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

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Date: Mon, 22 Apr 2024 11:06:44 +0200
From: Miguel Ojeda <ojeda@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Alex Gaynor <alex.gaynor@...il.com>
Cc: 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>,
	Alice Ryhl <aliceryhl@...gle.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: [PATCH] kbuild: rust: force `alloc` extern to allow "empty" Rust files

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>
---
 scripts/Makefile.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index baf86c0880b6..533a7799fdfe 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -273,7 +273,7 @@ rust_common_cmd = \
 	-Zallow-features=$(rust_allowed_features) \
 	-Zcrate-attr=no_std \
 	-Zcrate-attr='feature($(rust_allowed_features))' \
-	--extern alloc --extern kernel \
+	-Zunstable-options --extern force:alloc --extern kernel \
 	--crate-type rlib -L $(objtree)/rust/ \
 	--crate-name $(basename $(notdir $@)) \
 	--sysroot=/dev/null \

base-commit: 4cece764965020c22cff7665b18a012006359095
-- 
2.44.0