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

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

Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250502215133.1923676-3-ojeda@kernel.org
[ Used `::kernel` for paths. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/rust/macros/kunit.rs b/rust/macros/kunit.rs
index 971c2c3d849a..81d18149a0cc 100644
--- a/rust/macros/kunit.rs
+++ b/rust/macros/kunit.rs
@@ -102,8 +102,9 @@ 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 {kunit_wrapper_fn_name}(_test: *mut ::kernel::bindings::kunit) {{ {test}(); }}"
+            "unsafe extern \"C\" fn {kunit_wrapper_fn_name}(_test: *mut ::kernel::bindings::kunit) {{ use ::kernel::kunit::is_test_result_ok; assert!(is_test_result_ok({test}())); }}",
         );
         writeln!(kunit_macros, "{kunit_wrapper}").unwrap();
         writeln!(