error[E0502]: cannot borrow `*a` as mutable because `a` is also borrowed as immutable
  --> $DIR/E0502.rs:14:9
   |
LL |     let ref y = a;
   |         ----- immutable borrow occurs here
LL |     bar(a); //~ ERROR E0502
   |         ^ mutable borrow occurs here
LL | }
   | - immutable borrow ends here

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0502"