Patch "kbuild: rust: force `alloc` extern to allow "empty" Rust files" has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    kbuild: rust: force `alloc` extern to allow "empty" Rust files

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kbuild-rust-force-alloc-extern-to-allow-empty-rust-f.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 6f98bb1610157b1e6d70bf2beff2f68dd428b3eb
Author: Miguel Ojeda <ojeda@xxxxxxxxxx>
Date:   Mon Apr 22 11:06:44 2024 +0200

    kbuild: rust: force `alloc` extern to allow "empty" Rust files
    
    [ Upstream commit ded103c7eb23753f22597afa500a7c1ad34116ba ]
    
    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@xxxxxxxxxxxxxxx # v6.6+
    Reported-by: Daniel Almeida <daniel.almeida@xxxxxxxxxxxxx>
    Reported-by: Julian Stecklina <julian.stecklina@xxxxxxxxxxxxxxxxxxxxx>
    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]
    Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
    Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20240422090644.525520-1-ojeda@xxxxxxxxxx
    Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 9ae02542b9389..6afc2af8f7e67 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -282,7 +282,7 @@ 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 $@)) \
 	--emit=dep-info=$(depfile)




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