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

[Benno Lossin <benno.lossin@proton.me>]

On 26.07.24 17:15, Boqun Feng wrote:
> On Fri, Jul 26, 2024 at 02:42:36PM +0000, Benno Lossin wrote:
>> 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
> 
> Because no user really needs to convert a `raw_ptr` to a `&Self` whose
> lifetime is limited to a scope?

What if you have this:

    unsafe extern "C" fn called_from_c_via_vtable(foo: *mut bindings::foo) {
        // SAFETY: ...
        let foo = unsafe { Foo::from_raw(foo) };
        foo.bar();
    }

In this case, there is no need to take a refcount on `foo`.

> Why do we provide a function if no one needs and the solely purpose is
> to just avoid providing another function?

I don't think that there should be a lot of calls to that function
anyways and thus I don't think there is value in providing two functions
for almost the same behavior. Since one can be derived by the other, I
would go for only implementing the first one.

>> user can already get a `&Foo` reference, so what's the harm having a
>> function supplying that directly?
> 
> Getting a `&Foo` from a `ARef<Foo>` is totally different than getting a
> `&Foo` from a pointer, right? And it's OK for an abstraction provider to
> want to avoid that.
> 
> Another example that you may not want to provide a `-> &Self` function
> is:
>  	struct Foo(Opaque<foo>);
>  	impl Foo {
>  	    pub fn bar(&self) -> Bar { ... }
>  	    pub fn find_foo(idx: u32) -> ARef<Foo> { ... }
>  	}
> 
> in other words, you have a query function (idx -> *mut foo), and I think
> in this case, you would avoid `find_foo(idx: u32) -> &Foo`, right?

Yes, this is the exception I had in mind with "if possible (ie there is
a parameter that the lifetime can bind to)" (in this case there wouldn't
be such a parameter).

> Honestly, this discussion has been going to a rabit hole. I will mention
> and already mentioned the conversion `&Self` -> `ARef<Self>`. Leaving
> the preference part blank is fine to me, since if it's a good practice,
> then everybody will follow, otherwise, we are missing something here.
> Just trying to not make a descision for the users...

Sure.

---
Cheers,
Benno