Re: [PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

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

On Mon Jul 20, 2026 at 4:55 AM BST, FUJITA Tomonori wrote:
> From: FUJITA Tomonori <fujita.tomonori@gmail.com>
>
> 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.
>
> The warning only surfaces for callers inside the kernel crate itself:
> uses from other crates expand an external macro, so the lint is
> suppressed there.
>
> Rename the constant to _WARN_ON_FLAGS so the underscore marks it as
> possibly-unused, matching the existing _COND_STR constant in the same
> macro. The name is still referenceable, so configurations that do use
> the flags are unaffected.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/kernel/bug.rs | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
> index ed943960f851..2fd7ee25abcb 100644
> --- a/rust/kernel/bug.rs
> +++ b/rust/kernel/bug.rs
> @@ -123,9 +123,9 @@ macro_rules! warn_on {
>          const _COND_STR: &str = file!();
>  
>          if cond {
> -            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> +            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, _WARN_ON_FLAGS);

Hmm, why bother adding a const for this rather than just invoke

    $crate::warn_flags!(
        _COND_STR,
        const { ... }
    )

?

Also, the proper fix would be to mark it as used in the `warn_flags` macro
that discard the expression.

    if false {
        _ = $flags;
    }

Best,
Gary

>          }
>          cond
>      }};
>
> base-commit: 880c43b185ca52239e75bc546cc4f4d9154d0fed