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

[eddyb]

#[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

[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

[nikomatsakis]

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.