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

[Danilo Krummrich <dakr@kernel.org>]

On Tue, Apr 08, 2025 at 10:22:29AM +0000, Benno Lossin wrote:
> On Tue Apr 8, 2025 at 7:18 AM 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>
> > ---
> > 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> {}
> 
> You forgot to add the `A: Allocator` generic here and in the impl below.

Nit: Please use a where clause instead; I think it reads better and conforms
with the style of the rest of the file.