Auto merge of #14136 - stshine:orthogonal-symmetry, r=SimonSapin

style: Add a "start_end()" method to LogicalMargin

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

Add a `LogicalMargin::start_end()` method that receives a `Direction' parameter. This is useful for some layout that is symmetric in inline and block directions, like flexbox.

Part of #14123.

---
<!-- 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
- [ ] These changes fix #__ (github issue number if applicable).

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

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

r? @SimonSapin

<!-- Reviewable:start -->

---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14136)
<!-- Reviewable:end -->

1 files changed, 17 insertions, 0 deletions

diff --git a/components/style/logical_geometry.rs b/components/style/logical_geometry.rs
index 94dcd3881db..242a061b0da 100644
--- a/components/style/logical_geometry.rs
+++ b/components/style/logical_geometry.rs
@@ -213,6 +213,13 @@ impl Debug for DebugWritingMode {
 }
 
 
+// Used to specify the logical direction.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Direction {
+    Inline,
+    Block
+}
+
 /// A 2D size in flow-relative dimensions
 #[derive(PartialEq, Eq, Clone, Copy)]
 #[cfg_attr(feature = "servo", derive(Serialize))]
@@ -764,6 +771,16 @@ impl<T: Copy + Add<T, Output=T>> LogicalMargin<T> {
     }
 
     #[inline]
+    pub fn start_end(&self, direction: Direction) -> T {
+        match direction {
+            Direction::Inline =>
+                self.inline_start + self.inline_end,
+            Direction::Block =>
+                self.block_start + self.block_end
+        }
+    }
+
+    #[inline]
     pub fn top_bottom(&self, mode: WritingMode) -> T {
         self.debug_writing_mode.check(mode);
         if mode.is_vertical() {