rust: use derive(CoercePointee) on rustc >= 1.84.0

The `kernel` crate relies on both `coerce_unsized` and `dispatch_from_dyn`
unstable features.

Alice Ryhl has proposed [1] the introduction of the unstable macro
`SmartPointer` to reduce such dependence, along with a RFC patch [2].
Since Rust 1.81.0 this macro, later renamed to `CoercePointee` in
Rust 1.84.0 [3], has been fully implemented with the naming discussion
resolved.

This feature is now on track to stabilization in the language.
In order to do so, we shall start using this macro in the `kernel` crate
to prove the functionality and utility of the macro as the justification
of its stabilization.

This patch makes this switch in such a way that the crate remains
backward compatible with older Rust compiler versions,
via the new Kconfig option `RUSTC_HAS_COERCE_POINTEE`.

A minimal demonstration example is added to the
`samples/rust/rust_print_main.rs` module.

Link: https://rust-lang.github.io/rfcs/3621-derive-smart-pointer.html [1]
Link: https://lore.kernel.org/all/20240823-derive-smart-pointer-v1-1-53769cd37239@google.com/ [2]
Link: https://github.com/rust-lang/rust/pull/131284 [3]
Signed-off-by: Xiangfei Ding <dingxiangfei2009@gmail.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241203205050.679106-2-dingxiangfei2009@gmail.com
[ Fixed version to 1.84. Renamed option to `RUSTC_HAS_COERCE_POINTEE`
  to match `CC_HAS_*` ones. Moved up new config option, closer to the
  `CC_HAS_*` ones. Simplified Kconfig line. Fixed typos and slightly
  reworded example and commit. Added Link to PR. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/rust/kernel/list/arc.rs b/rust/kernel/list/arc.rs
index 3483d8c232c4..13c50df37b89 100644
--- a/rust/kernel/list/arc.rs
+++ b/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_RUSTC_HAS_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_RUSTC_HAS_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_RUSTC_HAS_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,
 {
 }