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

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Message-ID: <81ceeca9-8ae5-4a82-9a46-f47767e60f75@proton.me>
Date: Fri, 26 Jul 2024 14:42:36 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Boqun Feng <boqun.feng@...il.com>
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 26.07.24 16:26, Boqun Feng wrote:
> On Fri, Jul 26, 2024 at 01:43:38PM +0000, Benno Lossin wrote:
> [...]
>>>>
>>>> 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?
>>
>> I see... I am having a hard time coming up with an example where users
>> would exclusively want `ARef<Self>` though... What do you have in mind?
>> Normally types wrapped by `ARef` have `&self` methods.
>>
> 
> Having `&self` methods doesn't mean the necessarity of a `raw_ptr` ->
> `&Self` function, for example, a `Foo` is wrapped as follow:
> 
> 	struct Foo(Opaque<foo>);
> 	impl Foo {
> 	    pub fn bar(&self) -> Bar { ... }
> 	    pub unsafe fn get_foo(ptr: *mut foo) -> ARef<Foo> { ... }
> 	}
> 
> in this case, the abstration provider may not want user to get a
> `raw_ptr` -> `&Self` function, so no need to have it.

I don't understand this, why would the abstraction provider do that? The
user can already get a `&Foo` reference, so what's the harm having a
function supplying that directly?
I get the argument that you need to always convert to `ARef` if users
only use that, but when `Foo` provides `&self` methods, you're not
required to have an `ARef`.

---
Cheers,
Benno