Question

我为以下行为感到困惑：有人可以解释发生了什么事吗？

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：

即使发生了移动，也允许将 update 更新为p1.cx，
p2.x返回的值实际上不是更新的5000，而是旧的100。

由于没有复制特性，我一直期待新的价值（因此有所举动），因此，原本以为只有一个单元格的更新值（5000）应该打印出来。

但是，我必须缺少一些东西。有小费吗？预先感谢！

Answer 1

现在禁止这样做。

它曾经被允许。但是，这是旧借阅检查器中的一个错误，并且在引入新借阅检查器（NLL）时被警告，然后出错。

例如，在rustc 1.39.0和2015版中，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

还请注意，这是2018年版较长时间的错误（可能是自该版本创建以来）。

另请参阅：

The description of E0729
release notes for 1.40：

如先前所宣布，2015版中以前的任何NLL警告现在都是硬错误。

移动后使结构的字段突变

1 个答案: