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, useing 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 [email protected] # v6.6+

Reported-by Daniel Almeida <[email protected]>

Reported-by Julian Stecklina <[email protected]>

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 <[email protected]>

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, useing 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 [email protected] # v6.6+

Reported-by Daniel Almeida <[email protected]>

Reported-by Julian Stecklina <[email protected]>

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 <[email protected]>

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, useing 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 [email protected] # v6.6+

Reported-by Daniel Almeida <[email protected]>

Reported-by Julian Stecklina <[email protected]>

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 <[email protected]>

---

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

scripts/Makefile.build +1 -1

index baf86c0880b6..533a7799fdfe 100644
@@ -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