error: lifetime may not live long enough
  --> $DIR/ex2b-push-no-existing-names.rs:6:5
   |
LL | fn foo(x: &mut Vec<Ref<i32>>, y: Ref<i32>) {
   |        -                      - has type `Ref<'1, i32>`
   |        |
   |        has type `&mut Vec<Ref<'2, i32>>`
LL |     x.push(y);
   |     ^^^^^^^^^ argument requires that `'1` must outlive `'2`
   |
   = note: requirement occurs because of a mutable reference to `Vec<Ref<'_, i32>>`
   = note: mutable references are invariant over their type parameter
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
help: consider introducing a named lifetime parameter
   |
LL | fn foo<'a>(x: &mut Vec<Ref<'a, i32>>, y: Ref<'a, i32>) {
   |       ++++                 +++               +++

error: aborting due to 1 previous error