error[E0499]: cannot borrow `s.0` as mutable more than once at a time
  --> $DIR/fields-numeric-borrowck.rs:6:16
   |
LL |     let borrow1 = &mut s.0;
   |                   -------- first mutable borrow occurs here
LL |     let S { 0: ref mut borrow2 } = s;
   |                ^^^^^^^^^^^^^^^ second mutable borrow occurs here
...
LL |     borrow1.use_mut();
   |     ------- first borrow later used here

error: aborting due to 1 previous error

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