fix: service macro comments (#3474)

* fix: service macro comments #3472

* test: service macro comments #3472

* fix: add case for empty tuple seperately

* doc: add case for empty tuple seperately

* test: move test_services into lib

---------

Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-web/src/service.rs

@@ -662,6 +662,7 @@ where
 /// ```
 #[macro_export]
 macro_rules! services {
+    () => {()};
     ($($x:expr),+ $(,)?) => {
         ($($x,)+)
     }
@@ -870,4 +871,40 @@ mod tests {
         let req = test::TestRequest::default().to_request();
         let _res = test::call_service(&app, req).await;
     }
+
+    #[test]
+    fn define_services_macro_with_multiple_arguments() {
+        let result = services!(1, 2, 3);
+        assert_eq!(result, (1, 2, 3));
+    }
+
+    #[test]
+    fn define_services_macro_with_single_argument() {
+        let result = services!(1);
+        assert_eq!(result, (1,));
+    }
+
+    #[test]
+    fn define_services_macro_with_no_arguments() {
+        let result = services!();
+        let () = result;
+    }
+
+    #[test]
+    fn define_services_macro_with_trailing_comma() {
+        let result = services!(1, 2, 3,);
+        assert_eq!(result, (1, 2, 3));
+    }
+
+    #[test]
+    fn define_services_macro_with_comments_in_arguments() {
+        let result = services!(
+            1, // First comment
+            2, // Second comment
+            3  // Third comment
+        );
+
+        // Assert that comments are ignored and it correctly returns a tuple.
+        assert_eq!(result, (1, 2, 3));
+    }
 }