error[E0499]: cannot borrow `my_struct.field` as mutable more than once at a time
  --> $DIR/return_from_loop.rs:20:9
   |
LL |     let value = &mut my_struct.field;
   |                 -------------------- first mutable borrow occurs here
LL |     loop {
LL |         my_struct.field.push_str("Hello, world!");
   |         ^^^^^^^^^^^^^^^ second mutable borrow occurs here
LL |
LL |         value.len();
   |         ----- first borrow later used here

error: aborting due to previous error

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