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

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Message-ID: <CAJ-ks9=v8E_bV+oJ-bdm3KZW2dfrFdiCmeVLs+bgK8oVu6BCUA@mail.gmail.com>
Date: Sun, 4 May 2025 13:33:28 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Miguel Ojeda <ojeda@...nel.org>
Cc: Brendan Higgins <brendan.higgins@...ux.dev>, David Gow <davidgow@...gle.com>, 
	Alex Gaynor <alex.gaynor@...il.com>, Rae Moar <rmoar@...gle.com>, 
	linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH 2/7] rust: kunit: support checked `-> Result`s in KUnit `#[test]`s

On Fri, May 2, 2025 at 5:53 PM Miguel Ojeda <ojeda@...nel.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.

Why not restrict this to Result<(), E>?

>
> 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

Is it possible to include the error in the output?

>     [    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@...nel.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
>
>