auto merge of #1626 : kmcallister/servo/flowlist, r=pcwalton

r? @pcwalton

Some points of concern:

* Is this really memory-safe? Is there a way to access a stale `Rawlink` that doesn't exist in the normal `DList` code?
* Is it okay to coerce both `~Flow` and `&Flow` to `(uint, uint)` (or the moral equivalent) and compare them?  Will a given `Flow` always have the same (obj, vtable) regardless of whether it's obtained through `~Flow` or `&Flow`?

7 files changed, 386 insertions, 49 deletions

diff --git a/src/components/main/layout/block.rs b/src/components/main/layout/block.rs
index cc55ed3bb2c..89415f45745 100644
--- a/src/components/main/layout/block.rs
+++ b/src/components/main/layout/block.rs
@@ -285,9 +285,9 @@ impl BlockFlow {
             // last_child.floats_out -> self.floats_out (done at the end of this method)
             float_ctx = self.base.floats_in.translate(Point2D(-left_offset, -top_offset));
             for kid in self.base.child_iter() {
-                flow::mut_base(*kid).floats_in = float_ctx.clone();
+                flow::mut_base(kid).floats_in = float_ctx.clone();
                 kid.assign_height_inorder(ctx);
-                float_ctx = flow::mut_base(*kid).floats_out.clone();
+                float_ctx = flow::mut_base(kid).floats_out.clone();
             }
         }
 
@@ -329,7 +329,7 @@ impl BlockFlow {
                                  &mut collapsing,
                                  &mut collapsible);
 
-            let child_node = flow::mut_base(*kid);
+            let child_node = flow::mut_base(kid);
             cur_y = cur_y - collapsing;
             // At this point, after moving up by `collapsing`, cur_y is at the
             // top margin edge of kid
@@ -407,7 +407,7 @@ impl BlockFlow {
 
             if self.is_fixed {
                 for kid in self.base.child_iter() {
-                    let child_node = flow::mut_base(*kid);
+                    let child_node = flow::mut_base(kid);
                     child_node.position.origin.y = position.origin.y + top_offset;
                 }
             }
@@ -484,9 +484,9 @@ impl BlockFlow {
         if has_inorder_children {
             let mut float_ctx = FloatContext::new(self.float.get_ref().floated_children);
             for kid in self.base.child_iter() {
-                flow::mut_base(*kid).floats_in = float_ctx.clone();
+                flow::mut_base(kid).floats_in = float_ctx.clone();
                 kid.assign_height_inorder(ctx);
-                float_ctx = flow::mut_base(*kid).floats_out.clone();
+                float_ctx = flow::mut_base(kid).floats_out.clone();
             }
         }
         let mut cur_y = Au(0);
@@ -498,7 +498,7 @@ impl BlockFlow {
         }
 
         for kid in self.base.child_iter() {
-            let child_base = flow::mut_base(*kid);
+            let child_base = flow::mut_base(kid);
             child_base.position.origin.y = cur_y;
             cur_y = cur_y + child_base.position.size.height;
         }
@@ -573,7 +573,7 @@ impl BlockFlow {
         let this_position = self.base.abs_position;
 
         for child in self.base.child_iter() {
-            let child_base = flow::mut_base(*child);
+            let child_base = flow::mut_base(child);
             child_base.abs_position = this_position + child_base.position.origin + rel_offset;
         }
 
@@ -618,7 +618,7 @@ impl BlockFlow {
 
         // go deeper into the flow tree
         for child in self.base.child_iter() {
-            let child_base = flow::mut_base(*child);
+            let child_base = flow::mut_base(child);
             child_base.abs_position = offset + child_base.position.origin + rel_offset;
         }
 
@@ -652,7 +652,7 @@ impl Flow for BlockFlow {
         for child_ctx in self.base.child_iter() {
             assert!(child_ctx.starts_block_flow() || child_ctx.starts_inline_flow());
 
-            let child_base = flow::mut_base(*child_ctx);
+            let child_base = flow::mut_base(child_ctx);
             min_width = geometry::max(min_width, child_base.min_width);
             pref_width = geometry::max(pref_width, child_base.pref_width);
             num_floats = num_floats + child_base.num_floats;
@@ -783,7 +783,7 @@ impl Flow for BlockFlow {
         for kid in self.base.child_iter() {
             assert!(kid.starts_block_flow() || kid.starts_inline_flow());
 
-            let child_base = flow::mut_base(*kid);
+            let child_base = flow::mut_base(kid);
             child_base.position.origin.x = x_offset;
             child_base.position.size.width = remaining_width;
             child_base.flags_info.flags.set_inorder(has_inorder_children);
diff --git a/src/components/main/layout/flow.rs b/src/components/main/layout/flow.rs
index 6549a425d36..db38b3327ce 100644
--- a/src/components/main/layout/flow.rs
+++ b/src/components/main/layout/flow.rs
@@ -36,8 +36,8 @@ use layout::inline::InlineFlow;
 use layout::parallel::{FlowParallelInfo, UnsafeFlow};
 use layout::parallel;
 use layout::wrapper::ThreadSafeLayoutNode;
+use layout::flow_list::{FlowList, Link, Rawlink, FlowListIterator, MutFlowListIterator};
 
-use extra::dlist::{DList, DListIterator, MutDListIterator};
 use extra::container::Deque;
 use geom::point::Point2D;
 use geom::Size2D;
@@ -134,7 +134,7 @@ pub fn base<'a>(this: &'a Flow) -> &'a BaseFlow {
 }
 
 /// Iterates over the children of this immutable flow.
-pub fn imm_child_iter<'a>(flow: &'a Flow) -> DListIterator<'a,~Flow> {
+pub fn imm_child_iter<'a>(flow: &'a Flow) -> FlowListIterator<'a> {
     base(flow).children.iter()
 }
 
@@ -147,12 +147,12 @@ pub fn mut_base<'a>(this: &'a mut Flow) -> &'a mut BaseFlow {
 }
 
 /// Returns the last child of this flow.
-pub fn last_child<'a>(flow: &'a mut Flow) -> Option<&'a mut ~Flow> {
+pub fn last_child<'a>(flow: &'a mut Flow) -> Option<&'a mut Flow> {
     mut_base(flow).children.back_mut()
 }
 
 /// Iterates over the children of this flow.
-pub fn child_iter<'a>(flow: &'a mut Flow) -> MutDListIterator<'a,~Flow> {
+pub fn child_iter<'a>(flow: &'a mut Flow) -> MutFlowListIterator<'a> {
     mut_base(flow).children.mut_iter()
 }
 
@@ -196,10 +196,10 @@ pub trait MutableFlowUtils {
     // Mutators
 
     /// Invokes a closure with the first child of this flow.
-    fn with_first_child<R>(self, f: |Option<&mut ~Flow>| -> R) -> R;
+    fn with_first_child<R>(self, f: |Option<&mut Flow>| -> R) -> R;
 
     /// Invokes a closure with the last child of this flow.
-    fn with_last_child<R>(self, f: |Option<&mut ~Flow>| -> R) -> R;
+    fn with_last_child<R>(self, f: |Option<&mut Flow>| -> R) -> R;
 
     /// Computes the overflow region for this flow.
     fn store_overflow(self, _: &mut LayoutContext);
@@ -213,6 +213,9 @@ pub trait MutableFlowUtils {
                           index: uint,
                           mut list: &RefCell<DisplayListCollection<E>>)
                           -> bool;
+
+    /// Destroys the flow.
+    fn destroy(self, leaf_set: &FlowLeafSet);
 }
 
 pub trait MutableOwnedFlowUtils {
@@ -492,7 +495,9 @@ pub struct BaseFlow {
     restyle_damage: RestyleDamage,
 
     /// The children of this flow.
-    children: DList<~Flow>,
+    children: FlowList,
+    next_sibling: Link,
+    prev_sibling: Rawlink,
 
     /* TODO (Issue #87): debug only */
     id: int,
@@ -563,7 +568,9 @@ impl BaseFlow {
         BaseFlow {
             restyle_damage: node.restyle_damage(),
 
-            children: DList::new(),
+            children: FlowList::new(),
+            next_sibling: None,
+            prev_sibling: Rawlink::none(),
 
             id: id,
 
@@ -585,7 +592,7 @@ impl BaseFlow {
         }
     }
 
-    pub fn child_iter<'a>(&'a mut self) -> MutDListIterator<'a,~Flow> {
+    pub fn child_iter<'a>(&'a mut self) -> MutFlowListIterator<'a> {
         self.children.mut_iter()
     }
 }
@@ -700,12 +707,12 @@ impl<'a> MutableFlowUtils for &'a mut Flow {
     }
 
     /// Invokes a closure with the first child of this flow.
-    fn with_first_child<R>(self, f: |Option<&mut ~Flow>| -> R) -> R {
+    fn with_first_child<R>(self, f: |Option<&mut Flow>| -> R) -> R {
         f(mut_base(self).children.front_mut())
     }
 
     /// Invokes a closure with the last child of this flow.
-    fn with_last_child<R>(self, f: |Option<&mut ~Flow>| -> R) -> R {
+    fn with_last_child<R>(self, f: |Option<&mut Flow>| -> R) -> R {
         f(mut_base(self).children.back_mut())
     }
 
@@ -713,7 +720,7 @@ impl<'a> MutableFlowUtils for &'a mut Flow {
         let my_position = mut_base(self).position;
         let mut overflow = my_position;
         for kid in mut_base(self).child_iter() {
-            let mut kid_overflow = base(*kid).overflow;
+            let mut kid_overflow = base(kid).overflow;
             kid_overflow = kid_overflow.translate(&my_position.origin);
             overflow = overflow.union(&kid_overflow)
         }
@@ -789,6 +796,23 @@ impl<'a> MutableFlowUtils for &'a mut Flow {
         true
     }
 
+    /// Destroys the flow.
+    fn destroy(self, leaf_set: &FlowLeafSet) {
+        let is_leaf = {
+            let base = mut_base(self);
+            base.children.len() == 0
+        };
+
+        if is_leaf {
+            leaf_set.remove(self);
+        } else {
+            for kid in child_iter(self) {
+                kid.destroy(leaf_set)
+            }
+        }
+
+        mut_base(self).destroyed = true
+    }
 }
 
 impl MutableOwnedFlowUtils for ~Flow {
@@ -818,7 +842,8 @@ impl MutableOwnedFlowUtils for ~Flow {
             }
             base.flags_info.flags.set_is_leaf(true)
         }
-        leaf_set.insert(self)
+        let self_borrowed: &Flow = *self;
+        leaf_set.insert(self_borrowed);
     }
 
     /// Marks the flow as a nonleaf. The flow must not be marked as a leaf.
@@ -833,21 +858,8 @@ impl MutableOwnedFlowUtils for ~Flow {
 
     /// Destroys the flow.
     fn destroy(&mut self, leaf_set: &FlowLeafSet) {
-        let is_leaf = {
-            let base = mut_base(*self);
-            base.children.len() == 0
-        };
-
-        if is_leaf {
-            leaf_set.remove(self);
-        } else {
-            for kid in child_iter(*self) {
-                kid.destroy(leaf_set)
-            }
-        }
-
-        let base = mut_base(*self);
-        base.destroyed = true
+        let self_borrowed: &mut Flow = *self;
+        self_borrowed.destroy(leaf_set);
     }
 }
 
@@ -866,22 +878,22 @@ impl FlowLeafSet {
     }
 
     /// Inserts a newly-created flow into the leaf set.
-    fn insert(&self, flow: &~Flow) {
-        self.set.insert(parallel::owned_flow_to_unsafe_flow(flow), ());
+    fn insert(&self, flow: &Flow) {
+        self.set.insert(parallel::borrowed_flow_to_unsafe_flow(flow), ());
     }
 
     /// Removes a flow from the leaf set. Asserts that the flow was indeed in the leaf set. (This
     /// invariant is needed for memory safety, as there must always be exactly one leaf set.)
-    fn remove(&self, flow: &~Flow) {
+    fn remove(&self, flow: &Flow) {
         if !self.contains(flow) {
             fail!("attempted to remove a flow from the leaf set that wasn't in the set!")
         }
-        let flow = parallel::owned_flow_to_unsafe_flow(flow);
+        let flow = parallel::borrowed_flow_to_unsafe_flow(flow);
         self.set.remove(&flow);
     }
 
-    pub fn contains(&self, flow: &~Flow) -> bool {
-        let flow = parallel::owned_flow_to_unsafe_flow(flow);
+    pub fn contains(&self, flow: &Flow) -> bool {
+        let flow = parallel::borrowed_flow_to_unsafe_flow(flow);
         self.set.contains_key(&flow)
     }
 
diff --git a/src/components/main/layout/flow_list.rs b/src/components/main/layout/flow_list.rs
new file mode 100644
index 00000000000..232b718fba8
--- /dev/null
+++ b/src/components/main/layout/flow_list.rs
@@ -0,0 +1,312 @@
+/* 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/. */
+
+//! A variant of `DList` specialized to store `Flow`s without an extra
+//! indirection.
+
+use std::cast;
+use std::ptr;
+use std::util;
+
+use layout::flow::{Flow, base, mut_base};
+
+pub type Link = Option<~Flow>;
+
+#[deriving(Clone)]
+pub struct Rawlink {
+    priv vtable: *(),
+    priv obj: *mut (),
+}
+
+pub struct FlowList {
+    priv length: uint,
+    priv list_head: Link,
+    priv list_tail: Rawlink,
+}
+
+/// Double-ended FlowList iterator
+#[deriving(Clone)]
+pub struct FlowListIterator<'a> {
+    priv head: &'a Link,
+    priv tail: Rawlink,
+    priv nelem: uint,
+}
+
+/// Double-ended mutable FlowList iterator
+pub struct MutFlowListIterator<'a> {
+    priv list: &'a mut FlowList,
+    priv head: Rawlink,
+    priv tail: Rawlink,
+    priv nelem: uint,
+}
+
+impl Rawlink {
+    /// Like Option::None for Rawlink
+    pub fn none() -> Rawlink {
+        Rawlink {
+            vtable: ptr::null(),
+            obj: ptr::mut_null(),
+        }
+    }
+
+    /// Like Option::Some for Rawlink
+    fn some(n: &mut Flow) -> Rawlink {
+        unsafe { cast::transmute(n) }
+    }
+
+    /// Convert the `Rawlink` into an Option value
+    fn resolve_immut(&self) -> Option<&Flow> {
+        if self.obj.is_null() {
+            None
+        } else {
+            let me: &Flow = unsafe { cast::transmute_copy(self) };
+            Some(me)
+        }
+    }
+
+    fn resolve(&mut self) -> Option<&mut Flow> {
+        if self.obj.is_null() {
+            None
+        } else {
+            let me: &mut Flow = unsafe { cast::transmute_copy(self) };
+            Some(me)
+        }
+    }
+
+    fn is_none(&self) -> bool {
+        self.obj.is_null()
+    }
+
+    unsafe fn get<'a>(&'a mut self) -> &'a mut Flow {
+        assert!(self.obj.is_not_null());
+        cast::transmute_copy(self)
+    }
+}
+
+/// Set the .prev field on `next`, then return `Some(next)`
+fn link_with_prev(mut next: ~Flow, prev: Rawlink) -> Link {
+    mut_base(next).prev_sibling = prev;
+    Some(next)
+}
+
+impl Container for FlowList {
+    /// O(1)
+    #[inline]
+    fn is_empty(&self) -> bool {
+        self.list_head.is_none()
+    }
+    /// O(1)
+    #[inline]
+    fn len(&self) -> uint {
+        self.length
+    }
+}
+
+// This doesn't quite fit the Deque trait because of the need to switch between
+// &Flow and ~Flow.
+impl FlowList {
+    /// Provide a reference to the front element, or None if the list is empty
+    #[inline]
+    pub fn front<'a>(&'a self) -> Option<&'a Flow> {
+        self.list_head.as_ref().map(|head| { let x: &Flow = *head; x })
+    }
+
+    /// Provide a mutable reference to the front element, or None if the list is empty
+    #[inline]
+    pub fn front_mut<'a>(&'a mut self) -> Option<&'a mut Flow> {
+        self.list_head.as_mut().map(|head| { let x: &mut Flow = *head; x })
+    }
+
+    /// Provide a reference to the back element, or None if the list is empty
+    #[inline]
+    pub fn back<'a>(&'a self) -> Option<&'a Flow> {
+        let tmp = self.list_tail.resolve_immut();
+        tmp.as_ref().map(|tail| { let x: &Flow = *tail; x })
+    }
+
+    /// Provide a mutable reference to the back element, or None if the list is empty
+    #[inline]
+    pub fn back_mut<'a>(&'a mut self) -> Option<&'a mut Flow> {
+        // Can't use map() due to error:
+        // lifetime of `tail` is too short to guarantee its contents can be safely reborrowed
+        let tmp = self.list_tail.resolve();
+        match tmp {
+            None => None,
+            Some(tail) => {
+                let x: &mut Flow = tail;
+                Some(x)
+            }
+        }
+    }
+
+    /// Add an element first in the list
+    ///
+    /// O(1)
+    pub fn push_front(&mut self, mut new_head: ~Flow) {
+        match self.list_head {
+            None => {
+                self.list_tail = Rawlink::some(new_head);
+                self.list_head = link_with_prev(new_head, Rawlink::none());
+            }
+            Some(ref mut head) => {
+                mut_base(new_head).prev_sibling = Rawlink::none();
+                mut_base(*head).prev_sibling = Rawlink::some(new_head);
+                util::swap(head, &mut new_head);
+                mut_base(*head).next_sibling = Some(new_head);
+            }
+        }
+        self.length += 1;
+    }
+
+    /// Remove the first element and return it, or None if the list is empty
+    ///
+    /// O(1)
+    pub fn pop_front(&mut self) -> Option<~Flow> {
+        self.list_head.take().map(|mut front_node| {
+            self.length -= 1;
+            match mut_base(front_node).next_sibling.take() {
+                Some(node) => self.list_head = link_with_prev(node, Rawlink::none()),
+                None => self.list_tail = Rawlink::none()
+            }
+            front_node
+        })
+    }
+
+    /// Add an element last in the list
+    ///
+    /// O(1)
+    pub fn push_back(&mut self, mut new_tail: ~Flow) {
+        if self.list_tail.is_none() {
+            return self.push_front(new_tail);
+        } else {
+            let mut old_tail = self.list_tail;
+            self.list_tail = Rawlink::some(new_tail);
+            let tail = unsafe { old_tail.get() };
+            mut_base(tail).next_sibling = link_with_prev(new_tail, Rawlink::some(tail));
+        }
+        self.length += 1;
+    }
+
+    /// Remove the last element and return it, or None if the list is empty
+    ///
+    /// O(1)
+    pub fn pop_back(&mut self) -> Option<~Flow> {
+        if self.list_tail.is_none() {
+            None
+        } else {
+            self.length -= 1;
+
+            self.list_tail = base(unsafe { self.list_tail.get() }).prev_sibling;
+            if self.list_tail.is_none() {
+                self.list_head.take()
+            } else {
+                mut_base(unsafe { self.list_tail.get() }).next_sibling.take()
+            }
+        }
+    }
+
+    /// Create an empty list
+    #[inline]
+    pub fn new() -> FlowList {
+        FlowList {
+            list_head: None,
+            list_tail: Rawlink::none(),
+            length: 0,
+        }
+    }
+
+    /// Provide a forward iterator
+    #[inline]
+    pub fn iter<'a>(&'a self) -> FlowListIterator<'a> {
+        FlowListIterator {
+            nelem: self.len(),
+            head: &self.list_head,
+            tail: self.list_tail
+        }
+    }
+
+    /// Provide a forward iterator with mutable references
+    #[inline]
+    pub fn mut_iter<'a>(&'a mut self) -> MutFlowListIterator<'a> {
+        let head_raw = match self.list_head {
+            Some(ref mut h) => Rawlink::some(*h),
+            None => Rawlink::none(),
+        };
+        MutFlowListIterator {
+            nelem: self.len(),
+            head: head_raw,
+            tail: self.list_tail,
+            list: self
+        }
+    }
+}
+
+#[unsafe_destructor]
+impl Drop for FlowList {
+    fn drop(&mut self) {
+        // Dissolve the list in backwards direction
+        // Just dropping the list_head can lead to stack exhaustion
+        // when length is >> 1_000_000
+        let mut tail = self.list_tail;
+        loop {
+            match tail.resolve() {
+                None => break,
+                Some(prev) => {
+                    let prev_base = mut_base(prev);
+                    prev_base.next_sibling.take();
+                    tail = prev_base.prev_sibling;
+                }
+            }
+        }
+        self.length = 0;
+        self.list_head = None;
+        self.list_tail = Rawlink::none();
+    }
+}
+
+impl<'a> Iterator<&'a Flow> for FlowListIterator<'a> {
+    #[inline]
+    fn next(&mut self) -> Option<&'a Flow> {
+        if self.nelem == 0 {
+            return None;
+        }
+        self.head.as_ref().map(|head| {
+            let head_base = base(*head);
+            self.nelem -= 1;
+            self.head = &head_base.next_sibling;
+            let ret: &Flow = *head;
+            ret
+        })
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (uint, Option<uint>) {
+        (self.nelem, Some(self.nelem))
+    }
+}
+
+impl<'a> Iterator<&'a mut Flow> for MutFlowListIterator<'a> {
+    #[inline]
+    fn next(&mut self) -> Option<&'a mut Flow> {
+        if self.nelem == 0 {
+            return None;
+        }
+        self.head.resolve().map(|next| {
+            self.nelem -= 1;
+            self.head = match mut_base(next).next_sibling {
+                Some(ref mut node) => {
+                    let x: &mut Flow = *node;
+                    Rawlink::some(x)
+                }
+                None => Rawlink::none(),
+            };
+            next
+        })
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (uint, Option<uint>) {
+        (self.nelem, Some(self.nelem))
+    }
+}
diff --git a/src/components/main/layout/inline.rs b/src/components/main/layout/inline.rs
index 2135e104cc4..b14c3dc6be6 100644
--- a/src/components/main/layout/inline.rs
+++ b/src/components/main/layout/inline.rs
@@ -627,7 +627,7 @@ impl Flow for InlineFlow {
         let mut num_floats = 0;
 
         for kid in self.base.child_iter() {
-            let child_base = flow::mut_base(*kid);
+            let child_base = flow::mut_base(kid);
             num_floats += child_base.num_floats;
             child_base.floats_in = FloatContext::new(child_base.num_floats);
         }
@@ -668,7 +668,7 @@ impl Flow for InlineFlow {
         // FIXME(ksh8281) avoid copy
         let flags_info = self.base.flags_info.clone();
         for kid in self.base.child_iter() {
-            let child_base = flow::mut_base(*kid);
+            let child_base = flow::mut_base(kid);
             child_base.position.size.width = self.base.position.size.width;
             child_base.flags_info.flags.set_inorder(self.base.flags_info.flags.inorder());
             child_base.flags_info.propagate_text_alignment_from_parent(&flags_info)
diff --git a/src/components/main/layout/layout_task.rs b/src/components/main/layout/layout_task.rs
index cc8eae3e3db..d6aa0473d76 100644
--- a/src/components/main/layout/layout_task.rs
+++ b/src/components/main/layout/layout_task.rs
@@ -114,7 +114,7 @@ impl PostorderFlowTraversal for ComputeDamageTraversal {
     fn process(&mut self, flow: &mut Flow) -> bool {
         let mut damage = flow::base(flow).restyle_damage;
         for child in flow::child_iter(flow) {
-            damage.union_in_place(flow::base(*child).restyle_damage.propagate_up())
+            damage.union_in_place(flow::base(child).restyle_damage.propagate_up())
         }
         flow::mut_base(flow).restyle_damage = damage;
         true
@@ -139,7 +139,7 @@ impl PreorderFlowTraversal for PropagateDamageTraversal {
         let prop = flow::base(flow).restyle_damage.propagate_down();
         if prop.is_nonempty() {
             for kid_ctx in flow::child_iter(flow) {
-                flow::mut_base(*kid_ctx).restyle_damage.union_in_place(prop)
+                flow::mut_base(kid_ctx).restyle_damage.union_in_place(prop)
             }
         }
         true
diff --git a/src/components/main/layout/parallel.rs b/src/components/main/layout/parallel.rs
index 74c4a5c0073..589224bae4c 100644
--- a/src/components/main/layout/parallel.rs
+++ b/src/components/main/layout/parallel.rs
@@ -47,6 +47,18 @@ pub fn mut_owned_flow_to_unsafe_flow(flow: *mut ~Flow) -> UnsafeFlow {
     }
 }
 
+pub fn borrowed_flow_to_unsafe_flow(flow: &Flow) -> UnsafeFlow {
+    unsafe {
+        cast::transmute_copy(&flow)
+    }
+}
+
+pub fn mut_borrowed_flow_to_unsafe_flow(flow: &mut Flow) -> UnsafeFlow {
+    unsafe {
+        cast::transmute_copy(&flow)
+    }
+}
+
 /// Information that we need stored in each DOM node.
 pub struct DomParallelInfo {
     /// The number of children that still need work done.
diff --git a/src/components/main/servo.rc b/src/components/main/servo.rc
index 8b717f181f9..f45b83e4a53 100755
--- a/src/components/main/servo.rc
+++ b/src/components/main/servo.rc
@@ -92,6 +92,7 @@ pub mod layout {
     pub mod display_list_builder;
     pub mod float_context;
     pub mod flow;
+    pub mod flow_list;
     pub mod layout_task;
     pub mod inline;
     pub mod model;