On Mon, Apr 22, 2024 at 11:08 AM Miguel Ojeda ojeda@kernel.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@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/n... [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@kernel.org
Reviewed-by: Alice Ryhl aliceryhl@google.com