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

[Oliver Mangold]

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@xxxxx> 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.

> > 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`.

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`.