error[E0499]: cannot borrow `x` as mutable more than once at a time
  --> $DIR/borrowck-autoref-3261.rs:15:9
   |
LL |     (&mut x).with(
   |     -------- ---- first borrow later used by call
   |     |
   |     first mutable borrow occurs here
LL |         |opt| {
   |         ^^^^^ second mutable borrow occurs here
...
LL |                     x = X(Either::Left((0, 0)));
   |                     - second borrow occurs due to use of `x` in closure

error: aborting due to previous error

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