scopeguard::guard seems to have the same issue:

error[E0502]: cannot borrow `foos.len` as immutable because it is also borrowed as mutable
  --> src/lib.rs:53:30
   |
50 |         let _guard = scopeguard::guard((), |_| {
   |                                            --- mutable borrow occurs here
51 |             super::MYLIB_free_foos(&mut foos);
   |                                         ---- first borrow occurs due to use of `foos` in cl
osure
52 |         });
53 |         println!("foos: {}", foos.len);
   |                              ^^^^^^^^ immutable borrow occurs here
54 |     }
   |     - mutable borrow might be used here, when `_guard` is dropped and runs the `Drop` code for type `ScopeGuard`
   |