Re: [PATCH v1] rust: add mark_used macro

["Gary Guo" <gary@garyguo.net>]

On Mon Aug 10, 2026 at 8:51 AM BST, FUJITA Tomonori wrote:
> 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!`.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/bug.rs        | 18 ++++--------------
>  rust/kernel/lib.rs        | 12 ++++++++++++
>  rust/kernel/tracepoint.rs |  5 ++---
>  3 files changed, 18 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, $flags);
>      };
>  }
>  
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 9512af7156df..2f770d80bd8a 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -266,6 +266,18 @@ macro_rules! container_of {
>  #[doc(hidden)]
>  pub fn assert_same_type<T>(_: T, _: T) {}
>  
> +/// Marks the given expressions as used.
> +///
> +/// This avoids "unused" warnings in kernel configurations that do not otherwise use them.
> +#[macro_export]
> +macro_rules! mark_used {
> +    ($($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..cb06fec4c4d9 100644
> --- a/rust/kernel/tracepoint.rs
> +++ b/rust/kernel/tracepoint.rs
> @@ -38,9 +38,8 @@ 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; )*
> +                // If tracepoints are disabled, the arguments are unused.

nit: the comment seems redundant now that the macro name is self-documenting.

Best,
Gary

> +                $crate::mark_used!($($argname),*);
>              }
>          }
>      )*}
>
> base-commit: d8973c47544371613313ecb9a8c3e7b244aa9bbf