rust: sync: add support for dispatching on Arc and ArcBorrow.

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: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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)]
 

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 MyTrait {
+///     // Trait has a function whose `self` type is `Arc<Self>`.
+///     fn example1(self: Arc<Self>) {}
+///
+///     // 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