Re: [PATCH] kbuild: rust: force `alloc` extern to allow "empty" Rust files

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Apr 22, 2024 at 11:08 AM Miguel Ojeda <ojeda@kernel.org> wrote:
>
> If one attempts to build an essentially empty file somewhere in the
> kernel tree, it leads to a build error because the compiler does not
> recognize the `new_uninit` unstable feature:
>
>     error[E0635]: unknown feature `new_uninit`
>      --> <crate attribute>:1:9
>       |
>     1 | feature(new_uninit)
>       |         ^^^^^^^^^^
>
> The reason is that we pass `-Zcrate-attr='feature(new_uninit)'` (together
> with `-Zallow-features=new_uninit`) to let non-`rust/` code use that
> unstable feature.
>
> However, the compiler only recognizes the feature if the `alloc` crate
> is resolved (the feature is an `alloc` one). `--extern alloc`, which we
> pass, is not enough to resolve the crate.
>
> Introducing a reference like `use alloc;` or `extern crate alloc;`
> solves the issue, thus this is not seen in normal files. For instance,
> `use`ing the `kernel` prelude introduces such a reference, since `alloc`
> is used inside.
>
> While normal use of the build system is not impacted by this, it can still
> be fairly confusing for kernel developers [1], thus use the unstable
> `force` option of `--extern` [2] (added in Rust 1.71 [3]) to force the
> compiler to resolve `alloc`.
>
> This new unstable feature is only needed meanwhile we use the other
> unstable feature, since then we will not need `-Zcrate-attr`.
>
> Cc: stable@vger.kernel.org # v6.6+
> Reported-by: Daniel Almeida <daniel.almeida@collabora.com>
> Reported-by: Julian Stecklina <julian.stecklina@cyberus-technology.de>
> Closes: https://rust-for-linux.zulipchat.com/#narrow/stream/288089-General/topic/x/near/424096982 [1]
> Fixes: 2f7ab1267dc9 ("Kbuild: add Rust support")
> Link: https://github.com/rust-lang/rust/issues/111302 [2]
> Link: https://github.com/rust-lang/rust/pull/109421 [3]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>