Re: [PATCH v2] rust: kunit: fix warning when !CONFIG_PRINTK

[Alexandre Courbot]

On Tue Feb 24, 2026 at 8:51 PM JST, Andreas Hindborg wrote:
> "Alexandre Courbot" <acourbot@xxxxxxxxxx> writes:
>
>> If `CONFIG_PRINTK` is not set, then the following warnings are issued
>> during build:
>>
>>   warning: unused variable: `args`
>>     --> ../rust/kernel/kunit.rs:16:12
>>     |
>>   16 | pub fn err(args: fmt::Arguments<'_>) {
>>     |            ^^^^ help: if this is intentional, prefix it with an underscore: `_args`
>>     |
>>     = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default
>>
>>   warning: unused variable: `args`
>>     --> ../rust/kernel/kunit.rs:32:13
>>     |
>>   32 | pub fn info(args: fmt::Arguments<'_>) {
>>     |             ^^^^ help: if this is intentional, prefix it with an underscore: `_args`
>>
>> Fix this by adding a no-op assignment using `args` when `CONFIG_PRINTK`
>> is not set.
>>
>> Fixes: a66d733da801 ("rust: support running Rust documentation tests as KUnit ones")
>> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
>> ---
>> Changes in v2:
>> - Use a no-op assignment instead of the wider `#[allow(unused_variables)]`.
>> - Link to v1: https://patch.msgid.link/20260224-unused_var_err-v1-1-c51d805eceb5@xxxxxxxxxx
>> ---
>>  rust/kernel/kunit.rs | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
>> index f93f24a60bdd..a1edf7491579 100644
>> --- a/rust/kernel/kunit.rs
>> +++ b/rust/kernel/kunit.rs
>> @@ -14,6 +14,10 @@
>>  /// Public but hidden since it should only be used from KUnit generated code.
>>  #[doc(hidden)]
>>  pub fn err(args: fmt::Arguments<'_>) {
>> +    // `args` is unused if `CONFIG_PRINTK` is not set - this avoids a build-time warning.
>> +    #[cfg(not(CONFIG_PRINTK))]
>> +    let _ = args;
>
> I think (didn't test) that you can use a conditional attribute [1] instead:
>
>  pub fn err(
>      #[cfg_attr(not(CONFIG_PRINTK), expect(unused))]
>      args: fmt::Arguments<'_>
>  ) {

Yup, that works as well, and I think I like it better as it is more
localized. Alice, WDYT?