error: lifetime may not live long enough
  --> $DIR/issue-28848.rs:10:5
   |
LL | pub fn foo<'a, 'b>(u: &'b ()) -> &'a () {
   |            --  -- lifetime `'b` defined here
   |            |
   |            lifetime `'a` defined here
LL |     Foo::<'a, 'b>::xmute(u)
   |     ^^^^^^^^^^^^^^^^^^^^ requires that `'b` must outlive `'a`
   |
   = help: consider adding the following bound: `'b: 'a`

error: aborting due to previous error