Re: [PATCH v2] rust: alloc: allow coercion from `Box<T>` to `Box<dyn U>` if T implements U

[Benno Lossin]

On Fri Apr 11, 2025 at 2:07 PM CEST, Alexandre Courbot wrote:
> 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>

Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>

> ---
> 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 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 | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> index b77d32f3a58bab5ec73c612bdaaba0d79bfdff65..b9a905cd4bd285782b0db284b6771aec03e0c10b 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

I agree with Miguel, you could move the added line above down here and
also provide a simple example.

---
Cheers,
Benno

>  ///
>  /// ```