[PATCH v3] rust: alloc: allow coercion from `Box<T>` to `Box<dyn U>` if T implements U

[Alexandre Courbot]

This enables the creation of trait objects backed by a Box, similarly to
what can be done with the standard library.

Suggested-by: Benno Lossin <benno.lossin@xxxxxxxxx>
Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
---
>From this discussion on Zulip [1].

Heavily inspired from the similar feature on `Arc`.

[1] https://rust-for-linux.zulipchat.com/#narrow/channel/291565-Help/topic/Trait.20objects.3F/with/510689662
---
Changes in v3:
- Added trait object creation example.
- Link to v2: https://lore.kernel.org/r/20250411-box_trait_objs-v2-1-c5f31b8db847@xxxxxxxxxx

Changes in v2:
- Use where clauses to improve readability.
- Fix build with rustc 1.78.
- Link to v1: https://lore.kernel.org/r/20250408-box_trait_objs-v1-1-58d8e78b0fb2@xxxxxxxxxx
---
 rust/kernel/alloc/kbox.rs | 40 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index b77d32f3a58bab5ec73c612bdaaba0d79bfdff65..e043bf2f468774c4556e99d9d750f1596f0ed089 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -57,12 +57,50 @@
 /// assert!(KVBox::<Huge>::new_uninit(GFP_KERNEL).is_ok());
 /// ```
 ///
+/// [`Box`]es can also be used to store trait objects by coercing their type:
+///
+/// ```
+/// trait FooTrait {}
+///
+/// struct FooStruct;
+/// impl FooTrait for FooStruct {}
+///
+/// let _ = KBox::new(FooStruct, GFP_KERNEL)? as KBox<dyn FooTrait>;
+/// # Ok::<(), Error>(())
+/// ```
+///
 /// # Invariants
 ///
 /// `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)]
-pub struct Box<T: ?Sized, A: Allocator>(NonNull<T>, PhantomData<A>);
+#[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,
+{
+}
 
 /// Type alias for [`Box`] with a [`Kmalloc`] allocator.
 ///

---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20250408-box_trait_objs-02a700401f0b

Best regards,
-- 
Alexandre Courbot <acourbot@xxxxxxxxxx>