Re: [PATCH 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

[Wedson Almeida Filho]

On Sat, 23 Sept 2023 at 11:15, Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> On Sat, Sep 23, 2023 at 4:11 PM Wedson Almeida Filho <wedsonaf@xxxxxxxxx> wrote:
> >
> > On Fri, 22 Sept 2023 at 21:13, Gary Guo <gary@xxxxxxxxxxx> wrote:
> > >
> > > On Thu, 21 Sep 2023 18:34:40 -0300
> > > Wedson Almeida Filho <wedsonaf@xxxxxxxxx> wrote:
> > >
> > > > From: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
> > > >
> > > > With GATs, we don't need a separate type to represent a borrowed object
> > > > with a refcount, we can just use Rust's regular shared borrowing. In
> > > > this case, we use `&WithRef<T>` instead of `ArcBorrow<'_, T>`.
> > > >
> > > > Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > > Signed-off-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
> > >
> > > Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
> > >
> > > The implementation looks good to me, thanks Wedson.
> > >
> > > A minor thing that worth considering is to implement `AlwaysRefCounted`
> > > to `WithRef` and reimplement `Arc` with `ARef<TaskRef<T>>` or add
> > > conversion functions between them.
> > >
> > > It feels natural to have this this impl (because `WithRef` is indeed
> > > always ref counted), but on the other hand I don't currently foresee
> > > anyone to currently want to use this API :(
> >
> > Yes, I like the idea of defining `Arc<T>` as `ARef<WithRef<T>>`. My
> > concern had to do with error messages for type aliases but you or
> > Benno (or Bjorn?) had a link to plans from the compiler team to
> > improve this. And we're using type aliases for our locks anyway.
> >
> > So this change is in my queue. (As well as the changes to the guard we
> > discussed a couple of months ago.)
>
> Will this not cause issues with DispatchFromDyn and the Receiver
> trait? Maybe it's best to have Arc be a real type and just provide a
> conversion?

I don't think it will but I haven't really tried it yet so can't say for sure.

One thing I learned (obvious in hindsight) when doing the locks is
that we can have impl blocks for specialisations of a type, so
`DispatchFromDyn<Arc<T>>` will expand to
`DispatchFromDyn<ARef<WithRef<T>>>` and it should be ok.

Or are you worried about something else?