Auto merge of #12330 - stshine:flexitem, r=pcwalton

Implement flexible box layout for row container

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

This pull requests implements basic flexible box layout for row container.
It  has implemented most basic flexbox features, including grow, shrink, multi-line, *reverse properties, and alignment under `justify-content`, `align-items`, `align-self`, `align-content`.

---
<!-- 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]  There are tests for these changes

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

r? @pcwalton

<!-- 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/12330)
<!-- Reviewable:end -->

1 files changed, 16 insertions, 2 deletions

diff --git a/components/layout/fragment.rs b/components/layout/fragment.rs
index 0fb5c6579c1..0ddb083c1c0 100644
--- a/components/layout/fragment.rs
+++ b/components/layout/fragment.rs
@@ -23,7 +23,7 @@ use inline::{InlineMetrics, LAST_FRAGMENT_OF_ELEMENT};
 use ipc_channel::ipc::IpcSender;
 #[cfg(debug_assertions)]
 use layout_debug;
-use model::{self, IntrinsicISizes, IntrinsicISizesContribution, MaybeAuto, specified};
+use model::{self, Direction, IntrinsicISizes, IntrinsicISizesContribution, MaybeAuto, specified};
 use msg::constellation_msg::PipelineId;
 use net_traits::image::base::{Image, ImageMetadata};
 use net_traits::image_cache_thread::{ImageOrMetadataAvailable, UsePlaceholder};
@@ -39,7 +39,7 @@ use std::fmt;
 use std::sync::{Arc, Mutex};
 use style::arc_ptr_eq;
 use style::computed_values::content::ContentItem;
-use style::computed_values::{border_collapse, clear, color, display, mix_blend_mode};
+use style::computed_values::{border_collapse, box_sizing, clear, color, display, mix_blend_mode};
 use style::computed_values::{overflow_wrap, overflow_x, position, text_decoration};
 use style::computed_values::{transform_style, vertical_align, white_space, word_break, z_index};
 use style::dom::TRestyleDamage;
@@ -1109,6 +1109,20 @@ impl Fragment {
         }
     }
 
+    /// Returns the border width in given direction if this fragment has property
+    /// 'box-sizing: border-box'. The `border_padding` field should have been initialized.
+    pub fn box_sizing_boundary(&self, direction: Direction) -> Au {
+        match (self.style().get_position().box_sizing, direction) {
+            (box_sizing::T::border_box, Direction::Inline) => {
+                self.border_padding.inline_start_end()
+            }
+            (box_sizing::T::border_box, Direction::Block) => {
+                self.border_padding.block_start_end()
+            }
+            _ => Au(0)
+        }
+    }
+
     /// Computes the margins in the inline direction from the containing block inline-size and the
     /// style. After this call, the inline direction of the `margin` field will be correct.
     ///