error[E0502]: cannot borrow `foo` as mutable because it is also borrowed as immutable
  --> $DIR/borrowck-issue-49631.rs:20:9
   |
LL |     while let Some(Ok(string)) = foo.get() {
   |                                  --- immutable borrow occurs here
LL |         foo.mutate();
   |         ^^^^^^^^^^^^ mutable borrow occurs here
LL |
LL |         println!("foo={:?}", *string);
   |                              ------- immutable borrow later used here

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0502`.