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

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Message-ID: <ZqK1l05zcCwGforV@boqun-archlinux>
Date: Thu, 25 Jul 2024 13:29:11 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Jonathan Corbet <corbet@....net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Danilo Krummrich <dakr@...hat.com>,
	Trevor Gross <tmgross@...ch.edu>, gregkh@...uxfoundation.org
Subject: Re: [RFC PATCH] rust: types: Add explanation for ARef pattern

On Thu, Jul 25, 2024 at 06:12:56PM +0000, Benno Lossin wrote:
> On 24.07.24 19:44, Boqun Feng wrote:
> > On Tue, Jul 23, 2024 at 11:14:29AM +0200, Alice Ryhl wrote:
> >>> +/// 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?
> 
> You can always get a `&T` from `ARef<T>`, since it implements `Deref`.
> 

Yeah, but this is unrelated. I was talking about that API providers can
decide whether they want to only provide a `raw_ptr` -> `ARef<Self>` if
they don't need to provide a `raw_ptr` -> `&Self`.

> > 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?
> 
> I would argue that there should be a clear preference for functions
> returning `&Self` when possible (ie there is a parameter that the

If "possible" also means there's going to be `raw_ptr` -> `&Self`
function (as the same publicity level) anyway, then agreed. In other
words, if the users only need the `raw_ptr` -> `ARef<Self>`
functionality, we don't want to force people to provide a `raw_ptr` ->
`&Self` just because, right?

Regards,
Boqun

> lifetime can bind to). This is because then you get the two versions of
> the function (non-incrementing and incrementing) for the price of one
> function.
> 
> ---
> Cheers,
> Benno
>