Update style of flow and remove its reliance on `tree`

`tree` is an old API that predates the use of traits. It should be redone.
This commit breaks the dependency on it.

5 files changed, 223 insertions, 147 deletions

diff --git a/src/servo/layout/block.rs b/src/servo/layout/block.rs
index 130cc4da6ee..7b1fb67faa2 100644
--- a/src/servo/layout/block.rs
+++ b/src/servo/layout/block.rs
@@ -4,15 +4,14 @@
 
 // Block layout.
 
-use core::cell::Cell;
-
 use layout::box::{RenderBox};
 use layout::context::LayoutContext;
 use layout::display_list_builder::{DisplayListBuilder, FlowDisplayListBuilderMethods};
-use layout::flow::{FlowContext, FlowTree, InlineBlockFlow, BlockFlow, RootFlow};
+use layout::flow::{BlockFlow, FlowContext, InlineBlockFlow, RootFlow};
 use layout::inline::InlineLayout;
 
 use au = gfx::geometry;
+use core::cell::Cell;
 use geom::point::Point2D;
 use geom::rect::Rect;
 use gfx::display_list::DisplayList;
@@ -82,7 +81,7 @@ impl BlockLayout for FlowContext {
         let mut pref_width = Au(0);
 
         /* find max width from child block contexts */
-        for FlowTree.each_child(self) |child_ctx| {
+        for self.each_child |child_ctx| {
             assert!(child_ctx.starts_block_flow() || child_ctx.starts_inline_flow());
 
             min_width  = au::max(min_width, child_ctx.d().min_width);
@@ -122,7 +121,7 @@ impl BlockLayout for FlowContext {
             remaining_width -= left_used.add(&right_used);
         }
 
-        for FlowTree.each_child(self) |child_ctx| {
+        for self.each_child |child_ctx| {
             assert!(child_ctx.starts_block_flow() || child_ctx.starts_inline_flow());
             child_ctx.d().position.origin.x = left_used;
             child_ctx.d().position.size.width = remaining_width;
@@ -134,7 +133,7 @@ impl BlockLayout for FlowContext {
 
         let mut cur_y = Au(0);
 
-        for FlowTree.each_child(self) |child_ctx| {
+        for self.each_child |child_ctx| {
             child_ctx.d().position.origin.y = cur_y;
             cur_y += child_ctx.d().position.size.height;
         }
@@ -164,7 +163,7 @@ impl BlockLayout for FlowContext {
         // TODO: handle any out-of-flow elements
 
         // go deeper into the flow tree
-        for FlowTree.each_child(self) |child| {
+        for self.each_child |child| {
             self.build_display_list_for_child(builder, child, dirty, offset, list)
         }
     }
diff --git a/src/servo/layout/box_builder.rs b/src/servo/layout/box_builder.rs
index edaeaf5c00e..f127efa03e7 100644
--- a/src/servo/layout/box_builder.rs
+++ b/src/servo/layout/box_builder.rs
@@ -14,7 +14,6 @@ use layout::debug::{BoxedMutDebugMethods, DebugMethods};
 use layout::flow::*;
 use layout::inline::{InlineFlowData, InlineLayout};
 use layout::root::RootFlowData;
-use util::tree;
 
 use gfx::image::holder::ImageHolder;
 use servo_util::range::Range;
@@ -223,9 +222,8 @@ impl BuilderContext {
     priv fn attach_child_flow(&self, child: @mut FlowContext) {
         let d = self.default_collector.flow.d(); // FIXME: borrow checker workaround
         let cd = child.d(); // FIXME: borrow checker workaround
-        debug!("BuilderContext: Adding child flow f%? of f%?",
-               d.id, cd.id);
-        tree::add_child(&FlowTree, self.default_collector.flow, child);
+        debug!("BuilderContext: Adding child flow f%? of f%?", d.id, cd.id);
+        self.default_collector.flow.add_child(child);
     }
     
     priv fn create_child_flow_of_type(&self,
@@ -332,10 +330,10 @@ pub impl LayoutTreeBuilder {
         // eventually be elided or split, but the mapping between
         // nodes and FlowContexts should not change during layout.
         let flow = &mut this_ctx.default_collector.flow;
-        for tree::each_child(&FlowTree, flow) |child_flow: &@mut FlowContext| {
+        for flow.each_child |child_flow: @mut FlowContext| {
             let node = child_flow.d().node;
             assert!(node.has_layout_data());
-            node.layout_data().flow = Some(*child_flow);
+            node.layout_data().flow = Some(child_flow);
         }
     }
 
@@ -355,8 +353,8 @@ pub impl LayoutTreeBuilder {
                 let mut found_child_block = false;
 
                 let flow = &mut parent_ctx.default_collector.flow;
-                for tree::each_child(&FlowTree, flow) |child_ctx: &@mut FlowContext| {
-                    match **child_ctx {
+                for flow.each_child |child_ctx: @mut FlowContext| {
+                    match *child_ctx {
                         InlineFlow(*) | InlineBlockFlow(*) => found_child_inline = true,
                         BlockFlow(*) => found_child_block = true,
                         _ => {}
@@ -371,24 +369,36 @@ pub impl LayoutTreeBuilder {
                 // FIXME: this will create refcounted cycles between the removed flow and any
                 // of its RenderBox or FlowContext children, and possibly keep alive other junk
                 let parent_flow = parent_ctx.default_collector.flow;
+
+                // FIXME: Workaround for the borrow check.
+                let (first_child, last_child) = {
+                    let parent_flow: &mut FlowContext = parent_flow;
+                    let parent_flow_data = parent_flow.d();
+                    (parent_flow_data.first_child, parent_flow_data.last_child)
+                };
+
                 // check first/last child for whitespace-ness
-                for tree::first_child(&FlowTree, &parent_flow).each |first_flow: &@mut FlowContext| {
+                for first_child.each |first_flow: &@mut FlowContext| {
                     if first_flow.starts_inline_flow() {
                         let boxes = &mut first_flow.inline().boxes;
                         if boxes.len() == 1 && boxes[0].is_whitespace_only() {
-                            debug!("LayoutTreeBuilder: pruning whitespace-only first child flow f%d from parent f%d", 
-                                   first_flow.d().id, parent_flow.d().id);
-                            tree::remove_child(&FlowTree, parent_flow, *first_flow);
+                            debug!("LayoutTreeBuilder: pruning whitespace-only first child flow \
+                                    f%d from parent f%d", 
+                                   first_flow.d().id,
+                                   parent_flow.d().id);
+                            parent_flow.remove_child(*first_flow);
                         }
                     }
                 }
-                for tree::last_child(&FlowTree, &parent_flow).each |last_flow: &@mut FlowContext| {
+                for last_child.each |last_flow: &@mut FlowContext| {
                     if last_flow.starts_inline_flow() {
                         let boxes = &mut last_flow.inline().boxes;
                         if boxes.len() == 1 && boxes.last().is_whitespace_only() {
-                            debug!("LayoutTreeBuilder: pruning whitespace-only last child flow f%d from parent f%d", 
-                                   last_flow.d().id, parent_flow.d().id);
-                            tree::remove_child(&FlowTree, parent_flow, *last_flow);
+                            debug!("LayoutTreeBuilder: pruning whitespace-only last child flow \
+                                    f%d from parent f%d", 
+                                   last_flow.d().id,
+                                   parent_flow.d().id);
+                            parent_flow.remove_child(*last_flow);
                         }
                     }
                 }
@@ -416,7 +426,7 @@ pub impl LayoutTreeBuilder {
     }
 
     fn make_flow(&mut self, ty: FlowContextType, node: AbstractNode) -> @mut FlowContext {
-        let data = FlowData(self.next_flow_id(), node);
+        let data = FlowData::new(self.next_flow_id(), node);
         let ret = match ty {
             Flow_Absolute    => @mut AbsoluteFlow(data),
             Flow_Block       => @mut BlockFlow(data, BlockFlowData()),
diff --git a/src/servo/layout/flow.rs b/src/servo/layout/flow.rs
index a4ea3965a7b..701136c2db2 100644
--- a/src/servo/layout/flow.rs
+++ b/src/servo/layout/flow.rs
@@ -2,8 +2,28 @@
  * 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/. */
 
-use core;
-use core::cell::Cell;
+//! Servo's experimental layout system builds a tree of `FlowContext` and `RenderBox` objects and
+/// solves layout constraints to obtain positions and display attributes of tree nodes. Positions
+/// are computed in several tree traversals driven by the fundamental data dependencies required by
+/// inline and block layout.
+/// 
+/// Flows are interior nodes in the layout tree and correspond closely to *flow contexts* in the
+/// CSS specification. Flows are responsible for positioning their child flow contexts and render
+/// boxes. Flows have purpose-specific fields, such as auxiliary line box structs, out-of-flow
+/// child lists, and so on.
+///
+/// Currently, the important types of flows are:
+/// 
+/// * `BlockFlow`: a flow that establishes a block context. It has several child flows, each of
+///   which are positioned according to block formatting context rules (CSS block boxes). Block
+///   flows also contain a single `GenericBox` to represent their rendered borders, padding, etc.
+///   (In the future, this render box may be folded into `BlockFlow` to save space.)
+///
+/// * `InlineFlow`: a flow that establishes an inline context. It has a flat list of child
+///   boxes/flows that are subject to inline layout and line breaking and structs to represent
+///   line breaks and mapping to CSS boxes, for the purpose of handling `getClientRects()` and
+///   similar methods.
+
 use dom::node::AbstractNode;
 use layout::block::{BlockFlowData, BlockLayout};
 use layout::box::RenderBox;
@@ -12,41 +32,16 @@ use layout::debug::BoxedMutDebugMethods;
 use layout::display_list_builder::DisplayListBuilder;
 use layout::inline::{InlineFlowData, InlineLayout};
 use layout::root::{RootFlowData, RootLayout};
-use util::tree;
-use geom::rect::Rect;
+
+use core::cell::Cell;
+use core::ptr;
 use geom::point::Point2D;
+use geom::rect::Rect;
 use gfx::display_list::DisplayList;
 use gfx::geometry::Au;
 
-/** Servo's experimental layout system builds a tree of FlowContexts
-and RenderBoxes, and figures out positions and display attributes of
-tree nodes. Positions are computed in several tree traversals driven
-by fundamental data dependencies of inline and block layout.
-
-Flows are interior nodes in the layout tree, and correspond closely to
-flow contexts in the CSS specification. Flows are responsible for
-positioning their child flow contexts and render boxes. Flows have
-purpose-specific fields, such as auxilliary line box structs,
-out-of-flow child lists, and so on.
-
-Currently, the important types of flows are:
-
- * BlockFlow: a flow that establishes a block context. It has several
-   child flows, each of which are positioned according to block
-   formatting context rules (as if child flows CSS block boxes). Block
-   flows also contain a single GenericBox to represent their rendered
-   borders, padding, etc. (In the future, this render box may be
-   folded into BlockFlow to save space.)
-
- * InlineFlow: a flow that establishes an inline context. It has a
-   flat list of child boxes/flows that are subject to inline layout
-   and line breaking, and structs to represent line breaks and mapping
-   to CSS boxes, for the purpose of handling `getClientRects()`.
-
-*/
-
-/* The type of the formatting context, and data specific to each
-context, such as linebox structures or float lists */ 
+/// The type of the formatting context and data specific to each context, such as line box
+/// structures or float lists.
 pub enum FlowContext {
     AbsoluteFlow(FlowData), 
     BlockFlow(FlowData, BlockFlowData),
@@ -71,8 +66,13 @@ pub enum FlowContextType {
    render boxes within the context.  */
 pub struct FlowData {
     node: AbstractNode,
-    /* reference to parent, children flow contexts */
-    tree: tree::Tree<@mut FlowContext>,
+
+    parent: Option<@mut FlowContext>,
+    first_child: Option<@mut FlowContext>,
+    last_child: Option<@mut FlowContext>,
+    prev_sibling: Option<@mut FlowContext>,
+    next_sibling: Option<@mut FlowContext>,
+
     /* TODO (Issue #87): debug only */
     id: int,
 
@@ -84,55 +84,131 @@ pub struct FlowData {
     position: Rect<Au>,
 }
 
-pub fn FlowData(id: int, node: AbstractNode) -> FlowData {
-    FlowData {
-        node: node,
-        tree: tree::empty(),
-        id: id,
+impl FlowData {
+    pub fn new(id: int, node: AbstractNode) -> FlowData {
+        FlowData {
+            node: node,
+
+            parent: None,
+            first_child: None,
+            last_child: None,
+            prev_sibling: None,
+            next_sibling: None,
+
+            id: id,
 
-        min_width: Au(0),
-        pref_width: Au(0),
-        position: Au::zero_rect()
+            min_width: Au(0),
+            pref_width: Au(0),
+            position: Au::zero_rect(),
+        }
     }
 }
 
-pub impl<'self> FlowContext {
-    fn d(&'self mut self) -> &'self mut FlowData {
-      unsafe {
-        match *self {
-            AbsoluteFlow(ref d)    => cast::transmute(d),
-            BlockFlow(ref d, _)    => cast::transmute(d),
-            FloatFlow(ref d)       => cast::transmute(d),
-            InlineBlockFlow(ref d) => cast::transmute(d),
-            InlineFlow(ref d, _)   => cast::transmute(d),
-            RootFlow(ref d, _)     => cast::transmute(d),
-            TableFlow(ref d)       => cast::transmute(d)
+impl<'self> FlowContext {
+    pub fn d(&'self mut self) -> &'self mut FlowData {
+        unsafe {
+            match *self {
+                AbsoluteFlow(ref d)    => cast::transmute(d),
+                BlockFlow(ref d, _)    => cast::transmute(d),
+                FloatFlow(ref d)       => cast::transmute(d),
+                InlineBlockFlow(ref d) => cast::transmute(d),
+                InlineFlow(ref d, _)   => cast::transmute(d),
+                RootFlow(ref d, _)     => cast::transmute(d),
+                TableFlow(ref d)       => cast::transmute(d)
+            }
+        }
+    }
+
+    /// Iterates over the immediate children of this flow.
+    ///
+    /// TODO: Fold me into `util::tree`.
+    pub fn each_child(@mut self, f: &fn(@mut FlowContext) -> bool) {
+        let mut current_opt = self.d().first_child;
+        while !current_opt.is_none() {
+            let current = current_opt.get();
+            if !f(current) {
+                break;
+            }
+            current_opt = current.d().next_sibling;
         }
-      }
     }
 
-    fn inline(&'self mut self) -> &'self mut InlineFlowData {
+    /// Adds the given flow to the end of the list of this flow's children. The new child must be
+    /// detached from the tree before calling this method.
+    ///
+    /// TODO: Fold me into `util::tree`.
+    pub fn add_child(@mut self, child: @mut FlowContext) {
+        let self_data = self.d(), child_data = child.d();
+
+        assert!(child_data.parent.is_none());
+        assert!(child_data.prev_sibling.is_none());
+        assert!(child_data.next_sibling.is_none());
+
+        match self_data.last_child {
+            None => {
+                self_data.first_child = Some(child);
+            }
+            Some(last_child) => {
+                assert!(last_child.d().next_sibling.is_none());
+                last_child.d().next_sibling = Some(child);
+                child_data.prev_sibling = Some(last_child);
+            }
+        }
+
+        self_data.last_child = Some(child);
+        child_data.parent = Some(self);
+    }
+
+    /// Removes the given flow from the tree.
+    ///
+    /// TODO: Fold me into `util::tree`.
+    pub fn remove_child(@mut self, child: @mut FlowContext) {
+        let self_data = self.d(), child_data = child.d();
+
+        assert!(child_data.parent.is_some());
+        assert!(ptr::ref_eq(&*child_data.parent.get(), self));
+
+        match child_data.prev_sibling {
+            None => self_data.first_child = child_data.next_sibling,
+            Some(prev_sibling) => {
+                prev_sibling.d().next_sibling = child_data.next_sibling;
+                child_data.prev_sibling = None;
+            }
+        }
+
+        match child_data.next_sibling {
+            None => self_data.last_child = child.d().prev_sibling,
+            Some(next_sibling) => {
+                next_sibling.d().prev_sibling = Some(next_sibling);
+                child_data.next_sibling = None;
+            }
+        }
+
+        child_data.parent = None;
+    }
+
+    pub fn inline(&'self mut self) -> &'self mut InlineFlowData {
         match self {
             &InlineFlow(_, ref i) => unsafe { cast::transmute(i) },
             _ => fail!(fmt!("Tried to access inline data of non-inline: f%d", self.d().id))
         }
     }
 
-    fn block(&'self mut self) -> &'self mut BlockFlowData {
+    pub fn block(&'self mut self) -> &'self mut BlockFlowData {
         match self {
             &BlockFlow(_, ref mut b) => unsafe { cast::transmute(b) },
             _ => fail!(fmt!("Tried to access block data of non-block: f%d", self.d().id))
         }
     }
 
-    fn root(&'self mut self) -> &'self mut RootFlowData {
+    pub fn root(&'self mut self) -> &'self mut RootFlowData {
         match self {
             &RootFlow(_, ref r) => unsafe { cast::transmute(r) },
             _ => fail!(fmt!("Tried to access root data of non-root: f%d", self.d().id))
         }
     }
 
-    fn bubble_widths(@mut self, ctx: &mut LayoutContext) {
+    pub fn bubble_widths(@mut self, ctx: &mut LayoutContext) {
         match self {
             @BlockFlow(*)  => self.bubble_widths_block(ctx),
             @InlineFlow(*) => self.bubble_widths_inline(ctx),
@@ -141,7 +217,7 @@ pub impl<'self> FlowContext {
         }
     }
 
-    fn assign_widths(@mut self, ctx: &mut LayoutContext) {
+    pub fn assign_widths(@mut self, ctx: &mut LayoutContext) {
         match self {
             @BlockFlow(*)  => self.assign_widths_block(ctx),
             @InlineFlow(*) => self.assign_widths_inline(ctx),
@@ -150,7 +226,7 @@ pub impl<'self> FlowContext {
         }
     }
 
-    fn assign_height(@mut self, ctx: &mut LayoutContext) {
+    pub fn assign_height(@mut self, ctx: &mut LayoutContext) {
         match self {
             @BlockFlow(*)  => self.assign_height_block(ctx),
             @InlineFlow(*) => self.assign_height_inline(ctx),
@@ -159,8 +235,11 @@ pub impl<'self> FlowContext {
         }
     }
 
-    fn build_display_list_recurse(@mut self, builder: &DisplayListBuilder, dirty: &Rect<Au>,
-                                  offset: &Point2D<Au>, list: &Cell<DisplayList>) {
+    pub fn build_display_list_recurse(@mut self,
+                                      builder: &DisplayListBuilder,
+                                      dirty: &Rect<Au>,
+                                      offset: &Point2D<Au>,
+                                      list: &Cell<DisplayList>) {
         let d = self.d(); // FIXME: borrow checker workaround
         debug!("FlowContext::build_display_list at %?: %s", d.position, self.debug_str());
 
@@ -173,9 +252,10 @@ pub impl<'self> FlowContext {
     }
 
     // Actual methods that do not require much flow-specific logic
-    fn foldl_all_boxes<B: Copy>(&mut self,
-                                seed: B,
-                                cb: &fn(a: B, b: @mut RenderBox) -> B) -> B {
+    pub fn foldl_all_boxes<B:Copy>(&mut self,
+                                   seed: B,
+                                   cb: &fn(a: B, b: @mut RenderBox) -> B)
+                                   -> B {
         match self {
             &RootFlow(*)   => {
                 let root = self.root(); // FIXME: borrow checker workaround
@@ -193,85 +273,67 @@ pub impl<'self> FlowContext {
         }
     }
 
-    fn foldl_boxes_for_node<B: Copy>(&mut self,
-                                     node: AbstractNode,
-                                     seed: B,
-                                     cb: &fn(a: B,@mut RenderBox) -> B)
-            -> B {
+    pub fn foldl_boxes_for_node<B:Copy>(&mut self,
+                                        node: AbstractNode,
+                                        seed: B,
+                                        cb: &fn(a: B, @mut RenderBox) -> B)
+                                        -> B {
         do self.foldl_all_boxes(seed) |acc, box| {
             if box.d().node == node { cb(acc, box) }
             else { acc }
         }
     }
 
-    fn iter_all_boxes<T>(&mut self, cb: &fn(@mut RenderBox) -> T) {
+    pub fn iter_all_boxes(&mut self, cb: &fn(@mut RenderBox) -> bool) {
         match self {
             &RootFlow(*)   => {
                 let root = self.root(); // FIXME: borrow checker workaround
-                for root.box.each |box| { cb(*box); }
+                for root.box.each |box| {
+                    if !cb(*box) {
+                        break;
+                    }
+                }
             }
             &BlockFlow(*)  => {
                 let block = self.block(); // FIXME: borrow checker workaround
-                for block.box.each |box| { cb(*box); }
+                for block.box.each |box| {
+                    if !cb(*box) {
+                        break;
+                    }
+                }
             }
             &InlineFlow(*) => {
                 let inline = self.inline(); // FIXME: borrow checker workaround
-                for inline.boxes.each |box| { cb(*box); }
+                for inline.boxes.each |box| {
+                    if !cb(*box) {
+                        break;
+                    }
+                }
             }
             _ => fail!(fmt!("Don't know how to iterate node's RenderBoxes for %?", self))
         }
     }
 
-    fn iter_boxes_for_node<T>(&mut self,
-                              node: AbstractNode,
-                              cb: &fn(@mut RenderBox) -> T) {
-        do self.iter_all_boxes |box| {
-            if box.d().node == node { cb(box); }
+    pub fn iter_boxes_for_node(&mut self, node: AbstractNode, cb: &fn(@mut RenderBox) -> bool) {
+        for self.iter_all_boxes |box| {
+            if box.d().node == node {
+                if !cb(box) {
+                    break;
+                }
+            }
         }
     }
 }
 
-/* The tree holding FlowContexts */
-pub enum FlowTree { FlowTree }
-
-impl FlowTree {
-    fn each_child(&self, ctx: @mut FlowContext, f: &fn(box: @mut FlowContext) -> bool) {
-        tree::each_child(self, &ctx, |box| f(*box) )
-    }
-}
-
-impl tree::ReadMethods<@mut FlowContext> for FlowTree {
-    fn with_tree_fields<R>(&self, box: &@mut FlowContext, f: &fn(&mut tree::Tree<@mut FlowContext>) -> R) -> R {
-        let tree = &mut box.d().tree;
-        f(tree)
-    }
-}
-
-impl FlowTree {
-    fn add_child(self, parent: @mut FlowContext, child: @mut FlowContext) {
-        tree::add_child(&self, parent, child)
-    }
-}
-
-impl tree::WriteMethods<@mut FlowContext> for FlowTree {
-    fn tree_eq(&self, a: &@mut FlowContext, b: &@mut FlowContext) -> bool { core::managed::mut_ptr_eq(*a, *b) }
-
-    fn with_tree_fields<R>(&self, box: &@mut FlowContext, f: &fn(&mut tree::Tree<@mut FlowContext>) -> R) -> R {
-        let tree = &mut box.d().tree;
-        f(tree)
-    }
-}
-
-
 impl BoxedMutDebugMethods for FlowContext {
     fn dump(@mut self) {
-        self.dump_indent(0u);
+        self.dump_indent(0);
     }
 
-    /** Dumps the flow tree, for debugging, with indentation. */
+    /// Dumps the flow tree, for debugging, with indentation.
     fn dump_indent(@mut self, indent: uint) {
         let mut s = ~"|";
-        for uint::range(0u, indent) |_i| {
+        for uint::range(0, indent) |_i| {
             s += ~"---- ";
         }
 
@@ -279,8 +341,8 @@ impl BoxedMutDebugMethods for FlowContext {
         debug!("%s", s);
 
         // FIXME: this should have a pure/const version?
-        for FlowTree.each_child(self) |child| {
-            child.dump_indent(indent + 1u)
+        for self.each_child |child| {
+            child.dump_indent(indent + 1)
         }
     }
     
@@ -313,3 +375,4 @@ impl BoxedMutDebugMethods for FlowContext {
         fmt!("f%? %?", d.id, repr)
     }
 }
+
diff --git a/src/servo/layout/root.rs b/src/servo/layout/root.rs
index 98e1e49565b..af92b037d78 100644
--- a/src/servo/layout/root.rs
+++ b/src/servo/layout/root.rs
@@ -10,7 +10,7 @@ use gfx::geometry::Au;
 use layout::block::BlockLayout;
 use layout::box::RenderBox;
 use layout::context::LayoutContext;
-use layout::flow::{FlowContext, FlowTree, RootFlow};
+use layout::flow::{FlowContext, RootFlow};
 use layout::display_list_builder::DisplayListBuilder;
 
 pub struct RootFlowData {
@@ -63,7 +63,7 @@ impl RootLayout for FlowContext {
         // the root adjusts self height to at least cover the viewport.
         let mut cur_y = Au(0);
 
-        for FlowTree.each_child(self) |child_ctx| {
+        for self.each_child |child_ctx| {
             child_ctx.d().position.origin.y = cur_y;
             cur_y += child_ctx.d().position.size.height;
         }
diff --git a/src/servo/layout/traverse.rs b/src/servo/layout/traverse.rs
index 5d8e323ae17..b4710b974bc 100644
--- a/src/servo/layout/traverse.rs
+++ b/src/servo/layout/traverse.rs
@@ -2,9 +2,9 @@
  * 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/. */
 
-use layout::flow::{FlowContext, FlowTree};
+use layout::flow::FlowContext;
 
-/** Trait for running tree-based traversals over layout contexts */
+/// A trait for running tree-based traversals over flows contexts.
 pub trait FlowContextTraversals {
     fn traverse_preorder(@mut self, preorder_cb: &fn(@mut FlowContext));
     fn traverse_postorder(@mut self, postorder_cb: &fn(@mut FlowContext));
@@ -13,11 +13,15 @@ pub trait FlowContextTraversals {
 impl FlowContextTraversals for FlowContext {
     fn traverse_preorder(@mut self, preorder_cb: &fn(@mut FlowContext)) {
         preorder_cb(self);
-        do FlowTree.each_child(self) |child| { child.traverse_preorder(preorder_cb); true }
+        for self.each_child |child| {
+            child.traverse_preorder(preorder_cb);
+        }
     }
 
     fn traverse_postorder(@mut self, postorder_cb: &fn(@mut FlowContext)) {
-        do FlowTree.each_child(self) |child| { child.traverse_postorder(postorder_cb); true }
+        for self.each_child |child| {
+            child.traverse_postorder(postorder_cb);
+        }
         postorder_cb(self);
     }
 }