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

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Message-ID: <bcdd74f3-d0c0-4366-976a-5a935081a704@proton.me>
Date: Thu, 25 Jul 2024 18:12:56 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Boqun Feng <boqun.feng@...il.com>, Alice Ryhl <aliceryhl@...gle.com>
Cc: 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 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`.

> 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
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