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

[Alice Ryhl]

On Tue, Apr 8, 2025 at 3:19 PM Alexandre Courbot <acourbot@xxxxxxxxxx> wrote:
>
> On Tue Apr 8, 2025 at 7:35 PM JST, Miguel Ojeda wrote:
> > On Tue, Apr 8, 2025 at 12:22 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> >>
> >> You forgot to add the `A: Allocator` generic here and in the impl below.
> >
> > Yeah, for this sort of changes (ideally, all patches, really), please
> > test with both the minimum supported version and the latest.
>
> Apologies for that, I had no idea how to build using a specific
> toolchain and did the wrong thing, which is sending without proper
> testing.
>
> I had some trouble finding how to 1) find out the minimum supported Rust
> version, and 2) how to build using a specific toolchain.
>
> For 1) I eventually found a hardcoded version in
> scripts/min-tool-version.sh ; and 2) is somehow documentated in the
> Quick Start guide that mentions the `rustup override set stable`
> command.
>
> I can send a patch against the Coding Guidelines adding a section to
> encourage testing against the minimum version and explain how to force a
> specific Rust version if you can confirm this would be helpful (and that
> min-tool-version.sh is the correct way to check the minimum supported
> Rust version).
>
> >
> > For instance, there is another issue, `#[pointee]` is only applicable
> > when you have the feature:
> >
> >     error: cannot find attribute `pointee` in this scope
> >      --> rust/kernel/alloc/kbox.rs:66:18
> >       |
> >     66 | pub struct Box<#[pointee] T: ?Sized, A:
> > Allocator>(NonNull<T>, PhantomData<A>);
>
> Mmm that one is trickier to address - I don't know of a way to make this
> `#[pointee]` attribute depedent on `CONFIG_RUSTC_HAS_COERCE_POINTEE`...
> Only solution I see if having two separate declarations for `Box`, and
> then we have the problem of bindings the rustdoc to the declaration that
> is picked at build time... Any idea for how to best address this?

This might work:
struct Box<#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, pointee)] T>