Auto merge of #14896 - g1smo:tidy-assignments, r=jdm

Tidy assignments

<!-- Please describe your changes on the following line: -->
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.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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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components/style/thread_state.rs

@@ -51,8 +51,8 @@ macro_rules! thread_types ( ( $( $fun:ident = $flag:ident ; )* ) => (
     }
 
     #[cfg(debug_assertions)]
-    static TYPES: &'static [ThreadState]
-        = &[ $( $flag ),* ];
+    static TYPES: &'static [ThreadState] =
+        &[ $( $flag ),* ];
 ));
 
 thread_types! {