Trait fn implies borrow of self when I don't want it to

help
1

I need help with a mysterious lifetime issue. The error that the compiler is reporting is as follows:

   Compiling my-crate v0.6.0 (...path...)
error[E0515]: cannot return value referencing local variable `named_actor`
  --> (path)/assertion.rs:90:17
   |
88 |       let summary = named_actor.to_summary();
   |                     ----------- `named_actor` is borrowed here
89 |
90 | /     IdentityAssertionReport {
91 | |         signer_payload: SignerPayloadReport::from_signer_payload(&self.signer_payload),
92 | |         named_actor: Some(summary),
93 | |     }
   | |_________________^ returns a value referencing data owned by the current function

For more information about this error, try `rustc --explain E0515`.

The definition of the trait containing to_summary fn is here:

pub trait ToCredentialSummary {
    fn to_summary(&self) -> impl Serialize + 'static;
}

I'm not aware of anything that ties the output of to_summary to any ongoing lifetime of the type that implements ToCredentialSummary , but the compiler seems to think there is one.

Can you help me understand this implied lifetime or – even better – explain how to write to_summary to that the return type can outlive the implementation of ToCredentialSummary?

(edited to remove indentation that made it harder to read …)

2

You want to clone any data in named_actor referenced in to_summary so the reference is not alive when you hand named_actor to IdentityAssertionReport.

3

-> impl in traits -- and everywhere in edition 2024+ -- captures all borrows from the inputs. You can read more about it here. So you have to train your eyes to pick up on impl in a return the same way they pick up on & and '_.

In a trait, you need a nightly feature for the ideal fix, unfortunately.

#![feature(precise_capturing_in_traits)]
pub trait ToCredentialSummary {
    fn to_summary(&self) -> impl Serialize + use<Self>;
}

(Tracking issue.)

Another alternative is to use an associated type instead.

pub trait ToCredentialSummary {
    type Summary: Serialize;
    fn to_summary(&self) -> Self::Summary;
}

But that doesn't help if your type is unnameable unless you can type erase it (Box<dyn ..>). And it makes for a lot more boilerplate on your side if you want it to be opaque like -> impl is.

There's another feature that will help with that eventually.

#![feature(impl_trait_in_assoc_type)]
impl ToCredentialSummary for Something {
    type Summary: impl Serialize;
    fn to_summary(&self) -> Self::Summary { .. }
}

(Tracking issue, for type aliases generally, not just in traits. Edit: Also various discussions in this PR.)

1 Like
4

@quinedot been doing some reading since then and this looks like an accurate description of the problem space. I'm not able to use nightly features(*), so it looks like I'll be Boxing the result instead. Thanks for the confirmation.

(*) This code will be going into a public crate and I can't impose a requirement on our users to build with nightly Rust.

5

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