Re: [RFC PATCH] rust: types: Add explanation for ARef pattern

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Jul 23, 2024 at 11:14:29AM +0200, Alice Ryhl wrote:
[...]
> > +/// ## `ARef<Self>` vs `&Self`
> > +///
> > +/// For an `impl AlwaysRefCounted` type, `ARef<Self>` represents an owner of one reference count,
> > +/// e.g.
> > +///
> > +/// ```ignore
> > +/// impl Foo {
> > +///     /// Gets a ref-counted reference of [`Self`].
> > +///     ///
> > +///     /// # Safety
> > +///     ///
> > +///     /// - `ptr` must be a valid pointer to `foo` with at least one reference count.
> > +///     pub unsafe fn from_ptr(ptr: *mut foo) -> ARef<Self> {
> > +///         // SAFETY: `ptr` is a valid pointer per function safety requirement. The cast is OK
> > +///         // since `foo` is transparent to `Foo`.
> > +///         //
> > +///         // Note: `.into()` here increases the reference count, so the returned value has its own
> > +///         // reference count.
> > +///         unsafe { &*(ptr.cast::<Foo>()) }.into()

So I did use the `&Self` -> `ARef<Self>` conversion here,

> > +///     }
> > +/// }
> > +/// ```
> > +///
> > +/// Another function that returns an `ARef<Self>` but with a different semantics is
> > +/// [`ARef::from_raw`]: it takes away the refcount of the input pointer, i.e. no refcount
> > +/// incrementation inside the function.
> > +///

and mentioned the difference between .into() and `ARef::from_raw()`.

> > +/// However `&Self` represents a reference to the object, and the lifetime of the **reference** is
> > +/// known at compile-time. E.g. the `Foo::as_ref()` above.
> > +///
> > +/// ## `impl Drop` of an `impl AlwaysRefCounted` should not touch the refcount
> > +///
> > +/// [`ARef`] descreases the refcount automatically (in [`ARef::drop`]) when it goes out of the
> > +/// scope, therefore there's no need to `impl Drop` for the type of objects (e.g. `Foo`) to decrease
> > +/// the refcount.
> >  pub struct ARef<T: AlwaysRefCounted> {
> >      ptr: NonNull<T>,
> >      _p: PhantomData<T>,
> > --
> > 2.45.2
> >
> 
> I think this is missing some basic information related to `&Self` ->
> `ARef<Self>` conversions. We should explain that these conversions are
> possible, and that you usually don't want `raw_ptr` -> `ARef<Self>` to
> increment the refcount - instead provide a `raw_ptr` -> `&Self` and
> convert the `&Self` to `ARef<Self>`.
> 

I could be more explicit on this, but could there be a case where a `T`
only wants to return `ARef<T>` as a public API? In other words, the
author of `T` doesn't want to expose an `-> &T` function, therefore a
`-> ARef<T>` function makes more sense? If all the users of `T` want to
operate on an `ARef<T>` other than `&T`, I think it makes sense, right?

Overall, I feel like we don't necessarily make a preference between
`->&Self` and `->ARef<Self>` functions here, since it's up to the users'
design?

Regards,
Boqun

> Alice