[PATCH v2] rust: add mark_used macro

[FUJITA Tomonori <tomo@flapping.org>]

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

Macros whose expansion depends on the kernel configuration end up
ignoring some of their arguments, resulting in unused warnings.

Add a macro for the `if false { _ = ...; }` idiom used to avoid them,
and convert the existing open-coded users in `warn_flags!` and
`declare_trace!`.

Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
v2:
- Drop the comment in `declare_trace!`. The macro name says the same thing.
- Add Gary's Reviewed-by.
- Rebase on rust-next.
v1: https://lore.kernel.org/rust-for-linux/20260810075144.1245761-1-tomo@flapping.org/
---
 rust/kernel/bug.rs        | 18 ++++--------------
 rust/kernel/lib.rs        | 12 ++++++++++++
 rust/kernel/tracepoint.rs |  4 +---
 3 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
index 3566f0234ca4..c1e4eb7d4a70 100644
--- a/rust/kernel/bug.rs
+++ b/rust/kernel/bug.rs
@@ -55,9 +55,7 @@ macro_rules! warn_flags {
     ($file:expr, $flags:expr) => {
         const FLAGS: u32 = $crate::bindings::BUGFLAG_WARNING | $flags;
 
-        if false {
-            _ = $file;
-        }
+        $crate::mark_used!($file);
 
         // SAFETY:
         // - `flags` and `size` are all compile-time constants, preventing
@@ -83,9 +81,7 @@ macro_rules! warn_flags {
 #[cfg(all(CONFIG_BUG, CONFIG_UML))]
 macro_rules! warn_flags {
     ($file:expr, $flags:expr) => {
-        if false {
-            _ = $file;
-        }
+        $crate::mark_used!($file);
 
         // SAFETY: It is always safe to call `warn_slowpath_fmt()`
         // with a valid null-terminated string.
@@ -106,10 +102,7 @@ macro_rules! warn_flags {
 #[cfg(all(CONFIG_BUG, any(CONFIG_LOONGARCH, CONFIG_ARM)))]
 macro_rules! warn_flags {
     ($file:expr, $flags:expr) => {
-        if false {
-            _ = $file;
-            _ = $flags;
-        }
+        $crate::mark_used!($file, $flags);
 
         // SAFETY: It is always safe to call `WARN_ON()`.
         unsafe { $crate::bindings::WARN_ON(true) }
@@ -121,10 +114,7 @@ macro_rules! warn_flags {
 #[cfg(any(testlib, not(CONFIG_BUG)))]
 macro_rules! warn_flags {
     ($file:expr, $flags:expr) => {
-        if false {
-            _ = $file;
-            _ = $flags;
-        }
+        $crate::mark_used!($file, $flags);
     };
 }
 
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 4d5c96ddc49c..3b38e6b8196b 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -227,6 +227,18 @@ macro_rules! container_of {
 #[doc(hidden)]
 pub fn assert_same_type<T>(_: T, _: T) {}
 
+/// Marks the given expressions as used.
+///
+/// This avoids "unused" warnings in kernel configurations that do not otherwise use them.
+#[macro_export]
+macro_rules! mark_used {
+    ($($e:expr),* $(,)?) => {
+        if false {
+            $( _ = $e; )*
+        }
+    };
+}
+
 /// Helper for `.rs.S` files.
 #[doc(hidden)]
 #[macro_export]
diff --git a/rust/kernel/tracepoint.rs b/rust/kernel/tracepoint.rs
index c6e80aa99e8e..29d2d2d0aca8 100644
--- a/rust/kernel/tracepoint.rs
+++ b/rust/kernel/tracepoint.rs
@@ -38,9 +38,7 @@ macro_rules! declare_trace {
 
             #[cfg(not(CONFIG_TRACEPOINTS))]
             {
-                // If tracepoints are disabled, insert a trivial use of each argument
-                // to avoid unused argument warnings.
-                $( let _unused = $argname; )*
+                $crate::mark_used!($($argname),*);
             }
         }
     )*}

base-commit: 28924df2a08f440c73991b83028032c901de2ae4
-- 
2.43.0