rust 中的第二次借用問題

Question

我最近從 Rust 開始，我正在研究小游戲，不幸的是，由於多個可變借用的錯誤，我在實現一個簡單的機制時遇到了問題，例如，以下代碼有第二個可變借用的問題

struct Margin {
    x: f32,
    y: f32, 
    h: f32, 
    w: f32,
}

struct Level {
    margins: Vec<Margin>,
}

impl Level {
    fn new() -> Self {
        Level{
            margins: vec![]
        }
    }

    fn update(&mut self) {
        // Add a new margin on each tick
        self.margins.push(Margin {
          x:0., y:0., w:100., h:0.5,
        });

        // Iterate over margins moving each one down a little
        for (i, m) in self.margins.iter_mut().enumerate() {
            m.y += 1.;

            // Remove margins outside of the screen
            if m.y > 640. {
                self.margins.remove(i);
            }
        }
    }
}

fn main() {
    let mut l = Level::new();
    l.update();
}

錯誤

   Compiling playground v0.0.1 (/playground)
error[E0499]: cannot borrow `self.margins` as mutable more than once at a time
  --> src/main.rs:31:17
   |
26 |         for (i, m) in self.margins.iter_mut().enumerate() {
   |                       -----------------------------------
   |                       |
   |                       first mutable borrow occurs here
   |                       first borrow later used here
...
31 |                 self.margins.remove(i);
   |                 ^^^^^^^^^^^^ second mutable borrow occurs here

Answer 1

在Vec上找到有用的方法retain ，代碼固定如下：

struct Margin {
    x: f32,
    y: f32, 
    h: f32, 
    w: f32,
}

struct Level {
    margins: Vec<Margin>,
}

impl Level {
    fn new() -> Self {
        Level{
            margins: vec![]
        }
    }

    fn update(&mut self) {
        // Add a new margin on each tick
        self.margins.push(Margin {
          x:0., y:0., h:100., w:0.5,
        });

        // Iterate over margins moving each one down a little
        for m in self.margins.iter_mut() {
            m.y += 1.;
        }

        // Retains only the elements where y is lower than 640.
        self.margins.retain(|v| v.y < 640.);
    }
}

fn main() {
    let mut l = Level::new();
    l.update();
}

根據之前的答案回答這里刪除一個元素，同時迭代它