error[E0308]: mismatched types
  --> $DIR/match-ref-mut-invariance.rs:10:37
   |
LL |         match self.0 { ref mut x => x }
   |                                     ^ lifetime mismatch
   |
   = note: expected mutable reference `&'a mut &'a i32`
              found mutable reference `&'a mut &'b i32`
note: the lifetime `'a` as defined on the method body at 9:12...
  --> $DIR/match-ref-mut-invariance.rs:9:12
   |
LL |     fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
   |            ^^
note: ...does not necessarily outlive the lifetime `'b` as defined on the impl at 8:6
  --> $DIR/match-ref-mut-invariance.rs:8:6
   |
LL | impl<'b> S<'b> {
   |      ^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.