error[E0506]: cannot assign to `p` because it is borrowed
  --> $DIR/mut_while_borrow.rs:9:5
   |
LL |     let r = foo(&p);
   |                 -- borrow of `p` occurs here
LL |     p += 1;
   |     ^^^^^^ assignment to borrowed `p` occurs here
LL |     println!("{}", r);
   |                    - borrow later used here

error: aborting due to previous error

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