Add MulticolFlow and use it for multicol elements.

It currently "inherits" from BlockFlow and does not override anything.

components/layout/multicol.rs

@@ -0,0 +1,107 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! CSS Multi-column layout http://dev.w3.org/csswg/css-multicol/
+
+#![deny(unsafe_blocks)]
+
+use block::BlockFlow;
+use context::LayoutContext;
+use flow::{FlowClass, Flow};
+use fragment::{Fragment, FragmentBorderBoxIterator};
+use wrapper::ThreadSafeLayoutNode;
+
+use geom::{Point2D, Rect};
+use util::geometry::Au;
+use util::logical_geometry::LogicalRect;
+use std::fmt;
+use style::properties::ComputedValues;
+use std::sync::Arc;
+
+pub struct MulticolFlow {
+    pub block_flow: BlockFlow,
+}
+
+impl MulticolFlow {
+    pub fn from_node_and_fragment(node: &ThreadSafeLayoutNode, fragment: Fragment)
+                                  -> MulticolFlow {
+        MulticolFlow {
+            block_flow: BlockFlow::from_node_and_fragment(node, fragment)
+        }
+    }
+}
+
+impl Flow for MulticolFlow {
+    fn class(&self) -> FlowClass {
+        FlowClass::Multicol
+    }
+
+    fn as_multicol<'a>(&'a mut self) -> &'a mut MulticolFlow {
+        self
+    }
+
+    fn as_block<'a>(&'a mut self) -> &'a mut BlockFlow {
+        &mut self.block_flow
+    }
+
+    fn bubble_inline_sizes(&mut self) {
+        // FIXME(SimonSapin) http://dev.w3.org/csswg/css-sizing/#multicol-intrinsic
+        self.block_flow.bubble_inline_sizes();
+    }
+
+    fn assign_inline_sizes(&mut self, ctx: &LayoutContext) {
+        debug!("assign_inline_sizes({}): assigning inline_size for flow", "multicol");
+        self.block_flow.assign_inline_sizes(ctx);
+    }
+
+    fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
+        debug!("assign_block_size: assigning block_size for multicol");
+        self.block_flow.assign_block_size(ctx);
+    }
+
+    fn compute_absolute_position(&mut self) {
+        self.block_flow.compute_absolute_position()
+    }
+
+    fn update_late_computed_inline_position_if_necessary(&mut self, inline_position: Au) {
+        self.block_flow.update_late_computed_inline_position_if_necessary(inline_position)
+    }
+
+    fn update_late_computed_block_position_if_necessary(&mut self, block_position: Au) {
+        self.block_flow.update_late_computed_block_position_if_necessary(block_position)
+    }
+
+    fn build_display_list(&mut self, layout_context: &LayoutContext) {
+        debug!("build_display_list_multicol: same process as block flow");
+        self.block_flow.build_display_list(layout_context)
+    }
+
+    fn repair_style(&mut self, new_style: &Arc<ComputedValues>) {
+        self.block_flow.repair_style(new_style)
+    }
+
+    fn compute_overflow(&self) -> Rect<Au> {
+        self.block_flow.compute_overflow()
+    }
+
+    fn generated_containing_block_rect(&self) -> LogicalRect<Au> {
+        self.block_flow.generated_containing_block_rect()
+    }
+
+    fn iterate_through_fragment_border_boxes(&self,
+                                             iterator: &mut FragmentBorderBoxIterator,
+                                             stacking_context_position: &Point2D<Au>) {
+        self.block_flow.iterate_through_fragment_border_boxes(iterator, stacking_context_position)
+    }
+
+    fn mutate_fragments(&mut self, mutator: &mut FnMut(&mut Fragment)) {
+        self.block_flow.mutate_fragments(mutator)
+    }
+}
+
+impl fmt::Debug for MulticolFlow {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "MulticolFlow: {:?}", self.block_flow)
+    }
+}