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

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Message-ID: <CAH5fLgjoD+EVYCt-E_MVx2SA1RYRstEgFGXSQZe8aBbwVbLt+w@mail.gmail.com>
Date: Mon, 22 Apr 2024 11:13:00 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Alex Gaynor <alex.gaynor@...il.com>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Nicolas Schier <nicolas@...sle.eu>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	linux-kbuild@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev, stable@...r.kernel.org, 
	Daniel Almeida <daniel.almeida@...labora.com>, 
	Julian Stecklina <julian.stecklina@...erus-technology.de>
Subject: Re: [PATCH] kbuild: rust: force `alloc` extern to allow "empty" Rust files

On Mon, Apr 22, 2024 at 11:08 AM Miguel Ojeda <ojeda@...nel.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@...r.kernel.org # v6.6+
> Reported-by: Daniel Almeida <daniel.almeida@...labora.com>
> Reported-by: Julian Stecklina <julian.stecklina@...erus-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@...nel.org>

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