Re: [PATCH v4] rust: adding UniqueRefCounted and UniqueRef types

Alice Ryhl
2025-03-05T16:02:25Z

On Wed, Mar 5, 2025 at 4:39 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
> "Alice Ryhl" <aliceryhl@google.com> writes:
>
> > On Wed, Mar 5, 2025 at 3:56 PM Oliver Mangold <oliver.mangold@pm.me> wrote:
> >>
> >> Hi Alice,
> >>
> >> On 250305 1339, Alice Ryhl wrote:
> >> > On Wed, Mar 05, 2025 at 11:31:44AM +0000, Oliver Mangold wrote:
> >> >
> >> > > +impl<T: UniqueRefCounted> Deref for UniqueRef<T> {
> >> > > + type Target = T;
> >> > > +
> >> > > + fn deref(&self) -> &Self::Target {
> >> > > + // SAFETY: The type invariants guarantee that the object is valid.
> >> > > + unsafe { self.ptr.as_ref() }
> >> > > + }
> >> > > +}
> >> >
> >> > What stops people from doing this?
> >> >
> >> > let my_unique: UniqueRef<T> = ...;
> >> > let my_ref: &T = &*my_unique;
> >> > let my_shared: ARef<T> = ARef::from(my_ref);
> >> >
> >> > Now it is no longer unique.
> >> >
> >> Oh, indeed. That's a serious problem. I see 2 options to deal with that:
> >>
> >> 1. remove ARef::From<&T>
> >>
> >> I checked the users of this, and it looks to me like there is rather
> >> a limited number and they are easy to fix by replacing the &T with ARef<T>.
> >> But I assume that wouldn't be welcome as it is intrusive nonetheless
> >> and of course there is ergonomic value in having the function around.
> >
> > Definitely not an option. There are many users of this function that
> > are in the process of being upstreamed. The ability to go &T ->
> > ARef<T> is pretty fundamental for ARef.
>
> Not having `impl From<&T> for UniqueArc` seems to work out fine.
>
> It would be unfortunate if `impl From<&T> for ARef<T>` would prevent us
> from having a unique version of `ARef`. I would say that is a valid
> reason to consider removing that impl.

I think the impl is really important. It's required to do things such as:

let mm = ARef::from(&*current!().mm());

Without the impl (or something equivalent), it's not possible to
increment the refcount of the &Mm returned by `current!().mm()`. There
are many other examples of this.

Alice