added tidy check for -> ()

python/tidy/servo_tidy/tidy.py

@@ -565,6 +565,8 @@ def check_rust(file_name, lines):
             # This particular pattern is not reentrant-safe in script_thread.rs
             (r"match self.documents.borrow", "use a separate variable for the match expression",
              lambda match, line: file_name.endswith('script_thread.rs')),
+            # -> () is unnecessary
+            (r"-> \(\)", "encountered function signature with -> ()", no_filter),
         ]
 
         for pattern, message, filter_func in regex_rules:

python/tidy/servo_tidy_tests/rust_tidy.rs

@@ -38,7 +38,7 @@ impl test {
         }
     }
 
-    fn test_fun2(y : &String, z : &Vec<f32>, r: &Root<isize>) -> f32 {
+    fn test_fun2(y : &String, z : &Vec<f32>, r: &Root<isize>) -> () {
         let x = true;
         x
             && x;

python/tidy/servo_tidy_tests/test_tidy.py

@@ -123,6 +123,7 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('use &[T] instead of &Vec<T>', errors.next()[2])
         self.assertEqual('use &str instead of &String', errors.next()[2])
         self.assertEqual('use &T instead of &Root<T>', errors.next()[2])
+        self.assertEqual('encountered function signature with -> ()', errors.next()[2])
         self.assertEqual('operators should go at the end of the first line', errors.next()[2])
         self.assertEqual('else braces should be on the same line', errors.next()[2])
         self.assertEqual('extra space after (', errors.next()[2])