[PATCH] 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>
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
---
 rust/kernel/alloc/kbox.rs | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index b77d32f3a58bab5ec73c612bdaaba0d79bfdff65..969b9f9fd3149685e1d1ecdf1eed9c647c887397 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -32,6 +32,8 @@
 ///
 /// When dropping a [`Box`], the value is also dropped and the heap memory is automatically freed.
 ///
+/// [`Box`]es can also be used to store trait objects by coercing their type.
+///
 /// # Examples
 ///
 /// ```
@@ -62,7 +64,17 @@
 /// `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<#[pointee] T: ?Sized, A: Allocator>(NonNull<T>, PhantomData<A>);
+
+// This is to allow coercion from `Box<T>` to `Box<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<Box<U>> for Box<T> {}
+
+// This is to allow `Box<U>` to be dispatched on when `Box<T>` can be coerced into `Box<U>`.
+#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
+impl<T: ?Sized + core::marker::Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Box<U>> for Box<T> {}
 
 /// Type alias for [`Box`] with a [`Kmalloc`] allocator.
 ///

---
base-commit: a2cc6ff5ec8f91bc463fd3b0c26b61166a07eb11
change-id: 20250408-box_trait_objs-02a700401f0b

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