rust: bug: add a KUnit test for warn_on!

The test runs warn_on!(false) and warn_on!(true) inside a KUnit warning
suppression block, then checks that one warning was counted. The counter
is incremented by the kernel warning path, not by warn_on! itself, so a
count of one means the warning was really reported.

warn_flags! has a different definition per architecture. The test does not
look at any of them, so it runs on every architecture that supports Rust.

The new CONFIG_RUST_BUG_KUNIT_TEST depends on BUG. With CONFIG_BUG=n,
warn_on! does nothing and no warning is counted.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://patch.msgid.link/20260916140952.3290924-1-tomo@flapping.org
[ Added newlines for clarity. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/Kconfig.test

@@ -83,4 +83,15 @@ config RUST_BITFIELD_KUNIT_TEST
 
 	  If unsure, say N.
 
+config RUST_BUG_KUNIT_TEST
+	bool "KUnit tests for the Rust BUG/WARN functionality" if !KUNIT_ALL_TESTS
+	depends on BUG
+	default KUNIT_ALL_TESTS
+	help
+	  This option enables KUnit tests for the Rust BUG/WARN functionality.
+	  These are only for development and testing, not for regular
+	  kernel use cases.
+
+	  If unsure, say N.
+
 endif

rust/kernel/bug.rs

@@ -153,3 +153,33 @@ macro_rules! warn_on {
         cond
     }};
 }
+
+#[cfg(CONFIG_RUST_BUG_KUNIT_TEST)]
+#[macros::kunit_tests(rust_kernel_bug)]
+mod tests {
+    // The counter is incremented by the kernel warning path, not by `warn_on!`
+    // itself. A count of one means the warning was really reported.
+    #[test]
+    fn test_warn_on() {
+        // SAFETY: `kunit_get_current_test()` is always safe to call (it has
+        // fallbacks for when no KUnit test is running).
+        let test = unsafe { bindings::kunit_get_current_test() };
+
+        // SAFETY: This function runs only as a KUnit test case, so `test` is a
+        // valid pointer to the running test.
+        let handle = unsafe { bindings::kunit_start_suppress_warning(test) };
+
+        assert!(!warn_on!(false));
+        assert!(warn_on!(true));
+
+        // SAFETY: `kunit_suppressed_warning_count()` accepts any value returned by
+        // `kunit_start_suppress_warning()`.
+        let suppressed_count = unsafe { bindings::kunit_suppressed_warning_count(handle) };
+
+        // SAFETY: `test` is valid as above. `kunit_end_suppress_warning()` accepts any
+        // value returned by `kunit_start_suppress_warning()`.
+        unsafe { bindings::kunit_end_suppress_warning(test, handle) };
+
+        assert_eq!(suppressed_count, 1);
+    }
+}