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

["Alexandre Courbot" <acourbot@nvidia.com>]

On Fri Apr 11, 2025 at 9:43 PM JST, Benno Lossin wrote:
> 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@proton.me>
>> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
>
> Reviewed-by: Benno Lossin <benno.lossin@proton.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.

Oops, I completely forgot to address Miguel's comment, sorry about that.
Will send a new version.