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On the page you linked panics are only mentioned for load and store and the code below seems to work just fine?

    let x = atomic::AtomicU32::new(0);
    x.compare_exchange_weak(
        0,
        1,
        atomic::Ordering::AcqRel,
        atomic::Ordering::Relaxed).unwrap();
    println!("{}", x.load(atomic::Ordering::Relaxed));
Ah, you're right. I was using the same ordering for success and failure. It is possible to use AcqRel in the success case.
Looks like in C++ memory_order_acq_rel is treated like memory_order_acquire when it's a load and memory_order_release when it's a store. I would argue that this isn't really a difference in memory model but a difference in API.
I agree. Sorry for the misdirection - the panic is something I observed when I was experimenting with it, and I misinterpreted it.