error[E0621]: explicit lifetime required in the type of `x`
  --> $DIR/ex2a-push-one-existing-name-2.rs:16:12
   |
15 | fn foo<'a>(x: Ref<i32>, y: &mut Vec<Ref<'a, i32>>) {
   |            - consider changing the type of `x` to `Ref<'a, i32>`
16 |     y.push(x);
   |            ^ lifetime `'a` required

error: aborting due to previous error(s)