rust: kunit: allow cfg on tests

The `kunit_test` proc macro only checks for the `test` attribute
immediately preceding a `fn`. If the function is disabled via a `cfg`,
the generated code would result in a compile error referencing a
non-existent function [1].

This collects attributes and specifically cherry-picks `cfg` attributes
to be duplicated inside KUnit wrapper functions such that a test function
disabled via `cfg` compiles and is marked as skipped in KUnit correctly.

Link: https://lore.kernel.org/r/20250916021259.115578-1-ent3rm4n@gmail.com
Link: https://lore.kernel.org/rust-for-linux/CANiq72==48=69hYiDo1321pCzgn_n1_jg=ez5UYXX91c+g5JVQ@mail.gmail.com/ [1]
Closes: https://github.com/Rust-for-Linux/linux/issues/1185
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Suggested-by: David Gow <davidgow@google.com>
Signed-off-by: Kaibo Ma <ent3rm4n@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

rust/kernel/kunit.rs

@@ -361,4 +361,11 @@ fn rust_test_kunit_example_test() {
     fn rust_test_kunit_in_kunit_test() {
         assert!(in_kunit_test());
     }
+
+    #[test]
+    #[cfg(not(all()))]
+    fn rust_test_kunit_always_disabled_test() {
+        // This test should never run because of the `cfg`.
+        assert!(false);
+    }
 }

rust/macros/kunit.rs

@@ -5,6 +5,7 @@
 //! Copyright (c) 2023 José Expósito <jose.exposito89@gmail.com>
 
 use proc_macro::{Delimiter, Group, TokenStream, TokenTree};
+use std::collections::HashMap;
 use std::fmt::Write;
 
 pub(crate) fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
@@ -41,20 +42,32 @@ pub(crate) fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
     // Get the functions set as tests. Search for `[test]` -> `fn`.
     let mut body_it = body.stream().into_iter();
     let mut tests = Vec::new();
+    let mut attributes: HashMap<String, TokenStream> = HashMap::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);
+            TokenTree::Punct(ref p) if p.as_char() == '#' => match body_it.next() {
+                Some(TokenTree::Group(g)) if g.delimiter() == Delimiter::Bracket => {
+                    if let Some(TokenTree::Ident(name)) = g.stream().into_iter().next() {
+                        // Collect attributes because we need to find which are tests. We also
+                        // need to copy `cfg` attributes so tests can be conditionally enabled.
+                        attributes
+                            .entry(name.to_string())
+                            .or_default()
+                            .extend([token, TokenTree::Group(g)]);
+                    }
+                    continue;
                 }
-                _ => continue,
+                _ => (),
             },
+            TokenTree::Ident(i) if i.to_string() == "fn" && attributes.contains_key("test") => {
+                if let Some(TokenTree::Ident(test_name)) = body_it.next() {
+                    tests.push((test_name, attributes.remove("cfg").unwrap_or_default()))
+                }
+            }
+
             _ => (),
         }
+        attributes.clear();
     }
 
     // Add `#[cfg(CONFIG_KUNIT="y")]` before the module declaration.
@@ -100,11 +113,22 @@ pub(crate) fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
     let mut test_cases = "".to_owned();
     let mut assert_macros = "".to_owned();
     let path = crate::helpers::file();
-    for test in &tests {
+    let num_tests = tests.len();
+    for (test, cfg_attr) 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.
+        // Append any `cfg` attributes the user might have written on their tests so we don't
+        // attempt to call them when they are `cfg`'d out. An extra `use` is used here to reduce
+        // the length of the assert message.
         let kunit_wrapper = format!(
-            "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}())); }}",
+            r#"unsafe extern "C" fn {kunit_wrapper_fn_name}(_test: *mut ::kernel::bindings::kunit)
+            {{
+                (*_test).status = ::kernel::bindings::kunit_status_KUNIT_SKIPPED;
+                {cfg_attr} {{
+                    (*_test).status = ::kernel::bindings::kunit_status_KUNIT_SUCCESS;
+                    use ::kernel::kunit::is_test_result_ok;
+                    assert!(is_test_result_ok({test}()));
+                }}
+            }}"#,
         );
         writeln!(kunit_macros, "{kunit_wrapper}").unwrap();
         writeln!(
@@ -139,7 +163,7 @@ macro_rules! assert_eq {{
     writeln!(
         kunit_macros,
         "static mut TEST_CASES: [::kernel::bindings::kunit_case; {}] = [\n{test_cases}    ::kernel::kunit::kunit_case_null(),\n];",
-        tests.len() + 1
+        num_tests + 1
     )
     .unwrap();