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

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Message-ID: <D93TGVDQS6B6.1V6CKR5QSEWF6@proton.me>
Date: Fri, 11 Apr 2025 12:43:11 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alexandre Courbot <acourbot@...dia.com>, Danilo Krummrich <dakr@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rust: alloc: allow coercion from `Box<T>` to `Box<dyn U>` if T implements U

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@...ton.me>
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>

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

> ---
> 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@nvidia.com
> ---
>  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

>  ///
>  /// ```