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