rust: sync: allow type of `self` to be `Arc<T>` or variants

This allows associated functions whose `self` argument has `Arc<T>` or
variants as their type. This, in turn, allows callers to use the dot
syntax to make calls.
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

@@ -14,6 +14,7 @@
 #![no_std]
 #![feature(allocator_api)]
 #![feature(core_ffi_c)]
+#![feature(receiver_trait)]
 
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.

rust/kernel/sync/arc.rs

@@ -57,6 +57,31 @@
 ///
 /// // The refcount drops to zero when `cloned` goes out of scope, and the memory is freed.
 /// ```
+///
+/// Using `Arc<T>` as the type of `self`:
+///
+/// ```
+/// use kernel::sync::Arc;
+///
+/// struct Example {
+///     a: u32,
+///     b: u32,
+/// }
+///
+/// impl Example {
+///     fn take_over(self: Arc<Self>) {
+///         // ...
+///     }
+///
+///     fn use_reference(self: &Arc<Self>) {
+///         // ...
+///     }
+/// }
+///
+/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
+/// obj.use_reference();
+/// obj.take_over();
+/// ```
 pub struct Arc<T: ?Sized> {
     ptr: NonNull<ArcInner<T>>,
     _p: PhantomData<ArcInner<T>>,
@@ -68,6 +93,9 @@ struct ArcInner<T: ?Sized> {
     data: T,
 }
 
+// This is to allow [`Arc`] (and variants) to be used as the type of `self`.
+impl<T: ?Sized> core::ops::Receiver 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