error: lifetime may not live long enough
  --> $DIR/match-ref-mut-let-invariance.rs:11:9
   |
LL | impl<'b> S<'b> {
   |      -- lifetime `'b` defined here
LL |     fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
   |            -- lifetime `'a` defined here
LL |         let ref mut x = self.0;
LL |         x
   |         ^ method was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
   |
   = help: consider adding the following bound: `'a: 'b`
   = note: requirement occurs because of a mutable reference to `&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