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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@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: 2f7ab12 ("Kbuild: add Rust support") Link: rust-lang/rust#111302 [2] Link: rust-lang/rust#109421 [3] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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