[PATCH 6.18.y] rust: bug: prevent dead_code warning from warn_on!'s flags constant

[Sasha Levin <sashal@kernel.org>]

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

[ Upstream commit e66cfc29e0d06fec34c06bb40d4d281f595677b1 ]

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.

warn_flags! is the macro that accepts (and here discards) the flags
argument, so make it responsible for the argument it drops.

Also rename `_COND_STR` to `COND_STR` and consume `$file` for consistency.

Fixes: dff64b072708 ("rust: Add warn_on macro")
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260801024841.786664-1-tomo@flapping.org
[ Added newlines. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
[ adapted unreachable flag consumption to the older one-argument warn_flags! interface. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 rust/kernel/bug.rs | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
index 36aef43e5ebec..9de7e96eb6791 100644
--- a/rust/kernel/bug.rs
+++ b/rust/kernel/bug.rs
@@ -94,6 +94,10 @@ macro_rules! warn_flags {
 #[cfg(all(CONFIG_BUG, any(CONFIG_LOONGARCH, CONFIG_ARM)))]
 macro_rules! warn_flags {
     ($flags:expr) => {
+        if false {
+            _ = $flags;
+        }
+
         // SAFETY: It is always safe to call `WARN_ON()`.
         unsafe { $crate::bindings::WARN_ON(true) }
     };
@@ -103,7 +107,11 @@ macro_rules! warn_flags {
 #[doc(hidden)]
 #[cfg(not(CONFIG_BUG))]
 macro_rules! warn_flags {
-    ($flags:expr) => {};
+    ($flags:expr) => {
+        if false {
+            _ = $flags;
+        }
+    };
 }
 
 #[doc(hidden)]
-- 
2.53.0