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impl Trait should be able to capture long-lived associated types even if the trait contains a lifetime #54385

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RalfJung opened this issue Sep 20, 2018 · 6 comments
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A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. A-lifetimes Area: lifetime related C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.

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@RalfJung
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Basically, this should compile:

trait Trait<'foo> { type Assoc: 'static; }

fn test<'foo, T: Trait<'foo>>(x: T::Assoc) -> impl FnOnce() {
    move || { let _x = x; }
}

Thanks to @eddyb I can work around this, writing

trait Trait<'foo> { type Assoc: 'static; }

fn test<'foo, A: 'static, T: Trait<'foo, Assoc=A>>(x: T::Assoc) -> impl FnOnce() {
    move || { let _x = x; }
}

but that is almost impossible to discover.

@eddyb
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eddyb commented Sep 20, 2018

It doesn't compile right now because it contains <T as Trait<'foo>>::Assoc, which, despite being 'static, contains the lifetime 'foo, so for soundness reasons, the impl FnOnce() opaque type must also be parametrized over 'foo (to recover the concrete return type).

We could try some erasure scheme, but the "exporting" of the concrete type is already pretty complicated, and there's a huge risk here, of opening up a soundness hole.

cc @nikomatsakis @cramertj @oli-obk

@eddyb eddyb added the A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. label Sep 20, 2018
@Arnavion
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(Unexpected lifetime capture by impl-trait is #42940 )

@RalfJung
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@Mark-Simulacrum AFAIK this bug here is specifically about associated types, unlike #42940. Why did you close?

@Mark-Simulacrum
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I got the impression that the underlying bug is the same. But perhaps not. Reopening.

@Enselic Enselic added A-lifetimes Area: lifetime related T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. C-bug Category: This is a bug. labels Nov 24, 2023
@RalfJung RalfJung added the T-types Relevant to the types team, which will review and decide on the PR/issue. label Apr 24, 2024
@RalfJung
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This actually compiles with the 2024 edition. :)

@rust-lang/types I assume that means it can be closed?

@lcnr
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lcnr commented Apr 24, 2024

the underlying cause still exists:

#![feature(precise_capturing)]
trait Trait<'foo> { type Assoc: 'static; }
fn test<'foo, T: Trait<'foo>>(x: T::Assoc) -> impl use<T> FnOnce() {
    move || { let _x = x; }
}

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Labels
A-impl-trait Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch. A-lifetimes Area: lifetime related C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.
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