Lifetimes get replaced with 'static in Self within (return position) impl Trait.

@cramertj

#[derive(Copy, Clone)]
struct Invariant<'a>(std::marker::PhantomData<&'a mut &'a ()>);

impl<'a> Invariant<'a> {
    fn lazy_dup(self) -> impl Fn() -> (Invariant<'a>, Self) {
        || (self, self)
    }
}

error[E0308]: mismatched types
 --> src/lib.rs:6:19
  |
6 |         || (self, self)
  |                   ^^^^ lifetime mismatch
  |
  = note: expected type `Invariant<'static>`
             found type `Invariant<'a>`

I didn't write 'static anywhere - it comes from part of the implementation of impl Trait, and Self references lifetimes (from the impl header) "indirectly", hiding them from impl Trait logic.

cc @cramertj @oli-obk @nikomatsakis

There is a variant of this problem that was revealed in #62221. If you write T::Item, and we expand that (in type-checking) to <T as Foo<'a>>::Item, the 'a is overlooked as well (playground):

pub trait HasItem<'a> {
    type Item;
}

// Does not compile:
fn example1<'a, T: HasItem<'a>>(item: T::Item) -> impl IntoIterator<Item = T::Item> {
    Some(item)
}

// Compiles:
fn example2<'a, T: HasItem<'a>>(item: T::Item) -> impl IntoIterator<Item = <T as HasItem<'a>>::Item> {
    Some(item)
}

fn main() { }

UPDATE: This is #51525

I left some notes in this Zulip topic that explain one way to fix this problem. The idea is basically to detect Self during lowering and take it into account.

Auto merge of rust-lang#115659 - compiler-errors:itp, r=cjgillot

Auto merge of #115659 - compiler-errors:itp, r=cjgillot

Just like #61949 I guess this is also fixed by #115659 ? The test case in the OP compiles fine today, other than needing to add move to the closure to make it move || (self, self)

Closing this as it's fixed and it compiles to a different error with indication on how to fix it by adding move to the closure.

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Lifetimes get replaced with 'static in Self within (return position) impl Trait. #53613

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Lifetimes get replaced with 'static in Self within (return position) impl Trait. #53613

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nikomatsakis commented on Jul 5, 2019

nikomatsakis commented on Jul 9, 2019

Arnavion commented on Aug 8, 2024

Dylan-DPC commented on Dec 15, 2024

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