Re: [PATCH v4] rust: xarray: Add an abstraction for XArray

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Nov 27, 2023 at 02:51:33PM +0100, Alice Ryhl wrote:
> On Sun, Nov 26, 2023 at 2:13 PM Maíra Canal <mcanal@igalia.com> wrote:
> > +// Convenience impl for `ForeignOwnable` types whose `Borrowed`
> > +// form implements Deref.
> > +impl<'a, T: ForeignOwnable> Deref for Guard<'a, T>
> > +where
> > +    T::Borrowed<'static>: Deref,
> > +{
> > +    type Target = <T::Borrowed<'static> as Deref>::Target;
> > +
> > +    fn deref(&self) -> &Self::Target {
> > +        // SAFETY: See the `borrow()` method. The dereferenced `T::Borrowed` value
> > +        // must share the same lifetime, so we can return a reference to it.
> > +        unsafe { &*(T::borrow(self.0 as _).deref() as *const _) }
> > +    }
> > +}
> 
> I don't think this is sound. Deref could return a reference into the
> `T::Borrowed` itself, but it doesn't outlive this function. I would
> either omit this impl, or provide a sub-trait for ForeignOwnable that

Agreed. FWIW, there was some discussion around this:

	https://github.com/Rust-for-Linux/linux/pull/952#discussion_r1096791211

now think about it, how about the following?

	impl<'a, T: ForeignOwnable> Deref for Guard<'a, T>
	where
	    for<'b> T::Borrowed<'b>: Into<&'b T>,

	{
	    type Target = T;

	    fn deref(&self) -> &Self::Target {
		// SAFETY: See the `borrow()` method.
		unsafe { T::borrow(self.0 as _) }.into()
	    }
	}

Regards,
Boqun

> provides a borrow as a reference.
> 
> Alice