The lifespan of the boxed value link is not long enough

The following code won't compile:

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();
}

      

I am getting the following error:

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 | | }
   | |_^

      

I think that the value inside the field actually persists for as long as 'a

, since it is a member Foo

that has exactly that lifetime.

I was wondering if this movement is confusing Foo

at the end of the new function, so if I try to do append in derp

. I am getting another error:

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);
   |                           ^^^^^^^^^^

      

which gives me no indication of what the compiler thinks the lifetime is a short value.

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1 answer


I think the value inside the field really survives as long as "a", since it is a member of Foo that has exactly that lifetime.

You can assign a new field to an item derp

, after which the old field will be deleted and it will expire at the end.

I think what you are trying to do is not possible in safe Rust: cross-referencing between struct members is not supported. This often comes up as a question, but it just isn't available in that language.



You can use Rc

to get around this, possibly in conjunction with RefCell

.

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