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>

init/Kconfig

@@ -178,9 +178,6 @@ config LD_CAN_USE_KEEP_IN_OVERLAY
 	# https://github.com/llvm/llvm-project/pull/130661
 	def_bool LD_IS_BFD || LLD_VERSION >= 210000
 
-config RUSTC_HAS_COERCE_POINTEE
-	def_bool RUSTC_VERSION >= 108400
-
 config RUSTC_HAS_SPAN_FILE
 	def_bool RUSTC_VERSION >= 108800
 

rust/kernel/alloc/kbox.rs

@@ -77,33 +77,8 @@
 /// `self.0` is always properly aligned and either points to memory allocated with `A` or, for
 /// zero-sized types, is a dangling, well aligned pointer.
 #[repr(transparent)]
-#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, derive(core::marker::CoercePointee))]
-pub struct Box<#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, pointee)] T: ?Sized, A: Allocator>(
-    NonNull<T>,
-    PhantomData<A>,
-);
-
-// This is to allow coercion from `Box<T, A>` to `Box<U, A>` if `T` can be converted to the
-// dynamically-sized type (DST) `U`.
-#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
-impl<T, U, A> core::ops::CoerceUnsized<Box<U, A>> for Box<T, A>
-where
-    T: ?Sized + core::marker::Unsize<U>,
-    U: ?Sized,
-    A: Allocator,
-{
-}
-
-// This is to allow `Box<U, A>` to be dispatched on when `Box<T, A>` can be coerced into `Box<U,
-// A>`.
-#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
-impl<T, U, A> core::ops::DispatchFromDyn<Box<U, A>> for Box<T, A>
-where
-    T: ?Sized + core::marker::Unsize<U>,
-    U: ?Sized,
-    A: Allocator,
-{
-}
+#[derive(core::marker::CoercePointee)]
+pub struct Box<#[pointee] T: ?Sized, A: Allocator>(NonNull<T>, PhantomData<A>);
 
 /// Type alias for [`Box`] with a [`Kmalloc`] allocator.
 ///

rust/kernel/lib.rs

@@ -39,17 +39,11 @@
 //
 // Expected to become stable.
 #![feature(arbitrary_self_types)]
+#![feature(derive_coerce_pointee)]
 //
 // To be determined.
 #![feature(used_with_arg)]
 //
-// `feature(derive_coerce_pointee)` is expected to become stable. Before Rust
-// 1.84.0, it did not exist, so enable the predecessor features.
-#![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]
-#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
-#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
-#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(unsize))]
-//
 // `feature(file_with_nul)` is expected to become stable. Before Rust 1.89.0, it did not exist, so
 // enable it conditionally.
 #![cfg_attr(CONFIG_RUSTC_HAS_FILE_WITH_NUL, feature(file_with_nul))]

rust/kernel/list/arc.rs

@@ -160,7 +160,7 @@ fn try_new_list_arc(&self) -> bool {
 ///
 /// [`List`]: crate::list::List
 #[repr(transparent)]
-#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, derive(core::marker::CoercePointee))]
+#[derive(core::marker::CoercePointee)]
 pub struct ListArc<T, const ID: u64 = 0>
 where
     T: ListArcSafe<ID> + ?Sized,
@@ -443,26 +443,6 @@ fn as_ref(&self) -> &Arc<T> {
     }
 }
 
-// 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> + 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> + core::marker::Unsize<U> + ?Sized,
-    U: ListArcSafe<ID> + ?Sized,
-{
-}
-
 /// A utility for tracking whether a [`ListArc`] exists using an atomic.
 ///
 /// # Invariants

rust/kernel/sync/arc.rs

@@ -128,7 +128,7 @@
 /// # 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 @@ unsafe fn container_of(ptr: *const T) -> NonNull<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 @@ fn from(item: Pin<UniqueArc<T>>) -> Self {
 /// # 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