Comment by NobodyNada

It can be done safely with an upgrade method that requires an owned read guard. The RwLock implementation provided by the parking_lot crate supports this safely:

    let lock = RwLock::new(Box::new(111));
    let read = lock.upgradable_read();
    let r: &i32 = &**read; // points to 111
    *RwLockUpgradableReadGuard::upgrade(read) = Box::new(222); // error[E0505]: cannot move out of `read` because it is borrowed
    println!("{}", *r);