Cannot copy self-reference as of 0.18 #118

vbkaisetsu · 2024-07-14T04:25:05Z

The following code works in 0.17.2:

use ouroboros::self_referencing;

#[self_referencing]
struct Outer {
    data: String,

    #[borrows(data)]
    #[covariant]
    ref1: Option<&'this str>,

    #[borrows(data)]
    #[covariant]
    ref2: Option<&'this str>,
}

fn main() {
    let mut s = OuterBuilder {
        data: "test".to_string(),
        ref1_builder: |_| None,
        ref2_builder: |x| Some(x),
    }.build();

    s.with_mut(|f| {
        *f.ref1 = *f.ref2;
    });

    dbg!(s.borrow_ref1());
}

But in 0.18.4, it gives the following error:

error[E0597]: `s` does not live long enough
  --> src/main.rs:23:5
   |
17 |       let mut s = OuterBuilder {
   |           ----- binding `s` declared here
...
23 |       s.with_mut(|f| {
   |       ^ borrowed value does not live long enough
   |  _____|
   | |
24 | |         *f.ref1 = *f.ref2;
25 | |     });
   | |______- argument requires that `s` is borrowed for `'static`
...
28 |   }
   |   - `s` dropped here while still borrowed

error[E0502]: cannot borrow `s` as immutable because it is also borrowed as mutable
  --> src/main.rs:27:10
   |
23 |       s.with_mut(|f| {
   |       - mutable borrow occurs here
   |  _____|
   | |
24 | |         *f.ref1 = *f.ref2;
25 | |     });
   | |______- argument requires that `s` is borrowed for `'static`
26 |
27 |       dbg!(s.borrow_ref1());
   |            ^ immutable borrow occurs here

The text was updated successfully, but these errors were encountered:

someguynamedjosh · 2024-07-14T15:23:56Z

This is unfortunately a limitation of a check that was introduced to prevent a situation like the following:

#[self_referencing]
struct Outer {
    data: String,

    #[borrows(data)]
    #[covariant]
    ref1: Option<&'this str>,

    other: String,
}


fn main() {
    let mut s = OuterBuilder {
        data: "test".to_string(),
        ref1_builder: |_| None,
        other: "other".to_string(),
    }.build();

    s.with_mut(|f| {
        *f.ref1 = Some(f.other);
    });

    drop(s);
}

Which causes a use-after-free, since other is dropped before ref1 in the generated struct.

vbkaisetsu linked a pull request Jul 15, 2024 that will close this issue

Change lifetime bounds to allow copying references #119

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Cannot copy self-reference as of 0.18 #118

Cannot copy self-reference as of 0.18 #118

vbkaisetsu commented Jul 14, 2024

someguynamedjosh commented Jul 14, 2024

Cannot copy self-reference as of 0.18 #118

Cannot copy self-reference as of 0.18 #118

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vbkaisetsu commented Jul 14, 2024

someguynamedjosh commented Jul 14, 2024