error: lifetime may not live long enough
  --> $DIR/subset-relations.rs:10:5
   |
LL | fn missing_subset<'a, 'b>(x: &'a u32, y: &'b u32) -> &'a u32 {
   |                   --  -- lifetime `'b` defined here
   |                   |
   |                   lifetime `'a` defined here
LL |     y
   |     ^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
   |
   = help: consider adding the following bound: `'b: 'a`

error: aborting due to previous error