[PATCH v6 1/3] rust: kunit: add KUnit case and suite macros

[David Gow <davidgow@google.com>]

From: José Expósito <jose.exposito89@gmail.com>

Add a couple of Rust const functions and macros to allow to develop
KUnit tests without relying on generated C code:

 - The `kunit_unsafe_test_suite!` Rust macro is similar to the
   `kunit_test_suite` C macro. It requires a NULL-terminated array of
   test cases (see below).
 - The `kunit_case` Rust function is similar to the `KUNIT_CASE` C macro.
   It generates as case from the name and function.
 - The `kunit_case_null` Rust function generates a NULL test case, which
   is to be used as delimiter in `kunit_test_suite!`.

While these functions and macros can be used on their own, a future
patch will introduce another macro to create KUnit tests using a
user-space like syntax.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Co-developed-by: Matt Gilbride <mattgilbride@google.com>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
Co-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Co-developed-by: David Gow <davidgow@google.com>
Signed-off-by: David Gow <davidgow@google.com>
---

Changes since v5:
https://lore.kernel.org/all/20241213081035.2069066-2-davidgow@google.com/
- Rebased against 6.14-rc1
- Several documentation touch-ups, including noting that the example
  test function need not be unsafe. (Thanks, Miguel)
- Remove the need for static_mut_refs, by using core::addr_of_mut!()
  combined with a cast. (Thanks, Miguel)
  - While this should also avoid the need for const_mut_refs, it seems
    to have been enabled for other users anyway.
- Use ::kernel::ffi::c_char for C strings, rather than i8 directly.
  (Thanks, Miguel)

Changes since v4:
https://lore.kernel.org/linux-kselftest/20241101064505.3820737-2-davidgow@google.com/
- Rebased against 6.13-rc1
- Allowed an unused_unsafe warning after the behaviour of addr_of_mut!()
  changed in Rust 1.82. (Thanks Boqun, Miguel)
- Fix a couple of minor rustfmt issues which were triggering checkpatch
  warnings.

Changes since v3:
https://lore.kernel.org/linux-kselftest/20241030045719.3085147-4-davidgow@google.com/
- The kunit_unsafe_test_suite!() macro now panic!s if the suite name is
  too long, triggering a compile error. (Thanks, Alice!)

Changes since v2:
https://lore.kernel.org/linux-kselftest/20241029092422.2884505-2-davidgow@google.com/
- The kunit_unsafe_test_suite!() macro will truncate the name of the
  suite if it is too long. (Thanks Alice!)
- We no longer needlessly use UnsafeCell<> in
  kunit_unsafe_test_suite!(). (Thanks Alice!)

Changes since v1:
https://lore.kernel.org/lkml/20230720-rustbind-v1-1-c80db349e3b5@google.com/
- Rebase on top of rust-next
- As a result, KUnit attributes are new set. These are hardcoded to the
  defaults of "normal" speed and no module name.
- Split the kunit_case!() macro into two const functions, kunit_case()
  and kunit_case_null() (for the NULL terminator).

---
 rust/kernel/kunit.rs | 121 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 121 insertions(+)

diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
index 824da0e9738a..d34a92075174 100644
--- a/rust/kernel/kunit.rs
+++ b/rust/kernel/kunit.rs
@@ -161,3 +161,124 @@ macro_rules! kunit_assert_eq {
         $crate::kunit_assert!($name, $file, $diff, $left == $right);
     }};
 }
+
+/// Represents an individual test case.
+///
+/// The `kunit_unsafe_test_suite!` macro expects a NULL-terminated list of valid test cases.
+/// Use `kunit_case_null` to generate such a delimiter.
+#[doc(hidden)]
+pub const fn kunit_case(
+    name: &'static kernel::str::CStr,
+    run_case: unsafe extern "C" fn(*mut kernel::bindings::kunit),
+) -> kernel::bindings::kunit_case {
+    kernel::bindings::kunit_case {
+        run_case: Some(run_case),
+        name: name.as_char_ptr(),
+        attr: kernel::bindings::kunit_attributes {
+            speed: kernel::bindings::kunit_speed_KUNIT_SPEED_NORMAL,
+        },
+        generate_params: None,
+        status: kernel::bindings::kunit_status_KUNIT_SUCCESS,
+        module_name: core::ptr::null_mut(),
+        log: core::ptr::null_mut(),
+    }
+}
+
+/// Represents the NULL test case delimiter.
+///
+/// The `kunit_unsafe_test_suite!` macro expects a NULL-terminated list of test cases. This
+/// function returns such a delimiter.
+#[doc(hidden)]
+pub const fn kunit_case_null() -> kernel::bindings::kunit_case {
+    kernel::bindings::kunit_case {
+        run_case: None,
+        name: core::ptr::null_mut(),
+        generate_params: None,
+        attr: kernel::bindings::kunit_attributes {
+            speed: kernel::bindings::kunit_speed_KUNIT_SPEED_NORMAL,
+        },
+        status: kernel::bindings::kunit_status_KUNIT_SUCCESS,
+        module_name: core::ptr::null_mut(),
+        log: core::ptr::null_mut(),
+    }
+}
+
+/// Registers a KUnit test suite.
+///
+/// # Safety
+///
+/// `test_cases` must be a NULL terminated array of valid test cases.
+///
+/// # Examples
+///
+/// ```ignore
+/// extern "C" fn test_fn(_test: *mut kernel::bindings::kunit) {
+///     let actual = 1 + 1;
+///     let expected = 2;
+///     assert_eq!(actual, expected);
+/// }
+///
+/// static mut KUNIT_TEST_CASES: [kernel::bindings::kunit_case; 2] = [
+///     kernel::kunit::kunit_case(kernel::c_str!("name"), test_fn),
+///     kernel::kunit::kunit_case_null(),
+/// ];
+/// kernel::kunit_unsafe_test_suite!(suite_name, KUNIT_TEST_CASES);
+/// ```
+#[doc(hidden)]
+#[macro_export]
+macro_rules! kunit_unsafe_test_suite {
+    ($name:ident, $test_cases:ident) => {
+        const _: () = {
+            const KUNIT_TEST_SUITE_NAME: [::kernel::ffi::c_char; 256] = {
+                let name_u8 = ::core::stringify!($name).as_bytes();
+                let mut ret = [0; 256];
+
+                if name_u8.len() > 255 {
+                    panic!(concat!(
+                        "The test suite name `",
+                        ::core::stringify!($name),
+                        "` exceeds the maximum length of 255 bytes."
+                    ));
+                }
+
+                let mut i = 0;
+                while i < name_u8.len() {
+                    ret[i] = name_u8[i] as ::kernel::ffi::c_char;
+                    i += 1;
+                }
+
+                ret
+            };
+
+            #[allow(unused_unsafe)]
+            static mut KUNIT_TEST_SUITE: ::kernel::bindings::kunit_suite =
+                ::kernel::bindings::kunit_suite {
+                    name: KUNIT_TEST_SUITE_NAME,
+                    // SAFETY: User is expected to pass a correct `test_cases`, given the safety
+                    // precondition; hence this macro named `unsafe`.
+                    test_cases: unsafe {
+                        ::core::ptr::addr_of_mut!($test_cases)
+                            .cast::<::kernel::bindings::kunit_case>()
+                    },
+                    suite_init: None,
+                    suite_exit: None,
+                    init: None,
+                    exit: None,
+                    attr: ::kernel::bindings::kunit_attributes {
+                        speed: ::kernel::bindings::kunit_speed_KUNIT_SPEED_NORMAL,
+                    },
+                    status_comment: [0; 256usize],
+                    debugfs: ::core::ptr::null_mut(),
+                    log: ::core::ptr::null_mut(),
+                    suite_init_err: 0,
+                    is_init: false,
+                };
+
+            #[used]
+            #[link_section = ".kunit_test_suites"]
+            static mut KUNIT_TEST_SUITE_ENTRY: *const ::kernel::bindings::kunit_suite =
+                // SAFETY: `KUNIT_TEST_SUITE` is static.
+                unsafe { ::core::ptr::addr_of_mut!(KUNIT_TEST_SUITE) };
+        };
+    };
+}
-- 
2.48.1.601.g30ceb7b040-goog