引用解包属性失败：使用部分移动值：`self` [英] Reference to unwrapped property fails: use of partially moved value: `self`

问题描述

我尝试将未包装的字符串引用发送到为结构实现的静电方法。以下是简化的代码：

fn main() {
    let a = A {p: Some("p".to_string())};
    a.a();
}

struct A {
    p: Option<String>
}

impl A {
    fn a(self) -> Self {
        Self::b(&self.p.unwrap());
        self
    }
    fn b(b: &str) {
        print!("b: {}", b)
    }
}

失败：

error[E0382]: use of partially moved value: `self`
  --> src/main.rs:14:13
   |
13 |             Self::b(&self.p.unwrap());
   |                      ------ value moved here
14 |             self
   |             ^^^^ value used here after move
   |
   = note: move occurs because `self.p` has type `std::option::Option<std::string::String>`, which does not implement the `Copy` trait

我认为实现Copy特征不是解决方案。如何在该上下文中解开p并将其作为&str传递给b？

我按照Can't borrow File from &mut self (error msg: cannot move out of borrowed content)中的建议更改了代码：

fn main() {
    let a = A {p: Some("p".to_string())};
    a.a();
}

struct A {
    p: Option<String>
}

impl A {
    fn a(self) -> Self {
        let c = self.p.as_ref().unwrap();
        Self::b(&c);
        self
    }
    fn b(b: &str) {
        print!("b: {}", b)
    }
}

这会导致不同的错误：

error[E0505]: cannot move out of `self` because it is borrowed
  --> src/main.rs:15:13
   |
13 |             let c = self.p.as_ref().unwrap();
   |                     ------ borrow of `self.p` occurs here
14 |             Self::b(&c);
15 |             self
   |             ^^^^ move out of `self` occurs here

推荐答案

如Can't borrow File from &mut self (error msg: cannot move out of borrowed content)中所述，您不能对借用值调用unwrap，因为unwrap取得该值的所有权。

更改为as_ref会借用值self。当对值的任何引用都未完成时，不允许移动值(包括返回该值)。这意味着您需要将借用期限限制为在需要移动值之前结束：

fn a(self) -> Self {
    {
        let c = self.p.as_ref().unwrap();
        Self::b(c);
    }
    self
}

---

它可能是示例中的工件，但是代码非常奇怪。我会写成

impl A {
    fn a(self) -> Self {
        self.b();
        self
    }

    fn b(&self) {
        print!("b: {}", self.p.as_ref().unwrap())
    }
}

或

impl A {
    fn a(&self) {
        print!("a: {}", self.p.as_ref().unwrap())
    }
}

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