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/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: