[PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[FUJITA Tomonori]

On Sat,  8 Aug 2026 00:14:29 +0200
Miguel Ojeda <ojeda at kernel.org> wrote:

> On arm and loongarch64, or under `CONFIG_BUG=n`, the upcoming SRCU
> abstractions from commit
> 
>   59cf3a5bda61 ("rust: sync: add SRCU abstraction")
> 
> use `warn_on!`, which in turn warns:
> 
>     error: constant `WARN_ON_FLAGS` is never used
>        --> rust/kernel/bug.rs:126:19
>         |
>     126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>         |                     ^^^^^^^^^^^^^
>         |
>        ::: 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: `-D dead-code` implied by `-D warnings`
>         = help: to override `-D warnings` add `#[allow(dead_code)]`
>         = note: this error originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)
> 
> The reason is that `warn_flags!` discards both arguments in those cases,
> and so the `const` becomes unused. And since it is a macro, it went
> unnoticed so far.
> 
> Thus fix it by simplifying by removing the `const`.
> 
> Cc: stable at vger.kernel.org
> Fixes: dff64b072708 ("rust: Add warn_on macro")
> Signed-off-by: Miguel Ojeda <ojeda at kernel.org>
> ---
> Either this or adding an underscore.
> 
>  rust/kernel/bug.rs | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
> index ed943960f851..874adb93cdfe 100644
> --- a/rust/kernel/bug.rs
> +++ b/rust/kernel/bug.rs
> @@ -123,9 +123,10 @@ macro_rules! warn_on {
>          const _COND_STR: &str = file!();
> 
>          if cond {
> -            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,
> +                $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN)
> +            );
>          }
>          cond
>      }};
> 
> base-commit: dc01dfb37b34beeefcfe1c3055364d41a4070c7e
> --
> 2.55.0

I sent a fix for this bug a few days ago:

https://lore.kernel.org/all/20260801024841.786664-1-tomo@flapping.org/


If you prefer the simpler version, v1 might work.

https://lore.kernel.org/all/20260720035510.4048145-1-tomo@flapping.org/