compare_and_swap(None, ...) for ArcSwapOption

[Thomas B]

It seems we can't use compare_and_swap(None) for ArcSwapOption:

lazy_static::lazy_static! {
	static ref DATA_STORE: ArcSwapOption<SomeStruct> = ArcSwapOption::empty();
}
/// # Panics
/// - If has already been initialized
pub fn init_loading(loading_fn: impl Fn() -> InternalResult<Vec<SomeData>> + Send + Sync + 'static) {
	assert!(DATA_STORE
		.compare_and_swap(
			None,
			Some(Arc::new(SomeStruct {
				data: Vec::new(),
				reload: Arc::new(loading_fn)
			}))
		)
		.is_none())
}

error[E0277]: the trait bound `std::option::Option<_>: AsRaw<KnownCertificatesAndReload>` is not satisfied
  --> file.rs:80:4
   |
80 |         .compare_and_swap(
   |          ^^^^^^^^^^^^^^^^ the trait `AsRaw<KnownCertificatesAndReload>` is not implemented for `std::option::Option<_>

It seems this is not an technical limitation but rather an interface limitation, as AtomicPtr has compare_and_swap for NULL.
Is there any way we could achieve this?

[Michal 'vorner' Vaner]

Yes, this is not intentional. I'll have a look, hopefully soon.

(As a side note, if this code isn't just a minimized example for demonstration, it seems very similar to what once_cell does)

[Michal 'vorner' Vaner]

Quick discovery: It's not implemented for Option<Arc<_>>, but it is for &Option<Arc<_>>, so &None::<Arc<_>> or std::ptr::null() will work.

I'll have to check first if there's a good reason why it isn't implemented for Option directly before adding the implementation.

[Thomas B]

(As a side note, if this code isn't just a minimized example for demonstration, it seems very similar to what once_cell does)

Hmm it is but looks like for my use-case I can indeed use a combination of OnceCell and ArcSwap instead of ArcSwap and Arc. Thanks! :)

[Michal 'vorner' Vaner]

So, the reason why it's not for owned is that I get conflicting implementations :-(. I've at least tried to improve the documentation.