[PATCH v8 2/3] rust: macros: add macro to easily run KUnit tests

[David Gow <davidgow@google.com>]

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

Add a new procedural macro (`#[kunit_tests(kunit_test_suit_name)]`) to
run KUnit tests using a user-space like syntax.

The macro, that should be used on modules, transforms every `#[test]`
in a `kunit_case!` and adds a `kunit_unsafe_test_suite!` registering
all of them.

The only difference with user-space tests is that instead of using
`#[cfg(test)]`, `#[kunit_tests(kunit_test_suit_name)]` is used.

Note that `#[cfg(CONFIG_KUNIT)]` is added so the test module is not
compiled when `CONFIG_KUNIT` is set to `n`.

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Co-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: David Gow <davidgow@google.com>
---

No changes since v7:
https://lore.kernel.org/rust-for-linux/20250214074051.1619256-3-davidgow@google.com/

Changes since v6:
https://lore.kernel.org/rust-for-linux/20250214074051.1619256-3-davidgow@google.com/
- Fixed an [allow(unused_unsafe)] which ended up in patch 2 instead of
  patch 1. (Thanks, Tamir!)
- Add Tamir's Reviewed-by tag.

Changes since v5:
https://lore.kernel.org/all/20241213081035.2069066-3-davidgow@google.com/
- Fix some rustfmt-related formatting shenanigans. (Thanks, Miguel)
- Fix some documentation comment formatting as well. (Thanks, Miguel)
- Tidy up the generated code to avoid unneeded &mut[] everywhere.
  (Thanks, Miguel)
- Fix a new clippy warning for using .as_bytes().len() instead of .len()
  directly.

Changes since v4:
https://lore.kernel.org/linux-kselftest/20241101064505.3820737-3-davidgow@google.com/
- Rebased against 6.13-rc1
- "Expect" that the sample assert_eq!(1+1, 2) produces a clippy warning
  due to a redundant assertion. (Thanks Boqun, Miguel)

Changes since v3:
https://lore.kernel.org/linux-kselftest/20241030045719.3085147-6-davidgow@google.com/
- The #[kunit_tests()] macro now preserves span information, so
  errors can be better reported. (Thanks, Boqun!)
- The example test has been replaced to no longer use assert_eq!() with
  a constant bool argument (which triggered a clippy warning now we
  have the span info). It now checks 1 + 1 == 2, to match the C example.
  - (The in_kunit_test() test in the next patch uses assert!() to check
    a bool, so having something different here seemed to make a better
    example.)

Changes since v2:
https://lore.kernel.org/linux-kselftest/20241029092422.2884505-3-davidgow@google.com/
- Include missing rust/macros/kunit.rs file from v2. (Thanks Boqun!)
- The proc macro now emits an error if the suite name is too long.

Changes since v1:
https://lore.kernel.org/lkml/20230720-rustbind-v1-2-c80db349e3b5@google.com/
- Rebased on top of rust-next
- Make use of the new const functions, rather than the kunit_case!()
  macro.

---
 MAINTAINERS          |   1 +
 rust/kernel/kunit.rs |  11 +++
 rust/macros/kunit.rs | 161 +++++++++++++++++++++++++++++++++++++++++++
 rust/macros/lib.rs   |  29 ++++++++
 4 files changed, 202 insertions(+)
 create mode 100644 rust/macros/kunit.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index c8d9e8187eb0..4e7a6d2f2c49 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -12677,6 +12677,7 @@ F:	Documentation/dev-tools/kunit/
 F:	include/kunit/
 F:	lib/kunit/
 F:	rust/kernel/kunit.rs
+F:	rust/macros/kunit.rs
 F:	scripts/rustdoc_test_*
 F:	tools/testing/kunit/
 
diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
index 1b9b52892562..97e99b52e4a9 100644
--- a/rust/kernel/kunit.rs
+++ b/rust/kernel/kunit.rs
@@ -40,6 +40,8 @@ pub fn info(args: fmt::Arguments<'_>) {
     }
 }
 
+use macros::kunit_tests;
+
 /// Asserts that a boolean expression is `true` at runtime.
 ///
 /// Public but hidden since it should only be used from generated tests.
@@ -285,3 +287,12 @@ macro_rules! kunit_unsafe_test_suite {
         };
     };
 }
+
+#[kunit_tests(rust_kernel_kunit)]
+mod tests {
+    #[test]
+    fn rust_test_kunit_example_test() {
+        #![expect(clippy::eq_op)]
+        assert_eq!(1 + 1, 2);
+    }
+}
diff --git a/rust/macros/kunit.rs b/rust/macros/kunit.rs
new file mode 100644
index 000000000000..4f553ecf40c0
--- /dev/null
+++ b/rust/macros/kunit.rs
@@ -0,0 +1,161 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Procedural macro to run KUnit tests using a user-space like syntax.
+//!
+//! Copyright (c) 2023 José Expósito <jose.exposito89@gmail.com>
+
+use proc_macro::{Delimiter, Group, TokenStream, TokenTree};
+use std::fmt::Write;
+
+pub(crate) fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
+    let attr = attr.to_string();
+
+    if attr.is_empty() {
+        panic!("Missing test name in `#[kunit_tests(test_name)]` macro")
+    }
+
+    if attr.len() > 255 {
+        panic!(
+            "The test suite name `{}` exceeds the maximum length of 255 bytes",
+            attr
+        )
+    }
+
+    let mut tokens: Vec<_> = ts.into_iter().collect();
+
+    // Scan for the `mod` keyword.
+    tokens
+        .iter()
+        .find_map(|token| match token {
+            TokenTree::Ident(ident) => match ident.to_string().as_str() {
+                "mod" => Some(true),
+                _ => None,
+            },
+            _ => None,
+        })
+        .expect("`#[kunit_tests(test_name)]` attribute should only be applied to modules");
+
+    // Retrieve the main body. The main body should be the last token tree.
+    let body = match tokens.pop() {
+        Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Brace => group,
+        _ => panic!("Cannot locate main body of module"),
+    };
+
+    // Get the functions set as tests. Search for `[test]` -> `fn`.
+    let mut body_it = body.stream().into_iter();
+    let mut tests = Vec::new();
+    while let Some(token) = body_it.next() {
+        match token {
+            TokenTree::Group(ident) if ident.to_string() == "[test]" => match body_it.next() {
+                Some(TokenTree::Ident(ident)) if ident.to_string() == "fn" => {
+                    let test_name = match body_it.next() {
+                        Some(TokenTree::Ident(ident)) => ident.to_string(),
+                        _ => continue,
+                    };
+                    tests.push(test_name);
+                }
+                _ => continue,
+            },
+            _ => (),
+        }
+    }
+
+    // Add `#[cfg(CONFIG_KUNIT)]` before the module declaration.
+    let config_kunit = "#[cfg(CONFIG_KUNIT)]".to_owned().parse().unwrap();
+    tokens.insert(
+        0,
+        TokenTree::Group(Group::new(Delimiter::None, config_kunit)),
+    );
+
+    // Generate the test KUnit test suite and a test case for each `#[test]`.
+    // The code generated for the following test module:
+    //
+    // ```
+    // #[kunit_tests(kunit_test_suit_name)]
+    // mod tests {
+    //     #[test]
+    //     fn foo() {
+    //         assert_eq!(1, 1);
+    //     }
+    //
+    //     #[test]
+    //     fn bar() {
+    //         assert_eq!(2, 2);
+    //     }
+    // }
+    // ```
+    //
+    // Looks like:
+    //
+    // ```
+    // unsafe extern "C" fn kunit_rust_wrapper_foo(_test: *mut kernel::bindings::kunit) { foo(); }
+    // unsafe extern "C" fn kunit_rust_wrapper_bar(_test: *mut kernel::bindings::kunit) { bar(); }
+    //
+    // static mut TEST_CASES: [kernel::bindings::kunit_case; 3] = [
+    //     kernel::kunit::kunit_case(kernel::c_str!("foo"), kunit_rust_wrapper_foo),
+    //     kernel::kunit::kunit_case(kernel::c_str!("bar"), kunit_rust_wrapper_bar),
+    //     kernel::kunit::kunit_case_null(),
+    // ];
+    //
+    // kernel::kunit_unsafe_test_suite!(kunit_test_suit_name, TEST_CASES);
+    // ```
+    let mut kunit_macros = "".to_owned();
+    let mut test_cases = "".to_owned();
+    for test in &tests {
+        let kunit_wrapper_fn_name = format!("kunit_rust_wrapper_{}", test);
+        let kunit_wrapper = format!(
+            "unsafe extern \"C\" fn {}(_test: *mut kernel::bindings::kunit) {{ {}(); }}",
+            kunit_wrapper_fn_name, test
+        );
+        writeln!(kunit_macros, "{kunit_wrapper}").unwrap();
+        writeln!(
+            test_cases,
+            "    kernel::kunit::kunit_case(kernel::c_str!(\"{}\"), {}),",
+            test, kunit_wrapper_fn_name
+        )
+        .unwrap();
+    }
+
+    writeln!(kunit_macros).unwrap();
+    writeln!(
+        kunit_macros,
+        "static mut TEST_CASES: [kernel::bindings::kunit_case; {}] = [\n{test_cases}    kernel::kunit::kunit_case_null(),\n];",
+        tests.len() + 1
+    )
+    .unwrap();
+
+    writeln!(
+        kunit_macros,
+        "kernel::kunit_unsafe_test_suite!({attr}, TEST_CASES);"
+    )
+    .unwrap();
+
+    // Remove the `#[test]` macros.
+    // We do this at a token level, in order to preserve span information.
+    let mut new_body = vec![];
+    let mut body_it = body.stream().into_iter();
+
+    while let Some(token) = body_it.next() {
+        match token {
+            TokenTree::Punct(ref c) if c.as_char() == '#' => match body_it.next() {
+                Some(TokenTree::Group(group)) if group.to_string() == "[test]" => (),
+                Some(next) => {
+                    new_body.extend([token, next]);
+                }
+                _ => {
+                    new_body.push(token);
+                }
+            },
+            _ => {
+                new_body.push(token);
+            }
+        }
+    }
+
+    let mut new_body = TokenStream::from_iter(new_body);
+    new_body.extend::<TokenStream>(kunit_macros.parse().unwrap());
+
+    tokens.push(TokenTree::Group(Group::new(Delimiter::Brace, new_body)));
+
+    tokens.into_iter().collect()
+}
diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index d61bc6a56425..50b58259c577 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -10,6 +10,7 @@
 mod quote;
 mod concat_idents;
 mod helpers;
+mod kunit;
 mod module;
 mod paste;
 mod pin_data;
@@ -492,3 +493,31 @@ pub fn paste(input: TokenStream) -> TokenStream {
 pub fn derive_zeroable(input: TokenStream) -> TokenStream {
     zeroable::derive(input)
 }
+
+/// Registers a KUnit test suite and its test cases using a user-space like syntax.
+///
+/// This macro should be used on modules. If `CONFIG_KUNIT` (in `.config`) is `n`, the target module
+/// is ignored.
+///
+/// # Examples
+///
+/// ```ignore
+/// # use macros::kunit_tests;
+///
+/// #[kunit_tests(kunit_test_suit_name)]
+/// mod tests {
+///     #[test]
+///     fn foo() {
+///         assert_eq!(1, 1);
+///     }
+///
+///     #[test]
+///     fn bar() {
+///         assert_eq!(2, 2);
+///     }
+/// }
+/// ```
+#[proc_macro_attribute]
+pub fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
+    kunit::kunit_tests(attr, ts)
+}
-- 
2.49.0.rc0.332.g42c0ae87b1-goog