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From:   Wedson Almeida Filho <wedsonaf@gmail.com>
To:     rust-for-linux@vger.kernel.org
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?= <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org,
        Wedson Almeida Filho <wedsonaf@gmail.com>
Subject: [PATCH 7/7] rust: sync: add support for dispatching on Arc and ArcBorrow.
Date:   Wed, 28 Dec 2022 06:03:46 +0000
Message-Id: <20221228060346.352362-7-wedsonaf@gmail.com>
In-Reply-To: <20221228060346.352362-1-wedsonaf@gmail.com>
References: <20221228060346.352362-1-wedsonaf@gmail.com>
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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>
---
 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
-- 
2.34.1