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

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, May 05, 2025 at 02:02:09PM +0800, David Gow wrote:
> On Sat, 3 May 2025 at 05:51, Miguel Ojeda <ojeda@kernel.org> wrote:
> >
> > 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>
> > ---
> 
> This is a neat idea. I think a lot of tests will still want to go with
> the () return value, but having both as an option seems like a good
> idea to me.
> 

Handling the return value of a test is definitely a good thing, but I'm
not sure returning `Err` should be treated as assertion failure
though. Considering:

    #[test]
    fn foo() -> Result {
        let b = KBox::new(...)?; // need to allocate memory for the test
	use_box(b);
	assert!(...);
    }

if the test runs without enough memory, then KBox::new() would return a
Err(ENOMEM), and technically, it's not a test failure rather the test
has been skipped because of no enough resource.

I would suggest we handle the `Err` returns specially (mark as "SKIPPED"
maybe?).

Thoughts?

Regards,
Boqun

> Reviewed-by: David Gow <davidgow@google.com>
> 
> Cheers,
> -- David
> 
> 
> >  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
> >