bug: fetch_update never return

[flisky]

MRE:

use std::{
    sync::atomic::Ordering,
    time::{Duration, Instant},
};

#[test]
fn test() {
    let last_time = atomic::Atomic::new(Instant::now());
    let mut count = 0;
    loop {
        let update = last_time.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |old| {
            let now = std::time::Instant::now();
            (now.duration_since(old) >= Duration::from_millis(1)).then_some(now)
        });
        if update.is_ok() {
            println!("update: {update:?}"); // comment this line to see the difference
            count += 1;
            if count > 1 {
                break;
            }
        }
    }
}

It seems that accessing update will make the following compare_exchange_weak always failing. Please correct me if it's not a bug.

---

system: Darwin MacBook-Pro.local 22.5.0 Darwin Kernel Version 22.5.0: Thu Jun 8 22:22:19 PDT 2023; root:xnu-8796.121.3~7/RELEASE_ARM64_T8103 arm64
cargo: cargo 1.73.0-nightly (7ac9416d8 2023-07-24)

[Amanieu]

Atomic<T> should only be used on types with no padding bytes. This is now enforced in 0.6.0.