并且这是昂贵的(是很棘手的，因为通过可变引用在方法的整个长度(大约(上锁定self是很难管理的。