I think the reason it's not a compile-time error is that it's actually possible to select your ordering at runtime, like this:
use std::sync::atomic::*;
let x = AtomicU64::new(0);
let ordering = if rand::random() {
Ordering::Relaxed
} else {
Ordering::SeqCst
};
x.fetch_add(1, ordering);
Yes, I tried (a few months ago) mocking a PR for this that refactored the enum variants into ZST structs and that was the only sticking point for backwards compatibility.
