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

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

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.

>> > Good question, I have been thinking about it, too. But it might
>> > be, that it isn't needed at all. As I understand, usually Rust wrappers
>> > are around non-movable C structs. Do we actually have a useful application
>> > for OwnableMut?
>> 
>> Also, do we even need two different traits? Which types would only
>> implement `Ownable` but not `OwnableMut`?
>
> I'm not 100% sure, but on a quick glance it looks indeed be safe to
> substitute `OwnableMut` by `Unpin`.

We just have to change the safety requirements of `OwnableMut`.

> If we add `get_pin_mut(&mut self) -> Pin<&mut T>` as Andreas suggested,
> it would be possible to obtain an `&mut T` anyway, then, if T is `Unpin`.

Well the `DerefMut` impl still is convenient in the `Unpin` case.

---
Cheers,
Benno