error[E0502]: cannot borrow `db` as mutable because it is also borrowed as immutable
  --> tests/compile-fail/panic-when-reading-fields-of-tracked-structs-from-older-revisions.rs:22:21
   |
21 |     let tracked = tracked_fn(&db, input);
   |                              --- immutable borrow occurs here
22 |     input.set_field(&mut db).to(24);
   |                     ^^^^^^^ mutable borrow occurs here
23 |     tracked.field(&db); // tracked comes from prior revision
   |     ------- immutable borrow later used here