移动后改变结构的字段

Question

我对以下行为感到困惑：有人能解释一下这是怎么回事吗？

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，我感到困惑的是：

允许更新到p1.cx，即使移动已经发生，
p2.x返回的值实际上不是更新后的5000，而是旧的100。

我期待新的价值，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) 应该打印出来。

但是，我一定遗漏了什么。有小费吗？提前致谢！

Answer 1

这现在被禁止了。

以前是允许的。然而，这是旧的借用检查器中的一个错误，并且在引入新的借用检查器 (NLL) 时先是警告然后是错误。

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 版的一个错误，时间较长（可能自该版本创建以来）。

另请参阅：

The description of E0729
release notes for 1.40：

As previously announced, any previous NLL warnings in the 2015 edition are now hard errors.

移动后改变结构的字段

Mutating a struct's fields after a move

alias

move

ownership-semantics

rust