servo: Merge #15927 - added tidiness check for "-> ()" (from lucantrop:master); r=Ms2ger

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I added the check, modified the tests and removed all instances of "-> ()" that were already there.

---
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #15925 (github issue number if applicable).

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- [X] There are tests for these changes

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Source-Repo: https://github.com/servo/servo
Source-Revision: a6f89c503671726bcb0407cd84c380f3af4a7916

servo/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;

servo/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])