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Date:   Sat, 31 Dec 2022 19:52:20 +0000
From:   Gary Guo <gary@garyguo.net>
To:     Wedson Almeida Filho <wedsonaf@gmail.com>
Cc:     rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 7/7] rust: sync: add support for dispatching on Arc and
 ArcBorrow.
Message-ID: <20221231195220.42e0893d.gary@garyguo.net>
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        <20221228060346.352362-7-wedsonaf@gmail.com>
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On Wed, 28 Dec 2022 06:03:46 +0000
Wedson Almeida Filho <wedsonaf@gmail.com> wrote:

> Trait objects (`dyn T`) require trait `T` to be "object safe". One of
> the requirements for "object safety" is that the receiver have one of
> the allowed types. This commit adds `Arc<T>` and `ArcBorrow<'_, T>` to
> the list of allowed types.
> 
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/lib.rs      |  1 +
>  rust/kernel/sync/arc.rs | 20 ++++++++++++++++++--
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 4bde65e7b06b..e0b0e953907d 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -15,6 +15,7 @@
>  #![feature(allocator_api)]
>  #![feature(coerce_unsized)]
>  #![feature(core_ffi_c)]
> +#![feature(dispatch_from_dyn)]
>  #![feature(receiver_trait)]
>  #![feature(unsize)]
>  
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index 832bafc74a90..ff73f9240ca1 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -92,9 +92,15 @@ use core::{
>  /// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
>  ///
>  /// ```
> -/// use kernel::sync::Arc;
> +/// use kernel::sync::{Arc, ArcBorrow};
> +///
> +/// trait MyTrait {
> +///     // Trait has a function whose `self` type is `Arc<Self>`.
> +///     fn example1(self: Arc<Self>) {}
>  ///
> -/// trait MyTrait {}
> +///     // Trait has a function whose `self` type is `ArcBorrow<'_, Self>`.
> +///     fn example2(self: ArcBorrow<'_, Self>) {}
> +/// }
>  ///
>  /// struct Example;
>  /// impl MyTrait for Example {}
> @@ -123,6 +129,9 @@ impl<T: ?Sized> core::ops::Receiver for Arc<T> {}
>  // dynamically-sized type (DST) `U`.
>  impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Arc<U>> for Arc<T> {}
>  
> +// This is to allow `Arc<U>` to be dispatched on when `Arc<T>` can be coerced into `Arc<U>`.
> +impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Arc<U>> for Arc<T> {}
> +
>  // SAFETY: It is safe to send `Arc<T>` to another thread when the underlying `T` is `Sync` because
>  // it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
>  // `T` to be `Send` because any thread that has an `Arc<T>` may ultimately access `T` directly, for
> @@ -297,6 +306,13 @@ pub struct ArcBorrow<'a, T: ?Sized + 'a> {
>  // This is to allow [`ArcBorrow`] (and variants) to be used as the type of `self`.
>  impl<T: ?Sized> core::ops::Receiver for ArcBorrow<'_, T> {}
>  
> +// This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
> +// `ArcBorrow<U>`.
> +impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
> +    for ArcBorrow<'_, T>
> +{
> +}
> +
>  impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
>      fn clone(&self) -> Self {
>          *self