Re: [PATCH v1] rust: Add pr_*_once macros

[jens.korinth@tuta.io]

> Hmm... Isn't this derived from what Jens was working on in Zulip? (Cc'd)
>
Looks very similar. But more importantly it's missing the hotpath improvement suggested by Boqun Feng, which improves the perfomance significantly:

@_**Boqun Feng|411647** [said](https://rust-for-linux.zulipchat.com/#narrow/channel/291565-Help/topic/warn_once.2Fdo_once_lite_if/near/478155223):
````quote
FWIW, you can do this:
```diff
diff --git a/benches/once_cmp.rs b/benches/once_cmp.rs
index 322e420..1bfc9cd 100644
--- a/benches/once_cmp.rs
+++ b/benches/once_cmp.rs
@@ -124,7 +124,7 @@ macro_rules! do_once_lite_if_abool_swap {
     ($cond:expr, $e:expr) => {
         #[link_section = ".data.once"]
         static ALREADY_DONE: AtomicBool = AtomicBool::new(false);
-        if $cond && !ALREADY_DONE.swap(true, Ordering::Relaxed) {
+        if $cond && !ALREADY_DONE.load(Ordering::Relaxed) && !ALREADY_DONE.swap(true, Ordering::Relaxed) {
             $e;
         }
     }
```
to improve the hotpath, because you don't need to xchg() each time after you set the flag.
````

Jens