Re: [PATCH v10 1/5] rust: types: Add Ownable/Owned types

["Benno Lossin" <lossin@kernel.org>]

On Fri Jun 20, 2025 at 9:01 AM CEST, Oliver Mangold wrote:
> On 250618 2322, Benno Lossin wrote:
>> On Tue Jun 17, 2025 at 11:58 AM CEST, Oliver Mangold wrote:
>> > On 250514 1132, Benno Lossin wrote:
>> >> On Fri May 2, 2025 at 11:02 AM CEST, Oliver Mangold wrote:
>> >> > +///
>> >> > +/// # Safety
>> >> > +///
>> >> > +/// Implementers must ensure that:
>> >> > +/// - Any objects owned by Rust as [`Owned<T>`] stay alive while that owned reference exists (i.e.
>> >> > +///   until the [`release()`](Ownable::release) trait method is called).
>> >>
>> >> I don't immediately understand what this means. How about "Any value of
>> >> type `Self` needs to be stored as [`Owned<Self>`]."?
>> >
>> > Let me think. The safety requirements here talk about safety of
>> > implementing the trait.  But if you have a `Self` which is not wrapped, you
>> > still cannot create an `Owned<Self>` in safe code. It's different from an
>> > `AlwaysRefCounted`, where an `ARef<Self>` can be created from a `&Self`.
>> 
>> That might be true, but AFAIK this trait is designed to be used for
>> stuff that has a `create_foo` and `destroy_foo` function in C returning
>> and taking a raw pointer to `foo` respectively. So creating it on the
>> stack doesn't make sense.
>
> I didn't mean creating one on the stack, but keeping it in a raw pointer or
> `NonNull<T>`, not bothering to wrap in in an `Owned<T>`. But doesn't
> matter. In any case in v11 (which predates your answer), I moved this
> requirement to `Owned::from_raw()`, as, you asked below, which should be
> okay as that function is the only way to create an `Owned<T>`. But I can
> add the "needs to be stored as `Owned<Self>`" requirement, if you think it
> is important.

I'm not so sure, it depends on what we want `Owned<T>` to be. When I
take a look at v11, I might be able to figure it out.

---
Cheers,
Benno