error: lifetime may not live long enough
  --> $DIR/ex2e-push-inference-variable-3.rs:6:33
   |
LL | fn foo<'a, 'b, 'c>(x: &'a mut Vec<Ref<'b, i32>>, y: Ref<'c, i32>) {
   |            --  -- lifetime `'c` defined here
   |            |
   |            lifetime `'b` defined here
LL |     let a: &mut Vec<Ref<i32>> = x;
   |                                 ^ assignment requires that `'c` must outlive `'b`
   |
   = help: consider adding the following bound: `'c: 'b`
   = 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

error: aborting due to 1 previous error