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kbuild: rust: force alloc
extern to allow "empty" Rust files
commitded103c7eb
upstream. 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:2f7ab1267d
("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@google.com> Reviewed-by: Gary Guo <gary@garyguo.net> Link: https://lore.kernel.org/r/20240422090644.525520-1-ojeda@kernel.org Signed-off-by: Miguel Ojeda <ojeda@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@ -272,7 +272,7 @@ rust_common_cmd = \
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-Zallow-features=$(rust_allowed_features) \
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-Zallow-features=$(rust_allowed_features) \
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-Zcrate-attr=no_std \
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-Zcrate-attr=no_std \
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-Zcrate-attr='feature($(rust_allowed_features))' \
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-Zcrate-attr='feature($(rust_allowed_features))' \
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--extern alloc --extern kernel \
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-Zunstable-options --extern force:alloc --extern kernel \
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--crate-type rlib -L $(objtree)/rust/ \
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--crate-type rlib -L $(objtree)/rust/ \
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--crate-name $(basename $(notdir $@)) \
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--crate-name $(basename $(notdir $@)) \
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--sysroot=/dev/null \
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--sysroot=/dev/null \
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