Existential types have issues with lifetimes

[jethrogb]

#![feature(existential_type)]

trait MyTrait {
    type AssocType: Send;
    fn ret(&self) -> Self::AssocType;
}

impl MyTrait for () {
    existential type AssocType: Send;
    fn ret(&self) -> Self::AssocType {
        ()
    }
}

impl<'a> MyTrait for &'a () {
    existential type AssocType: Send;
    fn ret(&self) -> Self::AssocType {
        ()
    }
}

trait MyLifetimeTrait<'a> {
    type AssocType: Send + 'a;
    fn ret(&self) -> Self::AssocType;
}

impl<'a> MyLifetimeTrait<'a> for &'a () {
    existential type AssocType: Send + 'a;
    fn ret(&self) -> Self::AssocType {
        *self
    }
}

error: could not find defining uses
  --> src/lib.rs:16:5
   |
16 |     existential type AssocType: Send;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
  --> src/lib.rs:28:5
   |
28 |     existential type AssocType: Send + 'a;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: hidden type `&'a ()` captures the lifetime 'a as defined on the impl at 27:6
  --> src/lib.rs:27:6
   |
27 | impl<'a> MyLifetimeTrait<'a> for &'a () {
   |      ^^

error: could not find defining uses
  --> src/lib.rs:28:5
   |
28 |     existential type AssocType: Send + 'a;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

cc @cramertj

(original report at #34511 (comment))

@rustbot modify labels: A-impl-trait C-bug