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

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Message-Id: <D91ACTUAWQTF.2AZ98BUA5ZKJ6@nvidia.com>
Date: Tue, 08 Apr 2025 22:19:12 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Miguel Ojeda" <miguel.ojeda.sandonis@...il.com>, "Benno Lossin"
 <benno.lossin@...ton.me>
Cc: "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>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rust: alloc: allow coercion from `Box<T>` to `Box<dyn
 U>` if T implements U

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@...ton.me> 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?

Thanks,
Alex.