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).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

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components/layout/block.rs

@@ -1283,8 +1283,8 @@ impl BlockFlow {
                             content_block_size,
                             available_block_size));
 
-                    candidate_block_size_iterator.candidate_value
+                    candidate_block_size_iterator.candidate_value =
-                        = solution.unwrap().block_size;
+                        solution.unwrap().block_size;
                 }
             }
         }

components/layout/construct.rs

@@ -1290,8 +1290,8 @@ impl<'a, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode>
         for kid in node.children() {
             // CSS 2.1 § 17.2.1. Treat all non-column child fragments of `table-column-group`
             // as `display: none`.
-            if let ConstructionResult::ConstructionItem(ConstructionItem::TableColumnFragment(fragment))
+            if let ConstructionResult::ConstructionItem(ConstructionItem::TableColumnFragment(fragment)) =
-                   = kid.swap_out_construction_result() {
+                kid.swap_out_construction_result() {
                 col_fragments.push(fragment)
             }
         }

components/style/thread_state.rs

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

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),

python/tidy/servo_tidy_tests/rust_tidy.rs

@@ -59,4 +59,7 @@ impl test {
 
     // Should not be triggered
     macro_rules! test_macro ( ( $( $fun:ident = $flag:ident ; )* ) => ());
+
+    let var
+        = "val";
 }

python/tidy/servo_tidy_tests/test_tidy.py

@@ -125,6 +125,7 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('extra space after (', errors.next()[2])
         self.assertEqual('extra space after (', errors.next()[2])
         self.assertEqual('extra space after test_fun', errors.next()[2])
+        self.assertEqual('no = in the beginning of line', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
         feature_errors = tidy.collect_errors_for_files(iterFile('lib.rs'), [], [tidy.check_rust], print_text=False)