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

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Message-ID: <ZqOyMbi7xl67UfjY@Boquns-Mac-mini.home>
Date: Fri, 26 Jul 2024 07:26:57 -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 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.

Regards,
Boqun

> Cheers,
> Benno
>