[PATCH] rust: kbuild: allow `unused_features`

[PATCH] rust: kbuild: allow `unused_features`

[Miguel Ojeda]

Starting with the upcoming Rust 1.96.0 (to be released 2026-05-28),
`rustc` introduces the new lint `unused_features` [1], which warns [2]:

    warning: feature `used_with_arg` is declared but not used
     --> <crate attribute>:1:93
      |
    1 | #![feature(asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,used_with_arg)]
      |                                                                                             ^^^^^^^^^^^^^
      |
      = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default

The original goal of using `-Zcrate-attr` automatically was that there
is a consistent set of features enabled and managed globally for all
Rust kernel code (modulo exceptions like the `rust/` crated).

While we could require crates to enable features manually (even if we
still keep the `-Zallow-features=` list, i.e. removing the `-Zcrate-attr`
list), it is not really worth making all developers worry about it just
for a new lint.

The features are expected to eventually become stable anyway (most already
did), and thus having to remove features in every file that may use them
is not worth it either.

Thus just allow the new lint globally.

The lint actually existed for a long time, which is why `rustc` does
not complain about an unknown lint in the stable versions we support,
but it was "disabled" years ago [3], and now it was made to work again.

For extra context, the new implementation of the lint has already been
improved to avoid linting about features that became stable thanks to
Benno's report and the ensuing discussion [4] [5], but while that helps,
it is still the case that we may have features enabled that are not used
for one reason or another in a particular crate.

Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://github.com/rust-lang/rust/pull/152164 [1]
Link: https://github.com/Rust-for-Linux/pin-init/pull/114 [2]
Link: https://github.com/rust-lang/rust/issues/44232 [3]
Link: https://github.com/rust-lang/rust/issues/153523 [4]
Link: https://github.com/rust-lang/rust/pull/153610 [5]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Makefile b/Makefile
index 73a39592f112..587345f16c97 100644
--- a/Makefile
+++ b/Makefile
@@ -476,6 +476,7 @@ KBUILD_USERLDFLAGS := $(USERLDFLAGS)
 export rust_common_flags := --edition=2021 \
 			    -Zbinary_dep_depinfo=y \
 			    -Astable_features \
+			    -Aunused_features \
 			    -Dnon_ascii_idents \
 			    -Dunsafe_op_in_unsafe_fn \
 			    -Wmissing_docs \

base-commit: 487f9b3dc6e507a982f1b984aa6bfbd9dc4b0567
-- 
2.53.0

Re: [PATCH] rust: kbuild: allow `unused_features`

[Gary Guo]

On Thu Mar 12, 2026 at 11:10 AM GMT, Miguel Ojeda wrote:
> Starting with the upcoming Rust 1.96.0 (to be released 2026-05-28),
> `rustc` introduces the new lint `unused_features` [1], which warns [2]:
> 
>     warning: feature `used_with_arg` is declared but not used
>      --> <crate attribute>:1:93
>       |
>     1 | #![feature(asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,used_with_arg)]
>       |                                                                                             ^^^^^^^^^^^^^
>       |
>       = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default
> 
> The original goal of using `-Zcrate-attr` automatically was that there
> is a consistent set of features enabled and managed globally for all
> Rust kernel code (modulo exceptions like the `rust/` crated).
> 
> While we could require crates to enable features manually (even if we
> still keep the `-Zallow-features=` list, i.e. removing the `-Zcrate-attr`
> list), it is not really worth making all developers worry about it just
> for a new lint.
> 
> The features are expected to eventually become stable anyway (most already
> did), and thus having to remove features in every file that may use them
> is not worth it either.
> 
> Thus just allow the new lint globally.
> 
> The lint actually existed for a long time, which is why `rustc` does
> not complain about an unknown lint in the stable versions we support,
> but it was "disabled" years ago [3], and now it was made to work again.
> 
> For extra context, the new implementation of the lint has already been
> improved to avoid linting about features that became stable thanks to
> Benno's report and the ensuing discussion [4] [5], but while that helps,
> it is still the case that we may have features enabled that are not used
> for one reason or another in a particular crate.
> 
> Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
> Link: https://github.com/rust-lang/rust/pull/152164 [1]
> Link: https://github.com/Rust-for-Linux/pin-init/pull/114 [2]
> Link: https://github.com/rust-lang/rust/issues/44232 [3]
> Link: https://github.com/rust-lang/rust/issues/153523 [4]
> Link: https://github.com/rust-lang/rust/pull/153610 [5]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)

Re: [PATCH] rust: kbuild: allow `unused_features`

[Benno Lossin]

On Thu Mar 12, 2026 at 12:10 PM CET, Miguel Ojeda wrote:
> Starting with the upcoming Rust 1.96.0 (to be released 2026-05-28),
> `rustc` introduces the new lint `unused_features` [1], which warns [2]:
>
>     warning: feature `used_with_arg` is declared but not used
>      --> <crate attribute>:1:93
>       |
>     1 | #![feature(asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,used_with_arg)]
>       |                                                                                             ^^^^^^^^^^^^^
>       |
>       = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default
>
> The original goal of using `-Zcrate-attr` automatically was that there
> is a consistent set of features enabled and managed globally for all
> Rust kernel code (modulo exceptions like the `rust/` crated).
>
> While we could require crates to enable features manually (even if we
> still keep the `-Zallow-features=` list, i.e. removing the `-Zcrate-attr`
> list), it is not really worth making all developers worry about it just
> for a new lint.
>
> The features are expected to eventually become stable anyway (most already
> did), and thus having to remove features in every file that may use them
> is not worth it either.
>
> Thus just allow the new lint globally.
>
> The lint actually existed for a long time, which is why `rustc` does
> not complain about an unknown lint in the stable versions we support,
> but it was "disabled" years ago [3], and now it was made to work again.
>
> For extra context, the new implementation of the lint has already been
> improved to avoid linting about features that became stable thanks to
> Benno's report and the ensuing discussion [4] [5], but while that helps,
> it is still the case that we may have features enabled that are not used
> for one reason or another in a particular crate.
>
> Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
> Link: https://github.com/rust-lang/rust/pull/152164 [1]
> Link: https://github.com/Rust-for-Linux/pin-init/pull/114 [2]
> Link: https://github.com/rust-lang/rust/issues/44232 [3]
> Link: https://github.com/rust-lang/rust/issues/153523 [4]
> Link: https://github.com/rust-lang/rust/pull/153610 [5]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Reviewed-by: Benno Lossin <lossin@kernel.org>

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/Makefile b/Makefile
> index 73a39592f112..587345f16c97 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -476,6 +476,7 @@ KBUILD_USERLDFLAGS := $(USERLDFLAGS)
>  export rust_common_flags := --edition=2021 \
>  			    -Zbinary_dep_depinfo=y \
>  			    -Astable_features \
> +			    -Aunused_features \
>  			    -Dnon_ascii_idents \
>  			    -Dunsafe_op_in_unsafe_fn \
>  			    -Wmissing_docs \
>
> base-commit: 487f9b3dc6e507a982f1b984aa6bfbd9dc4b0567

Re: [PATCH] rust: kbuild: allow `unused_features`

[Miguel Ojeda]

On Thu, Mar 12, 2026 at 12:10 PM Miguel Ojeda <ojeda@kernel.org> wrote:
>
> Starting with the upcoming Rust 1.96.0 (to be released 2026-05-28),
> `rustc` introduces the new lint `unused_features` [1], which warns [2]:
>
>     warning: feature `used_with_arg` is declared but not used
>      --> <crate attribute>:1:93
>       |
>     1 | #![feature(asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,used_with_arg)]
>       |                                                                                             ^^^^^^^^^^^^^
>       |
>       = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default
>
> The original goal of using `-Zcrate-attr` automatically was that there
> is a consistent set of features enabled and managed globally for all
> Rust kernel code (modulo exceptions like the `rust/` crated).
>
> While we could require crates to enable features manually (even if we
> still keep the `-Zallow-features=` list, i.e. removing the `-Zcrate-attr`
> list), it is not really worth making all developers worry about it just
> for a new lint.
>
> The features are expected to eventually become stable anyway (most already
> did), and thus having to remove features in every file that may use them
> is not worth it either.
>
> Thus just allow the new lint globally.
>
> The lint actually existed for a long time, which is why `rustc` does
> not complain about an unknown lint in the stable versions we support,
> but it was "disabled" years ago [3], and now it was made to work again.
>
> For extra context, the new implementation of the lint has already been
> improved to avoid linting about features that became stable thanks to
> Benno's report and the ensuing discussion [4] [5], but while that helps,
> it is still the case that we may have features enabled that are not used
> for one reason or another in a particular crate.
>
> Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
> Link: https://github.com/rust-lang/rust/pull/152164 [1]
> Link: https://github.com/Rust-for-Linux/pin-init/pull/114 [2]
> Link: https://github.com/rust-lang/rust/issues/44232 [3]
> Link: https://github.com/rust-lang/rust/issues/153523 [4]
> Link: https://github.com/rust-lang/rust/pull/153610 [5]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Applied to `rust-fixes` -- thanks everyone!

We were already discussing this in the links above, and it is fairly
trivial on our side, so I am putting it in already so that it goes
into the next fixes PR.

Cheers,
Miguel