[PATCH 2/7] rust: kunit: support checked `-> Result`s in KUnit `#[test]`s

[Miguel Ojeda <ojeda@kernel.org>]

Currently, return values of KUnit `#[test]` functions are ignored.

Thus introduce support for `-> Result` functions by checking their
returned values.

At the same time, require that test functions return `()` or `Result<T,
E>`, which should avoid mistakes, especially with non-`#[must_use]`
types. Other types can be supported in the future if needed.

With this, a failing test like:

    #[test]
    fn my_test() -> Result {
        f()?;
        Ok(())
    }

will output:

    [    3.744214]     KTAP version 1
    [    3.744287]     # Subtest: my_test_suite
    [    3.744378]     # speed: normal
    [    3.744399]     1..1
    [    3.745817]     # my_test: ASSERTION FAILED at rust/kernel/lib.rs:321
    [    3.745817]     Expected is_test_result_ok(my_test()) to be true, but is false
    [    3.747152]     # my_test.speed: normal
    [    3.747199]     not ok 1 my_test
    [    3.747345] not ok 4 my_test_suite

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/kunit.rs | 25 +++++++++++++++++++++++++
 rust/macros/kunit.rs |  3 ++-
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
index 2659895d4c5d..f43e3ed460c2 100644
--- a/rust/kernel/kunit.rs
+++ b/rust/kernel/kunit.rs
@@ -164,6 +164,31 @@ macro_rules! kunit_assert_eq {
     }};
 }
 
+trait TestResult {
+    fn is_test_result_ok(&self) -> bool;
+}
+
+impl TestResult for () {
+    fn is_test_result_ok(&self) -> bool {
+        true
+    }
+}
+
+impl<T, E> TestResult for Result<T, E> {
+    fn is_test_result_ok(&self) -> bool {
+        self.is_ok()
+    }
+}
+
+/// Returns whether a test result is to be considered OK.
+///
+/// This will be `assert!`ed from the generated tests.
+#[doc(hidden)]
+#[expect(private_bounds)]
+pub fn is_test_result_ok(t: impl TestResult) -> bool {
+    t.is_test_result_ok()
+}
+
 /// Represents an individual test case.
 ///
 /// The [`kunit_unsafe_test_suite!`] macro expects a NULL-terminated list of valid test cases.
diff --git a/rust/macros/kunit.rs b/rust/macros/kunit.rs
index eb4f2afdbe43..9681775d160a 100644
--- a/rust/macros/kunit.rs
+++ b/rust/macros/kunit.rs
@@ -105,8 +105,9 @@ pub(crate) fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
     let path = crate::helpers::file();
     for test in &tests {
         let kunit_wrapper_fn_name = format!("kunit_rust_wrapper_{}", test);
+        // An extra `use` is used here to reduce the length of the message.
         let kunit_wrapper = format!(
-            "unsafe extern \"C\" fn {}(_test: *mut kernel::bindings::kunit) {{ {}(); }}",
+            "unsafe extern \"C\" fn {}(_test: *mut kernel::bindings::kunit) {{ use kernel::kunit::is_test_result_ok; assert!(is_test_result_ok({}())); }}",
             kunit_wrapper_fn_name, test
         );
         writeln!(kunit_macros, "{kunit_wrapper}").unwrap();
-- 
2.49.0