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

[Miguel Ojeda <ojeda@kernel.org>]

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@vger.kernel.org
Fixes: dff64b072708 ("rust: Add warn_on macro")
Signed-off-by: Miguel Ojeda <ojeda@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