移动后改变结构的字段

Question

我对以下行为感到困惑：有人可以解释发生了什么吗？

考虑代码：

struct Point {
    cx : u32,
}
fn main() {
    let mut p1 = Point { cx: 100 };
    let     p2 = p1; 
    p1.cx      = 5000;
    // println!("p1.x = {}", p1.cx); // disallowed as p1.cx is "moved" ... ok
    println!("p2.x = {}", p2.cx); // ==> prints 100 (!)
}

具体来说，我很困惑：

1. 即使发生移动，也允许更新到p1.cx，
2. p2.x返回的值其实不是更新后的5000，而是旧的100。

我期待新值，因为没有复制特征（因此移动）， 所以期望只有一个单元格的更新值（5000） 应该打印。

但是，我一定错过了什么。有小费吗？提前致谢！

Answer 1

现在禁止这样做。

以前是允许的。然而，这是旧借用检查器中的一个错误，它已发出警告，然后在引入新的借用检查器 (NLL) 时出现错误。

例如，对于rustc 1.39.0 和 2015 版，您会收到以下警告：

warning[E0382]: assign to part of moved value: `p1`
 --> a.rs:8:5
  |
6 |     let mut p1 = Point { cx: 100 };
  |         ------ move occurs because `p1` has type `Point`, which does not implement the `Copy` trait
7 |     let p2 = p1;
  |              -- value moved here
8 |     p1.cx = 5000;
  |     ^^^^^^^^^^^^ value partially assigned here after move
  |
  = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
  = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
  = note: for more information, try `rustc --explain E0729`

rustc 1.40.0变成了错误：

error[E0382]: assign to part of moved value: `p1`
 --> src/main.rs:7:5
  |
5 |     let mut p1 = Point { cx: 100 };
  |         ------ move occurs because `p1` has type `Point`, which does not implement the `Copy` trait
6 |     let p2 = p1;
  |              -- value moved here
7 |     p1.cx = 5000;
  |     ^^^^^^^^^^^^ value partially assigned here after move

error: aborting due to previous error

另请注意，这是 2018 版较长时间的错误（可能自该版本创建以来）。

另见：

• The description of E0729
• release notes for 1.40:

  如前所述，2015 版中之前的任何 NLL 警告现在都是硬错误。