servo: Merge #14896 - Tidy assignments (from g1smo:tidy-assignments); r=jdm

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I've added a new tidy rule (no = in the beginning of line) + tests for it. Also cleaned up a few rust source files to accord with the new rule.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14890 (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: b6c137b36f8b9f9cc5c0105bb94a4edeaf7d09be

servo/python/tidy/servo_tidy/tidy.py

@@ -520,6 +520,8 @@ def check_rust(file_name, lines):
                 lambda match, line: is_attribute),
             (r"=[A-Za-z0-9\"]", "missing space after =",
                 lambda match, line: is_attribute),
+            (r"^=\s", "no = in the beginning of line",
+                lambda match, line: not is_comment),
             # ignore scientific notation patterns like 1e-6
             (r"[A-DF-Za-df-z0-9]-", "missing space before -",
                 lambda match, line: not is_attribute),