rust: sync: allow coercion from Arc<T> to Arc<U>

The coercion is only allowed if `U` is a compatible dynamically-sized
type (DST). For example, if we have some type `X` that implements trait
`Y`, then this allows `Arc<X>` to be coerced into `Arc<dyn Y>`.

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

@@ -13,8 +13,10 @@
 
 #![no_std]
 #![feature(allocator_api)]
+#![feature(coerce_unsized)]
 #![feature(core_ffi_c)]
 #![feature(receiver_trait)]
+#![feature(unsize)]
 
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.

rust/kernel/sync/arc.rs

@@ -17,7 +17,11 @@
 
 use crate::{bindings, error::Result, types::Opaque};
 use alloc::boxed::Box;
-use core::{marker::PhantomData, ops::Deref, ptr::NonNull};
+use core::{
+    marker::{PhantomData, Unsize},
+    ops::Deref,
+    ptr::NonNull,
+};
 
 /// A reference-counted pointer to an instance of `T`.
 ///
@@ -82,6 +86,23 @@
 /// obj.use_reference();
 /// obj.take_over();
 /// ```
+///
+/// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
+///
+/// ```
+/// use kernel::sync::Arc;
+///
+/// trait MyTrait {}
+///
+/// struct Example;
+/// impl MyTrait for Example {}
+///
+/// // `obj` has type `Arc<Example>`.
+/// let obj: Arc<Example> = Arc::try_new(Example)?;
+///
+/// // `coerced` has type `Arc<dyn MyTrait>`.
+/// let coerced: Arc<dyn MyTrait> = obj;
+/// ```
 pub struct Arc<T: ?Sized> {
     ptr: NonNull<ArcInner<T>>,
     _p: PhantomData<ArcInner<T>>,
@@ -96,6 +117,10 @@ struct ArcInner<T: ?Sized> {
 // This is to allow [`Arc`] (and variants) to be used as the type of `self`.
 impl<T: ?Sized> core::ops::Receiver for Arc<T> {}
 
+// This is to allow coercion from `Arc<T>` to `Arc<U>` if `T` can be converted to the
+// dynamically-sized type (DST) `U`.
+impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<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