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

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Date:   Sat, 23 Sep 2023 11:20:15 -0300
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     Gary Guo <gary@...yguo.net>, rust-for-linux@...r.kernel.org,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

On Sat, 23 Sept 2023 at 11:15, Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> On Sat, Sep 23, 2023 at 4:11 PM Wedson Almeida Filho <wedsonaf@...il.com> wrote:
> >
> > On Fri, 22 Sept 2023 at 21:13, Gary Guo <gary@...yguo.net> wrote:
> > >
> > > On Thu, 21 Sep 2023 18:34:40 -0300
> > > Wedson Almeida Filho <wedsonaf@...il.com> wrote:
> > >
> > > > From: Wedson Almeida Filho <walmeida@...rosoft.com>
> > > >
> > > > 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@...il.com>
> > > > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > > > Signed-off-by: Wedson Almeida Filho <walmeida@...rosoft.com>
> > >
> > > Reviewed-by: Gary Guo <gary@...yguo.net>
> > >
> > > 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?