rust: remove `RUSTC_HAS_COERCE_POINTEE` and simplify code

With the Rust version bump in place, the `RUSTC_HAS_COERCE_POINTEE`
Kconfig (automatic) option is always true.

Thus remove the option and simplify the code.

In particular, this includes removing our use of the predecessor unstable
features we used with Rust < 1.84.0 (`coerce_unsized`, `dispatch_from_dyn`
and `unsize`).

Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-11-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 2 insertions, 19 deletions

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 921e19333b895..18d6c0d62ce03 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -128,7 +128,7 @@ mod std_vendor;
 /// # Ok::<(), Error>(())
 /// ```
 #[repr(transparent)]
-#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, derive(core::marker::CoercePointee))]
+#[derive(core::marker::CoercePointee)]
 pub struct Arc<T: ?Sized> {
     ptr: NonNull<ArcInner<T>>,
     // NB: this informs dropck that objects of type `ArcInner<T>` may be used in `<Arc<T> as
@@ -182,15 +182,6 @@ 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`.
-#[cfg(not(CONFIG_RUSTC_HAS_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>`.
-#[cfg(not(CONFIG_RUSTC_HAS_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
 // `T` to be `Send` because any thread that has an `Arc<T>` may ultimately access `T` using a
@@ -547,20 +538,12 @@ impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
 /// # Ok::<(), Error>(())
 /// ```
 #[repr(transparent)]
-#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, derive(core::marker::CoercePointee))]
+#[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>`.
-#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
-impl<T: ?Sized + core::marker::Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
-    for ArcBorrow<'_, T>
-{
-}
-
 impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
     fn clone(&self) -> Self {
         *self