error[E0499]: cannot borrow `y.0` as mutable more than once at a time
  --> $DIR/borrowck-anon-fields-tuple.rs:27:10
   |
LL |         (ref mut a, _) => a
   |          --------- first mutable borrow occurs here
...
LL |         (ref mut b, _) => b
   |          ^^^^^^^^^ second mutable borrow occurs here
...
LL |     *a += 1;
   |     ------- first borrow later used here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0499`.
