Re: [PATCH v11 1/4] rust: types: Add Ownable/Owned types

[Oliver Mangold <oliver.mangold@pm.me>]

On 250819 1913, Benno Lossin wrote:
> On Tue Aug 19, 2025 at 10:53 AM CEST, Andreas Hindborg wrote:
> > "Benno Lossin" <lossin@kernel.org> writes:
> >> On Tue Aug 19, 2025 at 8:04 AM CEST, Oliver Mangold wrote:
> >>> On 250819 0027, Benno Lossin wrote:
> >>>> On Mon Aug 18, 2025 at 3:04 PM CEST, Oliver Mangold wrote:
> >>>> > On 250818 1446, Andreas Hindborg wrote:
> >>>> >> "Oliver Mangold" <oliver.mangold@pm.me> writes:
> >>>> >> > +impl<T: OwnableMut> DerefMut for Owned<T> {
> >>>> >> > +    fn deref_mut(&mut self) -> &mut Self::Target {
> >>>> >> > +        // SAFETY: The type invariants guarantee that the object is valid, and that we can safely
> >>>> >> > +        // return a mutable reference to it.
> >>>> >> > +        unsafe { self.ptr.as_mut() }
> >>>> >> > +    }
> >>>> >> > +}
> >>>> >>
> >>>> >> I think someone mentioned this before, but handing out mutable
> >>>> >> references can be a problem if `T: !Unpin`. For instance, we don't want
> >>>> >> to hand out `&mut Page` in case of `Owned<Page>`.
> >>>> >>
> >>>> >
> >>>> > That was the reason, why `OwnableMut` was introduced in the first place.
> >>>> > It's clear, I guess, that as-is it cannot be implemented on many classes.
> >>>>
> >>>> Yeah the safety requirements ensure that you can't implement it on
> >>>> `!Unpin` types.
> >>>>
> >>>> But I'm not sure it's useful then? As you said there aren't many types
> >>>> that will implement the type then, so how about we change the meaning
> >>>> and make it give out a pinned mutable reference instead?
> >>>
> >>> Making `deref_mut()` give out a pinned type won't work. The return types of
> >>> deref() are required to match.
> >>
> >> I meant the changes that Andreas suggested.
> >
> > Not sure what you are asking, but I need to assert exclusive access to
> > an `Page`. I could either get this by taking a `&mut Owned<Page>` or a
> > `Pin<&mut Page>`. I think the latter is more agnostic.
> 
> The former isn't really correct? It's like having a `&mut Box<Page>`
> which is weird. I was saying we can have a `DerefMut` impl gated on `T:
> Unpin` and a `fn get_pin_mut(&mut self) -> Pin<&mut T>`.

Yes. I think `Page` is the wrong example, as it already has owned semantics
and does its own cleanup. Wrapping it in an Owned would be redundant.

Best,

Oliver