Re: [PATCH v10 5/5] rust: Add OwnableRefCounted and SimpleOwnableRefCounted

[Oliver Mangold <oliver.mangold@pm.me>]

On 250502 1143, Alice Ryhl wrote:
> On Fri, May 02, 2025 at 09:03:04AM +0000, Oliver Mangold wrote:
> > +pub unsafe trait SimpleOwnableRefCounted: RefCounted {
> > +    /// Checks if exactly one [`ARef`] to the object exists. In case the object is [`Sync`], the
> > +    /// check needs to be race-free.
> > +    fn is_unique(&self) -> bool;
> > +}
> > +
> > +#[cfg_attr(RUSTC_HAS_DO_NOT_RECOMMEND, diagnostic::do_not_recommend)]
> > +// SAFETY: Safe by the requirements on implementation of [`SimpleOwnableRefCounted`].
> > +unsafe impl<T: SimpleOwnableRefCounted> OwnableRefCounted for T {
> > +    fn try_from_shared(this: ARef<Self>) -> Result<Owned<Self>, ARef<Self>> {
> > +        if T::is_unique(&*this) {
> > +            // SAFETY: Safe by the requirements on implementation of [`SimpleOwnable`].
> > +            Ok(unsafe { Owned::from_raw(ARef::into_raw(this)) })
> > +        } else {
> > +            Err(this)
> > +        }
> > +    }
> > +}
> > +
> > +#[cfg_attr(RUSTC_HAS_DO_NOT_RECOMMEND, diagnostic::do_not_recommend)]
> > +// SAFETY: Safe by the requirements on implementation of [`SimpleOwnableRefCounted`].
> > +unsafe impl<T: SimpleOwnableRefCounted> Ownable for T {
> > +    unsafe fn release(this: NonNull<Self>) {
> > +        // SAFETY: Safe by the requirements on implementation of
> > +        // [`SimpleOwnableRefCounted::dec_ref()`].
> > +        unsafe { RefCounted::dec_ref(this) };
> > +    }
> > +}
> 
> I wonder if this is too limiting. It will limit our ability to write
> other blanket impls for Ownable and OwnableRefCounted. Using e.g. a
> derive macro might be better?
> 
> Alice

You might be right. I don't have a strong opinion on the matter.

I'm not really familiar with procmacros, though. So maybe I should just do
away with SimpleOwnableRefCounted for now?

I mean Andreas doesn't need it for his current use case and this or a derive
macro can always be added later if it makes sense.

Best regards,

Oliver