[PATCH] rust: kunit: avoid retain_mut for test attr filtering

[Elsanti]

Remove the unnecessary mutable borrow when stripping #[test] attributes:
retain() is sufficient to drop test markers while tracking their presence.
This keeps the logic compatible with the current MSRV and avoids extra
mutability.

Signed-off-by: Elsanti <santiagojoselea...@gmail.com>
---
 rust/macros/kunit.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rust/macros/kunit.rs b/rust/macros/kunit.rs
index e064419bfc10..083b8fe39e6c 100644
--- a/rust/macros/kunit.rs
+++ b/rust/macros/kunit.rs
@@ -88,7 +88,7 @@ pub(crate) fn kunit_tests(test_suite: Ident, mut module: 
ItemMod) -> Result<Toke
         };
 
         let mut had_test_attr = false;
-        f.attrs.retain_mut(|attr| {
+        f.attrs.retain(|attr| {
             let is_test = attr.path().is_ident("test");
             if is_test {
                 had_test_attr = true;
-- 
2.53.0