[PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[FUJITA Tomonori]

From: FUJITA Tomonori <fujita.tomonori@gmail.com>

Fix the following dead_code warning on some configurations in an
atomic development branch:

warning: constant `WARN_ON_FLAGS` is never used
   --> /linux/rust/kernel/bug.rs:126:19
    |
126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
    |                     ^^^^^^^^^^^^^
    |
   ::: /linux/rust/kernel/sync/srcu.rs:106:12
    |
106 |           if crate::warn_on!(
    |  ____________-
107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
108 | |             // and `srcu_readers_active()` only checks the active reader count.
109 | |             unsafe { bindings::srcu_readers_active(ptr) }
110 | |         ) {
    | |_________- in this macro invocation
    |
    = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
    = note: this warning originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)

The warn_on! macro always defines a WARN_ON_FLAGS constant and hands it
to warn_flags!. On configurations where warn_flags! does not reference
its flags argument (the LOONGARCH/ARM variant, which only calls
WARN_ON(), and the !CONFIG_BUG no-op variant), the constant is left
unused and triggers a dead_code warning.

The warning only surfaces for callers inside the kernel crate itself:
uses from other crates expand an external macro, so the lint is
suppressed there.

Rename the constant to _WARN_ON_FLAGS so the underscore marks it as
possibly-unused, matching the existing _COND_STR constant in the same
macro. The name is still referenceable, so configurations that do use
the flags are unaffected.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/bug.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
index ed943960f851..2fd7ee25abcb 100644
--- a/rust/kernel/bug.rs
+++ b/rust/kernel/bug.rs
@@ -123,9 +123,9 @@ macro_rules! warn_on {
         const _COND_STR: &str = file!();
 
         if cond {
-            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
+            const _WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
 
-            $crate::warn_flags!(_COND_STR, WARN_ON_FLAGS);
+            $crate::warn_flags!(_COND_STR, _WARN_ON_FLAGS);
         }
         cond
     }};

base-commit: 880c43b185ca52239e75bc546cc4f4d9154d0fed
-- 
2.43.0

Re: [PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[Boqun Feng]

On Mon, Jul 20, 2026 at 12:55:10PM +0900, FUJITA Tomonori wrote:
> From: FUJITA Tomonori <fujita.tomonori@gmail.com>
> 
> Fix the following dead_code warning on some configurations in an
> atomic development branch:
> 
> warning: constant `WARN_ON_FLAGS` is never used
>    --> /linux/rust/kernel/bug.rs:126:19
>     |
> 126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>     |                     ^^^^^^^^^^^^^
>     |
>    ::: /linux/rust/kernel/sync/srcu.rs:106:12
>     |
> 106 |           if crate::warn_on!(
>     |  ____________-
> 107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
> 108 | |             // and `srcu_readers_active()` only checks the active reader count.
> 109 | |             unsafe { bindings::srcu_readers_active(ptr) }
> 110 | |         ) {
>     | |_________- in this macro invocation
>     |
>     = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
>     = note: this warning originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)
> 
> The warn_on! macro always defines a WARN_ON_FLAGS constant and hands it
> to warn_flags!. On configurations where warn_flags! does not reference
> its flags argument (the LOONGARCH/ARM variant, which only calls
> WARN_ON(), and the !CONFIG_BUG no-op variant), the constant is left
> unused and triggers a dead_code warning.
> 
> The warning only surfaces for callers inside the kernel crate itself:
> uses from other crates expand an external macro, so the lint is
> suppressed there.
> 
> Rename the constant to _WARN_ON_FLAGS so the underscore marks it as
> possibly-unused, matching the existing _COND_STR constant in the same
> macro. The name is still referenceable, so configurations that do use
> the flags are unaffected.
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>

[Cc Paul]

Do we need a "Fixes" tag here?

FWIW:

Reviewed-by: Boqun Feng <boqun@kernel.org>

Regards,
Boqun

> ---
>  rust/kernel/bug.rs | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
> index ed943960f851..2fd7ee25abcb 100644
> --- a/rust/kernel/bug.rs
> +++ b/rust/kernel/bug.rs
> @@ -123,9 +123,9 @@ macro_rules! warn_on {
>          const _COND_STR: &str = file!();
>  
>          if cond {
> -            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> +            const _WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>  
> -            $crate::warn_flags!(_COND_STR, WARN_ON_FLAGS);
> +            $crate::warn_flags!(_COND_STR, _WARN_ON_FLAGS);
>          }
>          cond
>      }};
> 
> base-commit: 880c43b185ca52239e75bc546cc4f4d9154d0fed
> -- 
> 2.43.0
> 

Re: [PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[FUJITA Tomonori]

On Sun, 19 Jul 2026 22:54:24 -0700
Boqun Feng <boqun@kernel.org> wrote:

> On Mon, Jul 20, 2026 at 12:55:10PM +0900, FUJITA Tomonori wrote:
>> From: FUJITA Tomonori <fujita.tomonori@gmail.com>
>> 
>> Fix the following dead_code warning on some configurations in an
>> atomic development branch:
>> 
>> warning: constant `WARN_ON_FLAGS` is never used
>>    --> /linux/rust/kernel/bug.rs:126:19
>>     |
>> 126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>>     |                     ^^^^^^^^^^^^^
>>     |
>>    ::: /linux/rust/kernel/sync/srcu.rs:106:12
>>     |
>> 106 |           if crate::warn_on!(
>>     |  ____________-
>> 107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
>> 108 | |             // and `srcu_readers_active()` only checks the active reader count.
>> 109 | |             unsafe { bindings::srcu_readers_active(ptr) }
>> 110 | |         ) {
>>     | |_________- in this macro invocation
>>     |
>>     = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
>>     = note: this warning originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)
>> 
>> The warn_on! macro always defines a WARN_ON_FLAGS constant and hands it
>> to warn_flags!. On configurations where warn_flags! does not reference
>> its flags argument (the LOONGARCH/ARM variant, which only calls
>> WARN_ON(), and the !CONFIG_BUG no-op variant), the constant is left
>> unused and triggers a dead_code warning.
>> 
>> The warning only surfaces for callers inside the kernel crate itself:
>> uses from other crates expand an external macro, so the lint is
>> suppressed there.
>> 
>> Rename the constant to _WARN_ON_FLAGS so the underscore marks it as
>> possibly-unused, matching the existing _COND_STR constant in the same
>> macro. The name is still referenceable, so configurations that do use
>> the flags are unaffected.
>> 
>> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> 
> [Cc Paul]
> 
> Do we need a "Fixes" tag here?

Right, we likely do.

Fixes: dff64b072708 ("rust: Add warn_on macro")


> FWIW:
> 
> Reviewed-by: Boqun Feng <boqun@kernel.org>

Thanks!

Re: [PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[Paul E. McKenney]

On Sun, Jul 19, 2026 at 10:54:24PM -0700, Boqun Feng wrote:
> On Mon, Jul 20, 2026 at 12:55:10PM +0900, FUJITA Tomonori wrote:
> > From: FUJITA Tomonori <fujita.tomonori@gmail.com>
> > 
> > Fix the following dead_code warning on some configurations in an
> > atomic development branch:
> > 
> > warning: constant `WARN_ON_FLAGS` is never used
> >    --> /linux/rust/kernel/bug.rs:126:19
> >     |
> > 126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> >     |                     ^^^^^^^^^^^^^
> >     |
> >    ::: /linux/rust/kernel/sync/srcu.rs:106:12
> >     |
> > 106 |           if crate::warn_on!(
> >     |  ____________-
> > 107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
> > 108 | |             // and `srcu_readers_active()` only checks the active reader count.
> > 109 | |             unsafe { bindings::srcu_readers_active(ptr) }
> > 110 | |         ) {
> >     | |_________- in this macro invocation
> >     |
> >     = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
> >     = note: this warning originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)
> > 
> > The warn_on! macro always defines a WARN_ON_FLAGS constant and hands it
> > to warn_flags!. On configurations where warn_flags! does not reference
> > its flags argument (the LOONGARCH/ARM variant, which only calls
> > WARN_ON(), and the !CONFIG_BUG no-op variant), the constant is left
> > unused and triggers a dead_code warning.
> > 
> > The warning only surfaces for callers inside the kernel crate itself:
> > uses from other crates expand an external macro, so the lint is
> > suppressed there.
> > 
> > Rename the constant to _WARN_ON_FLAGS so the underscore marks it as
> > possibly-unused, matching the existing _COND_STR constant in the same
> > macro. The name is still referenceable, so configurations that do use
> > the flags are unaffected.
> > 
> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> 
> [Cc Paul]
> 
> Do we need a "Fixes" tag here?

Future bug-chasers would thank us for a "Fixes" tag.  ;-)

> FWIW:
> 
> Reviewed-by: Boqun Feng <boqun@kernel.org>

And thank you both for the fix.  I will keep the Rust-SRCU series.
Until further notice, anyway.  ;-)

							Thanx, Paul
> Regards,
> Boqun
> 
> > ---
> >  rust/kernel/bug.rs | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
> > index ed943960f851..2fd7ee25abcb 100644
> > --- a/rust/kernel/bug.rs
> > +++ b/rust/kernel/bug.rs
> > @@ -123,9 +123,9 @@ macro_rules! warn_on {
> >          const _COND_STR: &str = file!();
> >  
> >          if cond {
> > -            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> > +            const _WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> >  
> > -            $crate::warn_flags!(_COND_STR, WARN_ON_FLAGS);
> > +            $crate::warn_flags!(_COND_STR, _WARN_ON_FLAGS);
> >          }
> >          cond
> >      }};
> > 
> > base-commit: 880c43b185ca52239e75bc546cc4f4d9154d0fed
> > -- 
> > 2.43.0
> > 

Re: [PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[Gary Guo]

On Mon Jul 20, 2026 at 4:55 AM BST, FUJITA Tomonori wrote:
> From: FUJITA Tomonori <fujita.tomonori@gmail.com>
>
> Fix the following dead_code warning on some configurations in an
> atomic development branch:
>
> warning: constant `WARN_ON_FLAGS` is never used
>    --> /linux/rust/kernel/bug.rs:126:19
>     |
> 126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>     |                     ^^^^^^^^^^^^^
>     |
>    ::: /linux/rust/kernel/sync/srcu.rs:106:12
>     |
> 106 |           if crate::warn_on!(
>     |  ____________-
> 107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
> 108 | |             // and `srcu_readers_active()` only checks the active reader count.
> 109 | |             unsafe { bindings::srcu_readers_active(ptr) }
> 110 | |         ) {
>     | |_________- in this macro invocation
>     |
>     = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
>     = note: this warning originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> The warn_on! macro always defines a WARN_ON_FLAGS constant and hands it
> to warn_flags!. On configurations where warn_flags! does not reference
> its flags argument (the LOONGARCH/ARM variant, which only calls
> WARN_ON(), and the !CONFIG_BUG no-op variant), the constant is left
> unused and triggers a dead_code warning.
>
> The warning only surfaces for callers inside the kernel crate itself:
> uses from other crates expand an external macro, so the lint is
> suppressed there.
>
> Rename the constant to _WARN_ON_FLAGS so the underscore marks it as
> possibly-unused, matching the existing _COND_STR constant in the same
> macro. The name is still referenceable, so configurations that do use
> the flags are unaffected.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/kernel/bug.rs | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
> index ed943960f851..2fd7ee25abcb 100644
> --- a/rust/kernel/bug.rs
> +++ b/rust/kernel/bug.rs
> @@ -123,9 +123,9 @@ macro_rules! warn_on {
>          const _COND_STR: &str = file!();
>  
>          if cond {
> -            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> +            const _WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>  
> -            $crate::warn_flags!(_COND_STR, WARN_ON_FLAGS);
> +            $crate::warn_flags!(_COND_STR, _WARN_ON_FLAGS);

Hmm, why bother adding a const for this rather than just invoke

    $crate::warn_flags!(
        _COND_STR,
        const { ... }
    )

?

Also, the proper fix would be to mark it as used in the `warn_flags` macro
that discard the expression.

    if false {
        _ = $flags;
    }

Best,
Gary

>          }
>          cond
>      }};
>
> base-commit: 880c43b185ca52239e75bc546cc4f4d9154d0fed

Re: [PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[FUJITA Tomonori]

On Fri, 24 Jul 2026 00:40:04 +0100
"Gary Guo" <gary@garyguo.net> wrote:

>> diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
>> index ed943960f851..2fd7ee25abcb 100644
>> --- a/rust/kernel/bug.rs
>> +++ b/rust/kernel/bug.rs
>> @@ -123,9 +123,9 @@ macro_rules! warn_on {
>>          const _COND_STR: &str = file!();
>>  
>>          if cond {
>> -            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>> +            const _WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>>  
>> -            $crate::warn_flags!(_COND_STR, WARN_ON_FLAGS);
>> +            $crate::warn_flags!(_COND_STR, _WARN_ON_FLAGS);
> 
> Hmm, why bother adding a const for this rather than just invoke
> 
>     $crate::warn_flags!(
>         _COND_STR,
>         const { ... }
>     )
> 
> ?

Ah, `(const { ... })` works.


> Also, the proper fix would be to mark it as used in the `warn_flags` macro
> that discard the expression.
> 
>     if false {
>         _ = $flags;
>     }


Yeah, having the callee that discards the argument consume it is more
logical than working around it at the caller. I'll send v2 shortly.

The same reasoning applies to `$file`. Since there is no warning right
now, I'll rename `_COND_STR` to `COND_STR` and consume `$file` in a
separate patch for consistency.

Re: [PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[Gary Guo]

On Fri Jul 24, 2026 at 5:08 AM BST, FUJITA Tomonori wrote:
> On Fri, 24 Jul 2026 00:40:04 +0100
> "Gary Guo" <gary@garyguo.net> wrote:
>
>>> diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
>>> index ed943960f851..2fd7ee25abcb 100644
>>> --- a/rust/kernel/bug.rs
>>> +++ b/rust/kernel/bug.rs
>>> @@ -123,9 +123,9 @@ macro_rules! warn_on {
>>>          const _COND_STR: &str = file!();
>>>  
>>>          if cond {
>>> -            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>>> +            const _WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>>>  
>>> -            $crate::warn_flags!(_COND_STR, WARN_ON_FLAGS);
>>> +            $crate::warn_flags!(_COND_STR, _WARN_ON_FLAGS);
>> 
>> Hmm, why bother adding a const for this rather than just invoke
>> 
>>     $crate::warn_flags!(
>>         _COND_STR,
>>         const { ... }
>>     )
>> 
>> ?
>
> Ah, `(const { ... })` works.
>
>
>> Also, the proper fix would be to mark it as used in the `warn_flags` macro
>> that discard the expression.
>> 
>>     if false {
>>         _ = $flags;
>>     }
>
>
> Yeah, having the callee that discards the argument consume it is more
> logical than working around it at the caller. I'll send v2 shortly.
>
> The same reasoning applies to `$file`. Since there is no warning right
> now, I'll rename `_COND_STR` to `COND_STR` and consume `$file` in a
> separate patch for consistency.

Given that this is closely related to $flags I'd do it in a single patch.

Best,
Gary