引用盒装值的生命周期不够长

Question

以下代码无法编译：

use std::borrow::Borrow;

struct Inner<'a> {
    v: Vec<&'a u8>,
}

struct Foo<'a> {
    inner: Inner<'a>,
    derp: Box<u8>,
}

impl<'a> Foo<'a> {
    fn new() -> Foo<'a> {
        let mut e = Foo {
            inner: Inner { v: vec![] },
            derp: Box::new(128),
        };
        e.inner.v.push(&*e.derp);

        return e;
    }

    fn derp(&mut self) {
        println!("{:?}", self.inner.v);
    }
}

fn main() {
    let mut f = Foo::new();

    f.derp();
}

我收到以下错误：

error[E0597]: `*e.derp` does not live long enough
  --> src/main.rs:18:25
   |
18 |         e.inner.v.push(&*e.derp);
   |                         ^^^^^^^ does not live long enough
...
21 |     }
   |     - borrowed value only lives until here
   |
note: borrowed value must be valid for the lifetime 'a as defined on the impl at 12:1...
  --> src/main.rs:12:1
   |
12 | / impl<'a> Foo<'a> {
13 | |     fn new() -> Foo<'a> {
14 | |         let mut e = Foo {
15 | |             inner: Inner { v: vec![] },
...  |
25 | |     }
26 | | }
   | |_^

我认为框内的值的寿命与 'a 一样长，因为它是 Foo 的成员，而 Foo 的寿命恰好如此。

我想知道 Foo 在新函数末尾的移动是否混淆了它，所以如果尝试在 derp 中进行追加。我得到一个不同的错误：

error[E0495]: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
  --> main.rs:20:27
   |
20 |         self.inner.v.push(& *self.derp);
   |                           ^^^^^^^^^^

这没有告诉我编译器认为装箱值的生命周期有多长。

Answer 1

I think though that the value inside of the box does live for as long as 'a since it is a member of Foo which has exactly that lifetime.

可以为 derp 成员分配一个新框，此时旧框将被删除，其中值的生命周期结束。

我认为您尝试做的事情在安全 Rust 中是不可能的：不支持结构成员之间的交叉引用。这作为一个问题经常出现，但它在该语言中不可用。

How to initialize struct fields which reference each other

您可以使用 Rc to work around this, perhaps in combination with RefCell.

引用盒装值的生命周期不够长

Lifetime of reference to boxed value does not live long enough

rust

borrow-checker