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

[Miguel Ojeda]

On Sat, Aug 8, 2026 at 12:14 AM 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>

Sorry, I forgot the Cc to Tomonori:

Cc: FUJITA Tomonori <fujita.tomonori at gmail.com>

Cheers,
Miguel