error[E0506]: cannot assign to `fancy_num` because it is borrowed
  --> $DIR/E0506.rs:21:5
   |
LL |     let fancy_ref = &fancy_num;
   |                      --------- borrow of `fancy_num` occurs here
LL |     fancy_num = FancyNum { num: 6 }; //[ast]~ ERROR E0506
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `fancy_num` occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0506`.