Switch to CoercePointee macro, with examples

init/Kconfig

@@ -1978,6 +1978,9 @@ config RUST
 
 	  If unsure, say N.
 
+config RUST_COERCE_POINTEE
+	def_bool y if RUSTC_VERSION >= 108300
+
 config RUSTC_VERSION_TEXT
 	string
 	depends on RUST

rust/kernel/lib.rs

@@ -13,11 +13,12 @@
 
 #![no_std]
 #![feature(arbitrary_self_types)]
-#![feature(coerce_unsized)]
-#![feature(dispatch_from_dyn)]
+#![cfg_attr(CONFIG_RUST_COERCE_POINTEE, feature(derive_coerce_pointee))]
+#![cfg_attr(not(CONFIG_RUST_COERCE_POINTEE), feature(coerce_unsized))]
+#![cfg_attr(not(CONFIG_RUST_COERCE_POINTEE), feature(dispatch_from_dyn))]
+#![cfg_attr(not(CONFIG_RUST_COERCE_POINTEE), feature(unsize))]
 #![feature(inline_const)]
 #![feature(lint_reasons)]
-#![feature(unsize)]
 
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.

rust/kernel/list/arc.rs

@@ -7,7 +7,7 @@
 use crate::alloc::{AllocError, Flags};
 use crate::prelude::*;
 use crate::sync::{Arc, ArcBorrow, UniqueArc};
-use core::marker::{PhantomPinned, Unsize};
+use core::marker::PhantomPinned;
 use core::ops::Deref;
 use core::pin::Pin;
 use core::sync::atomic::{AtomicBool, Ordering};
@@ -159,6 +159,7 @@ pub use impl_list_arc_safe;
 ///
 /// [`List`]: crate::list::List
 #[repr(transparent)]
+#[cfg_attr(CONFIG_RUST_COERCE_POINTEE, derive(core::marker::CoercePointee))]
 pub struct ListArc<T, const ID: u64 = 0>
 where
     T: ListArcSafe<ID> + ?Sized,
@@ -443,18 +444,20 @@ where
 
 // This is to allow coercion from `ListArc<T>` to `ListArc<U>` if `T` can be converted to the
 // dynamically-sized type (DST) `U`.
+#[cfg(not(CONFIG_RUST_COERCE_POINTEE))]
 impl<T, U, const ID: u64> core::ops::CoerceUnsized<ListArc<U, ID>> for ListArc<T, ID>
 where
-    T: ListArcSafe<ID> + Unsize<U> + ?Sized,
+    T: ListArcSafe<ID> + core::marker::Unsize<U> + ?Sized,
     U: ListArcSafe<ID> + ?Sized,
 {
 }
 
 // This is to allow `ListArc<U>` to be dispatched on when `ListArc<T>` can be coerced into
 // `ListArc<U>`.
+#[cfg(not(CONFIG_RUST_COERCE_POINTEE))]
 impl<T, U, const ID: u64> core::ops::DispatchFromDyn<ListArc<U, ID>> for ListArc<T, ID>
 where
-    T: ListArcSafe<ID> + Unsize<U> + ?Sized,
+    T: ListArcSafe<ID> + core::marker::Unsize<U> + ?Sized,
     U: ListArcSafe<ID> + ?Sized,
 {
 }

rust/kernel/sync/arc.rs

@@ -26,7 +26,7 @@ use crate::{
 use core::{
     alloc::Layout,
     fmt,
-    marker::{PhantomData, Unsize},
+    marker::PhantomData,
     mem::{ManuallyDrop, MaybeUninit},
     ops::{Deref, DerefMut},
     pin::Pin,
@@ -125,6 +125,8 @@ mod std_vendor;
 /// let coerced: Arc<dyn MyTrait> = obj;
 /// # Ok::<(), Error>(())
 /// ```
+#[repr(transparent)]
+#[cfg_attr(CONFIG_RUST_COERCE_POINTEE, derive(core::marker::CoercePointee))]
 pub struct Arc<T: ?Sized> {
     ptr: NonNull<ArcInner<T>>,
     _p: PhantomData<ArcInner<T>>,
@@ -172,10 +174,12 @@ impl<T: ?Sized> ArcInner<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> {}
+#[cfg(not(CONFIG_RUST_COERCE_POINTEE))]
+impl<T: ?Sized + core::marker::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> {}
+#[cfg(not(CONFIG_RUST_COERCE_POINTEE))]
+impl<T: ?Sized + core::marker::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
@@ -471,14 +475,17 @@ impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
 /// obj.as_arc_borrow().use_reference();
 /// # Ok::<(), Error>(())
 /// ```
+#[repr(transparent)]
+#[cfg_attr(CONFIG_RUST_COERCE_POINTEE, derive(core::marker::CoercePointee))]
 pub struct ArcBorrow<'a, T: ?Sized + 'a> {
     inner: NonNull<ArcInner<T>>,
     _p: PhantomData<&'a ()>,
 }
 
 // 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>>
+#[cfg(not(CONFIG_RUST_COERCE_POINTEE))]
+impl<T: ?Sized + core::marker::Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
     for ArcBorrow<'_, T>
 {
 }

samples/rust/rust_print_main.rs

@@ -34,6 +34,21 @@ fn arc_print() -> Result {
     // Uses `dbg` to print, will move `c` (for temporary debugging purposes).
     dbg!(c);
 
+    {
+        use core::fmt::Display;
+        fn arc_dyn_print(arc: &Arc<dyn Display>) {
+            pr_info!("Arc<dyn Display> says {arc}");
+        }
+        // `Arc` can be used to delegate dynamic dispatch and the following is an example.
+        // Both `i32` and `&str` implements `Display`.
+        // This enables us to express a unified behaviour, contract or protocol
+        // on both `i32` and `&str` into a single `Arc` type `Arc<dyn Display>`.
+        let a_i32_display: Arc<dyn Display> = Arc::new(42i32, GFP_KERNEL)?;
+        let a_str_display: Arc<dyn Display> = a.clone();
+        arc_dyn_print(&a_i32_display);
+        arc_dyn_print(&a_str_display);
+    }
+
     // Pretty-prints the debug formatting with lower-case hexadecimal integers.
     pr_info!("{:#x?}", a);