Replace FlowRef with Arc<Flow>, now that Arc supports DST.

… and WeakFlowRef with Weak<Flow>.

components/layout/construct.rs

@@ -375,7 +375,7 @@ impl<'a> FlowConstructor<'a> {
                                                                 style,
                                                                 child_node.restyle_damage(),
                                                                 SpecificFragmentInfo::TableRow);
-            let mut new_child = FlowRef::new(box TableRowFlow::from_fragment(fragment));
+            let mut new_child: FlowRef = Arc::new(TableRowFlow::from_fragment(fragment));
             new_child.add_new_child(child.clone());
             child.finish();
             *child = new_child
@@ -388,7 +388,7 @@ impl<'a> FlowConstructor<'a> {
                                                                 style,
                                                                 child_node.restyle_damage(),
                                                                 SpecificFragmentInfo::Table);
-            let mut new_child = FlowRef::new(box TableFlow::from_fragment(fragment));
+            let mut new_child: FlowRef = Arc::new(TableFlow::from_fragment(fragment));
             new_child.add_new_child(child.clone());
             child.finish();
             *child = new_child
@@ -402,7 +402,7 @@ impl<'a> FlowConstructor<'a> {
                                                      style,
                                                      child_node.restyle_damage(),
                                                      SpecificFragmentInfo::TableWrapper);
-            let mut new_child = FlowRef::new(box TableWrapperFlow::from_fragment(fragment, None));
+            let mut new_child: FlowRef = Arc::new(TableWrapperFlow::from_fragment(fragment, None));
             new_child.add_new_child(child.clone());
             child.finish();
             *child = new_child
@@ -457,9 +457,8 @@ impl<'a> FlowConstructor<'a> {
         let scanned_fragments =
             TextRunScanner::new().scan_for_runs(&mut self.layout_context.font_context(),
                                                 fragments.fragments);
-        let mut inline_flow_ref =
-            FlowRef::new(box InlineFlow::from_fragments(scanned_fragments,
-                                                        node.style().writing_mode));
+        let mut inline_flow_ref: FlowRef = Arc::new(
+            InlineFlow::from_fragments(scanned_fragments, node.style().writing_mode));
 
         // Add all the inline-block fragments as children of the inline flow.
         for inline_block_flow in &inline_block_flows {
@@ -753,12 +752,12 @@ impl<'a> FlowConstructor<'a> {
     fn build_flow_for_block(&mut self, node: &ThreadSafeLayoutNode, float_kind: Option<FloatKind>)
                             -> ConstructionResult {
         let fragment = self.build_fragment_for_block(node);
-        let flow = if node.style().is_multicol() {
-            box MulticolFlow::from_fragment(fragment, float_kind) as Box<Flow>
+        let flow: FlowRef = if node.style().is_multicol() {
+            Arc::new(MulticolFlow::from_fragment(fragment, float_kind))
         } else {
-            box BlockFlow::from_fragment(fragment, float_kind) as Box<Flow>
+            Arc::new(BlockFlow::from_fragment(fragment, float_kind))
         };
-        self.build_flow_for_block_like(FlowRef::new(flow), node)
+        self.build_flow_for_block_like(flow, node)
     }
 
     /// Bubbles up {ib} splits.
@@ -1072,13 +1071,11 @@ impl<'a> FlowConstructor<'a> {
     fn build_flow_for_table_wrapper(&mut self, node: &ThreadSafeLayoutNode, float_value: float::T)
                                     -> ConstructionResult {
         let fragment = Fragment::new(node, SpecificFragmentInfo::TableWrapper);
-        let wrapper_flow =
-            box TableWrapperFlow::from_fragment(fragment, FloatKind::from_property(float_value));
-        let mut wrapper_flow = FlowRef::new(wrapper_flow as Box<Flow>);
+        let mut wrapper_flow: FlowRef = Arc::new(
+            TableWrapperFlow::from_fragment(fragment, FloatKind::from_property(float_value)));
 
         let table_fragment = Fragment::new(node, SpecificFragmentInfo::Table);
-        let table_flow = box TableFlow::from_fragment(table_fragment);
-        let table_flow = FlowRef::new(table_flow as Box<Flow>);
+        let table_flow = Arc::new(TableFlow::from_fragment(table_fragment));
 
         // First populate the table flow with its children.
         let construction_result = self.build_flow_for_block_like(table_flow, node);
@@ -1135,8 +1132,8 @@ impl<'a> FlowConstructor<'a> {
     /// with possibly other `BlockFlow`s or `InlineFlow`s underneath it.
     fn build_flow_for_table_caption(&mut self, node: &ThreadSafeLayoutNode) -> ConstructionResult {
         let fragment = self.build_fragment_for_block(node);
-        let flow = box TableCaptionFlow::from_fragment(fragment) as Box<Flow>;
-        self.build_flow_for_block_like(FlowRef::new(flow), node)
+        let flow = Arc::new(TableCaptionFlow::from_fragment(fragment));
+        self.build_flow_for_block_like(flow, node)
     }
 
     /// Builds a flow for a node with `display: table-row-group`. This yields a `TableRowGroupFlow`
@@ -1144,16 +1141,16 @@ impl<'a> FlowConstructor<'a> {
     fn build_flow_for_table_rowgroup(&mut self, node: &ThreadSafeLayoutNode)
                                      -> ConstructionResult {
         let fragment = Fragment::new(node, SpecificFragmentInfo::TableRow);
-        let flow = box TableRowGroupFlow::from_fragment(fragment) as Box<Flow>;
-        self.build_flow_for_block_like(FlowRef::new(flow), node)
+        let flow = Arc::new(TableRowGroupFlow::from_fragment(fragment));
+        self.build_flow_for_block_like(flow, node)
     }
 
     /// Builds a flow for a node with `display: table-row`. This yields a `TableRowFlow` with
     /// possibly other `TableCellFlow`s underneath it.
     fn build_flow_for_table_row(&mut self, node: &ThreadSafeLayoutNode) -> ConstructionResult {
         let fragment = Fragment::new(node, SpecificFragmentInfo::TableRow);
-        let flow = box TableRowFlow::from_fragment(fragment) as Box<Flow>;
-        self.build_flow_for_block_like(FlowRef::new(flow), node)
+        let flow = Arc::new(TableRowFlow::from_fragment(fragment));
+        self.build_flow_for_block_like(flow, node)
     }
 
     /// Builds a flow for a node with `display: table-cell`. This yields a `TableCellFlow` with
@@ -1172,9 +1169,9 @@ impl<'a> FlowConstructor<'a> {
                 position == position::T::fixed
             });
 
-        let flow = box TableCellFlow::from_node_fragment_and_visibility_flag(node, fragment, !hide)
-            as Box<Flow>;
-        self.build_flow_for_block_like(FlowRef::new(flow), node)
+        let flow = Arc::new(
+            TableCellFlow::from_node_fragment_and_visibility_flag(node, fragment, !hide));
+        self.build_flow_for_block_like(flow, node)
     }
 
     /// Builds a flow for a node with `display: list-item`. This yields a `ListItemFlow` with
@@ -1222,21 +1219,19 @@ impl<'a> FlowConstructor<'a> {
         let main_fragment = self.build_fragment_for_block(node);
         let flow = match node.style().get_list().list_style_position {
             list_style_position::T::outside => {
-                box ListItemFlow::from_fragments_and_flotation(main_fragment,
-                                                               marker_fragments,
-                                                               flotation)
+                Arc::new(ListItemFlow::from_fragments_and_flotation(
+                    main_fragment, marker_fragments, flotation))
             }
             list_style_position::T::inside => {
                 for marker_fragment in marker_fragments {
                     initial_fragments.fragments.push_back(marker_fragment)
                 }
-                box ListItemFlow::from_fragments_and_flotation(main_fragment, vec![], flotation)
+                Arc::new(ListItemFlow::from_fragments_and_flotation(
+                    main_fragment, vec![], flotation))
             }
         };
 
-        self.build_flow_for_block_starting_with_fragments(FlowRef::new(flow as Box<Flow>),
-                                                          node,
-                                                          initial_fragments)
+        self.build_flow_for_block_starting_with_fragments(flow, node, initial_fragments)
     }
 
     /// Creates a fragment for a node with `display: table-column`.
@@ -1277,8 +1272,7 @@ impl<'a> FlowConstructor<'a> {
             let specific = SpecificFragmentInfo::TableColumn(TableColumnFragmentInfo::new(node));
             col_fragments.push(Fragment::new(node, specific));
         }
-        let flow = box TableColGroupFlow::from_fragments(fragment, col_fragments);
-        let mut flow = FlowRef::new(flow as Box<Flow>);
+        let mut flow: FlowRef = Arc::new(TableColGroupFlow::from_fragments(fragment, col_fragments));
         flow.finish();
 
         ConstructionResult::Flow(flow, AbsoluteDescendants::new())

components/layout/flow.rs

@@ -57,7 +57,7 @@ use std::mem;
 use std::raw;
 use std::slice::IterMut;
 use std::sync::Arc;
-use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::atomic::Ordering;
 use style::computed_values::{clear, display, empty_cells, float, position, text_align};
 use style::properties::{self, ComputedValues};
 use style::values::computed::LengthOrPercentageOrAuto;
@@ -68,7 +68,7 @@ use util::logical_geometry::{LogicalRect, LogicalSize, WritingMode};
 ///
 /// Note that virtual methods have a cost; we should not overuse them in Servo. Consider adding
 /// methods to `ImmutableFlowUtils` or `MutableFlowUtils` before adding more methods here.
-pub trait Flow: fmt::Debug + Sync {
+pub trait Flow: fmt::Debug + Sync + Send + 'static {
     // RTTI
     //
     // TODO(pcwalton): Use Rust's RTTI, once that works.
@@ -770,9 +770,9 @@ pub struct AbsoluteDescendantIter<'a> {
 }
 
 impl<'a> Iterator for AbsoluteDescendantIter<'a> {
-    type Item = &'a mut (Flow + 'a);
+    type Item = &'a mut Flow;
     #[allow(unsafe_code)]
-    fn next(&mut self) -> Option<&'a mut (Flow + 'a)> {
+    fn next(&mut self) -> Option<&'a mut Flow> {
         self.iter.next().map(|info| unsafe { flow_ref::deref_mut(&mut info.flow) })
     }
 }
@@ -815,13 +815,6 @@ impl AbsolutePositionInfo {
 
 /// Data common to all flows.
 pub struct BaseFlow {
-    /// NB: Must be the first element.
-    ///
-    /// The necessity of this will disappear once we have dynamically-sized types.
-    strong_ref_count: AtomicUsize,
-
-    weak_ref_count: AtomicUsize,
-
     pub restyle_damage: RestyleDamage,
 
     /// The children of this flow.
@@ -963,15 +956,6 @@ impl Encodable for BaseFlow {
     }
 }
 
-impl Drop for BaseFlow {
-    fn drop(&mut self) {
-        if self.strong_ref_count.load(Ordering::SeqCst) != 0 &&
-           self.weak_ref_count.load(Ordering::SeqCst) != 0 {
-            panic!("Flow destroyed before its ref count hit zero—this is unsafe!")
-        }
-    }
-}
-
 /// Whether a base flow should be forced to be nonfloated. This can affect e.g. `TableFlow`, which
 /// is never floated because the table wrapper flow is the floated one.
 #[derive(Clone, PartialEq)]
@@ -1039,8 +1023,6 @@ impl BaseFlow {
         damage.remove(RECONSTRUCT_FLOW);
 
         BaseFlow {
-            strong_ref_count: AtomicUsize::new(1),
-            weak_ref_count: AtomicUsize::new(1),
             restyle_damage: damage,
             children: FlowList::new(),
             intrinsic_inline_sizes: IntrinsicISizes::new(),
@@ -1069,16 +1051,6 @@ impl BaseFlow {
         self.children.iter_mut()
     }
 
-    #[allow(unsafe_code)]
-    pub unsafe fn strong_ref_count<'a>(&'a self) -> &'a AtomicUsize {
-        &self.strong_ref_count
-    }
-
-    #[allow(unsafe_code)]
-    pub unsafe fn weak_ref_count<'a>(&'a self) -> &'a AtomicUsize {
-        &self.weak_ref_count
-    }
-
     pub fn debug_id(&self) -> usize {
         let p = self as *const _;
         p as usize
@@ -1115,7 +1087,7 @@ impl BaseFlow {
     }
 }
 
-impl<'a> ImmutableFlowUtils for &'a (Flow + 'a) {
+impl<'a> ImmutableFlowUtils for &'a Flow {
     /// Returns true if this flow is a block flow or subclass thereof.
     fn is_block_like(self) -> bool {
         match self.class() {
@@ -1213,7 +1185,7 @@ impl<'a> ImmutableFlowUtils for &'a (Flow + 'a) {
     /// as it's harder to understand.
     fn generate_missing_child_flow(self, node: &ThreadSafeLayoutNode) -> FlowRef {
         let mut style = node.style().clone();
-        let flow = match self.class() {
+        match self.class() {
             FlowClass::Table | FlowClass::TableRowGroup => {
                 properties::modify_style_for_anonymous_table_object(
                     &mut style,
@@ -1224,7 +1196,7 @@ impl<'a> ImmutableFlowUtils for &'a (Flow + 'a) {
                     style,
                     node.restyle_damage(),
                     SpecificFragmentInfo::TableRow);
-                box TableRowFlow::from_fragment(fragment) as Box<Flow>
+                Arc::new(TableRowFlow::from_fragment(fragment))
             },
             FlowClass::TableRow => {
                 properties::modify_style_for_anonymous_table_object(
@@ -1237,14 +1209,12 @@ impl<'a> ImmutableFlowUtils for &'a (Flow + 'a) {
                     node.restyle_damage(),
                     SpecificFragmentInfo::TableCell);
                 let hide = node.style().get_inheritedtable().empty_cells == empty_cells::T::hide;
-                box TableCellFlow::from_node_fragment_and_visibility_flag(node, fragment, !hide) as
-                    Box<Flow>
+                Arc::new(TableCellFlow::from_node_fragment_and_visibility_flag(node, fragment, !hide))
             },
             _ => {
                 panic!("no need to generate a missing child")
             }
-        };
-        FlowRef::new(flow)
+        }
     }
 
     /// Returns true if this flow contains fragments that are roots of an absolute flow tree.
@@ -1315,7 +1285,7 @@ impl<'a> ImmutableFlowUtils for &'a (Flow + 'a) {
     }
 }
 
-impl<'a> MutableFlowUtils for &'a mut (Flow + 'a) {
+impl<'a> MutableFlowUtils for &'a mut Flow {
     /// Traverses the tree in preorder.
     fn traverse_preorder<T: PreorderFlowTraversal>(self, traversal: &T) {
         if traversal.should_process(self) {

components/layout/flow_list.rs

@@ -108,9 +108,9 @@ impl FlowList {
 }
 
 impl<'a> Iterator for FlowListIterator<'a> {
-    type Item = &'a (Flow + 'a);
+    type Item = &'a Flow;
     #[inline]
-    fn next(&mut self) -> Option<&'a (Flow + 'a)> {
+    fn next(&mut self) -> Option<&'a Flow> {
         self.it.next().map(|x| &**x)
     }
 
@@ -121,10 +121,10 @@ impl<'a> Iterator for FlowListIterator<'a> {
 }
 
 impl<'a> Iterator for MutFlowListIterator<'a> {
-    type Item = &'a mut (Flow + 'a);
+    type Item = &'a mut Flow;
     #[inline]
     #[allow(unsafe_code)]
-    fn next(&mut self) -> Option<&'a mut (Flow + 'a)> {
+    fn next(&mut self) -> Option<&'a mut Flow> {
         self.it.next().map(|x| unsafe { flow_ref::deref_mut(x) })
     }
 

components/layout/flow_ref.rs

@@ -10,190 +10,15 @@
 
 #![allow(unsafe_code)]
 
-use flow;
-use flow::{Flow, BaseFlow};
+use flow::Flow;
+use std::sync::{Arc, Weak};
 
-use std::mem;
-use std::ops::Deref;
-use std::ptr;
-use std::raw;
-use std::rt::heap;
-use std::sync::atomic::{self, Ordering};
-
-#[unsafe_no_drop_flag]
-pub struct FlowRef {
-    object: raw::TraitObject,
-}
-
-unsafe impl Send for FlowRef {}
-unsafe impl Sync for FlowRef {}
-
-#[unsafe_no_drop_flag]
-pub struct WeakFlowRef {
-    object: raw::TraitObject,
-}
-
-unsafe impl Send for WeakFlowRef {}
-unsafe impl Sync for WeakFlowRef {}
-
-impl FlowRef {
-    pub fn new(mut flow: Box<Flow>) -> FlowRef {
-        unsafe {
-            let result = {
-                let flow_ref: &mut Flow = &mut *flow;
-                let object = mem::transmute::<&mut Flow, raw::TraitObject>(flow_ref);
-                FlowRef { object: object }
-            };
-            mem::forget(flow);
-            result
-        }
-    }
-
-    /// Downgrades the FlowRef to a WeakFlowRef.
-    pub fn downgrade(&self) -> WeakFlowRef {
-        unsafe {
-            flow::base(&**self).weak_ref_count().fetch_add(1, Ordering::Relaxed);
-        }
-        WeakFlowRef { object: self.object }
-    }
-}
-
-impl<'a> Deref for FlowRef {
-    type Target = Flow + 'a;
-    fn deref(&self) -> &(Flow + 'a) {
-        unsafe {
-            mem::transmute_copy::<raw::TraitObject, &(Flow + 'a)>(&self.object)
-        }
-    }
-}
+pub type FlowRef = Arc<Flow>;
+pub type WeakFlowRef = Weak<Flow>;
 
 // FIXME(https://github.com/servo/servo/issues/6503) This introduces unsound mutable aliasing.
 // Try to replace it with Arc::get_mut (which checks that the reference count is 1).
-pub unsafe fn deref_mut<'a>(flow_ref: &mut FlowRef) -> &mut (Flow + 'a) {
-    mem::transmute_copy::<raw::TraitObject, &mut (Flow + 'a)>(&flow_ref.object)
-}
-
-impl Drop for FlowRef {
-    fn drop(&mut self) {
-        unsafe {
-            if self.object.vtable.is_null() ||
-               self.object.vtable as usize == mem::POST_DROP_USIZE {
-                return
-            }
-            if flow::base(&**self).strong_ref_count().fetch_sub(1, Ordering::Release) != 1 {
-                return
-            }
-            atomic::fence(Ordering::Acquire);
-
-            // Normally we'd call the underlying Drop logic but not free the
-            // allocation, but this is almost impossible without DST in
-            // Rust. Instead we make a fake trait object to run the drop logic
-            // on.
-            let flow_ref: FlowRef = mem::replace(self, FlowRef {
-                object: raw::TraitObject {
-                    vtable: ptr::null_mut(),
-                    data: ptr::null_mut(),
-                }
-            });
-
-            let vtable: &[usize; 3] = mem::transmute::<*mut (), &[usize; 3]>(flow_ref.object.vtable);
-            let object_size = vtable[1];
-            let object_align = vtable[2];
-
-            let fake_data = heap::allocate(object_size, object_align);
-            ptr::copy(flow_ref.object.data as *const u8, fake_data, object_size);
-
-            let fake_box = raw::TraitObject { vtable: flow_ref.object.vtable, data: fake_data as *mut () };
-            let fake_flow = mem::transmute::<raw::TraitObject, Box<Flow>>(fake_box);
-            drop(fake_flow);
-
-            if flow::base(&*flow_ref).weak_ref_count().fetch_sub(1, Ordering::Release) == 1 {
-                atomic::fence(Ordering::Acquire);
-                heap::deallocate(flow_ref.object.data as *mut u8, object_size, object_align);
-            }
-
-            mem::forget(flow_ref);
-        }
-    }
-}
-
-impl Clone for FlowRef {
-    fn clone(&self) -> FlowRef {
-        unsafe {
-            let _ = flow::base(&**self).strong_ref_count().fetch_add(1, Ordering::Relaxed);
-            FlowRef {
-                object: raw::TraitObject {
-                    vtable: self.object.vtable,
-                    data: self.object.data,
-                }
-            }
-        }
-    }
-}
-
-fn base<'a>(r: &WeakFlowRef) -> &'a BaseFlow {
-    let data = r.object.data;
-    debug_assert!(!data.is_null());
-    unsafe {
-        mem::transmute::<*mut (), &'a BaseFlow>(data)
-    }
-}
-
-impl WeakFlowRef {
-    /// Upgrades a WeakFlowRef to a FlowRef.
-    pub fn upgrade(&self) -> Option<FlowRef> {
-        unsafe {
-            let object = base(self);
-            // We use a CAS loop to increment the strong count instead of a
-            // fetch_add because once the count hits 0 is must never be above
-            // 0.
-            loop {
-                let n = object.strong_ref_count().load(Ordering::SeqCst);
-                if n == 0 { return None }
-                let old = object.strong_ref_count().compare_and_swap(n, n + 1, Ordering::SeqCst);
-                if old == n {
-                    return Some(FlowRef { object: self.object })
-                }
-            }
-        }
-    }
-}
-
-impl Clone for WeakFlowRef {
-    fn clone(&self) -> WeakFlowRef {
-        unsafe {
-            base(self).weak_ref_count().fetch_add(1, Ordering::Relaxed);
-        }
-        WeakFlowRef { object: self. object }
-    }
-}
-
-impl Drop for WeakFlowRef {
-    fn drop(&mut self) {
-        unsafe {
-            if self.object.vtable.is_null() ||
-               self.object.vtable as usize == mem::POST_DROP_USIZE {
-                return
-            }
-
-            if base(self).weak_ref_count().fetch_sub(1, Ordering::Release) == 1 {
-                atomic::fence(Ordering::Acquire);
-
-                // This dance deallocates the Box<Flow> without running its
-                // drop glue. The drop glue is run when the last strong
-                // reference is released.
-                let weak_ref: WeakFlowRef = mem::replace(self, WeakFlowRef {
-                    object: raw::TraitObject {
-                        vtable: ptr::null_mut(),
-                        data: ptr::null_mut(),
-                    }
-                });
-                let vtable: &[usize; 3] = mem::transmute::<*mut (), &[usize; 3]>(weak_ref.object.vtable);
-                let object_size = vtable[1];
-                let object_align = vtable[2];
-                heap::deallocate(weak_ref.object.data as *mut u8, object_size, object_align);
-                mem::forget(weak_ref);
-            }
-        }
-    }
+pub unsafe fn deref_mut<'a>(r: &'a mut FlowRef) -> &'a mut Flow {
+    let ptr: *const Flow = &**r;
+    &mut *(ptr as *mut Flow)
 }

components/layout/incremental.rs

@@ -192,7 +192,7 @@ pub trait LayoutDamageComputation {
     fn reflow_entire_document(self);
 }
 
-impl<'a> LayoutDamageComputation for &'a mut (Flow + 'a) {
+impl<'a> LayoutDamageComputation for &'a mut Flow {
     fn compute_layout_damage(self) -> SpecialRestyleDamage {
         let mut special_damage = SpecialRestyleDamage::empty();
         let is_absolutely_positioned = flow::base(self).flags.contains(IS_ABSOLUTELY_POSITIONED);

components/layout/lib.rs

@@ -4,13 +4,12 @@
 
 #![feature(append)]
 #![feature(arc_unique)]
+#![feature(arc_weak)]
 #![feature(box_str)]
 #![feature(box_syntax)]
 #![feature(cell_extras)]
 #![feature(custom_derive)]
-#![feature(filling_drop)]
 #![feature(hashmap_hasher)]
-#![feature(heap_api)]
 #![feature(mpsc_select)]
 #![feature(plugin)]
 #![feature(raw)]

components/layout/parallel.rs

@@ -47,25 +47,25 @@ fn null_unsafe_flow() -> UnsafeFlow {
 
 pub fn owned_flow_to_unsafe_flow(flow: *const FlowRef) -> UnsafeFlow {
     unsafe {
-        mem::transmute_copy(&*flow)
+        mem::transmute::<&Flow, UnsafeFlow>(&**flow)
     }
 }
 
 pub fn mut_owned_flow_to_unsafe_flow(flow: *mut FlowRef) -> UnsafeFlow {
     unsafe {
-        mem::transmute_copy(&*flow)
+        mem::transmute::<&Flow, UnsafeFlow>(&**flow)
     }
 }
 
 pub fn borrowed_flow_to_unsafe_flow(flow: &Flow) -> UnsafeFlow {
     unsafe {
-        mem::transmute_copy(&flow)
+        mem::transmute::<&Flow, UnsafeFlow>(flow)
     }
 }
 
 pub fn mut_borrowed_flow_to_unsafe_flow(flow: &mut Flow) -> UnsafeFlow {
     unsafe {
-        mem::transmute_copy(&flow)
+        mem::transmute::<&Flow, UnsafeFlow>(flow)
     }
 }
 
@@ -239,10 +239,9 @@ trait ParallelPostorderFlowTraversal : PostorderFlowTraversal {
     fn run_parallel(&self, mut unsafe_flow: UnsafeFlow) {
         loop {
             // Get a real flow.
-            let flow: &mut FlowRef = unsafe {
-                mem::transmute(&mut unsafe_flow)
+            let flow: &mut Flow = unsafe {
+                mem::transmute(unsafe_flow)
             };
-            let flow = unsafe { flow_ref::deref_mut(flow) };
 
             // Perform the appropriate traversal.
             if self.should_process(flow) {
@@ -257,7 +256,7 @@ trait ParallelPostorderFlowTraversal : PostorderFlowTraversal {
                                                Ordering::Relaxed);
 
             // Possibly enqueue the parent.
-            let mut unsafe_parent = base.parallel.parent;
+            let unsafe_parent = base.parallel.parent;
             if unsafe_parent == null_unsafe_flow() {
                 // We're done!
                 break
@@ -266,10 +265,10 @@ trait ParallelPostorderFlowTraversal : PostorderFlowTraversal {
             // No, we're not at the root yet. Then are we the last child
             // of our parent to finish processing? If so, we can continue
             // on with our parent; otherwise, we've gotta wait.
-            let parent: &mut FlowRef = unsafe {
-                mem::transmute(&mut unsafe_parent)
+            let parent: &mut Flow = unsafe {
+                mem::transmute(unsafe_parent)
             };
-            let parent_base = flow::mut_base(unsafe { flow_ref::deref_mut(parent) });
+            let parent_base = flow::mut_base(parent);
             if parent_base.parallel.children_count.fetch_sub(1, Ordering::Relaxed) == 1 {
                 // We were the last child of our parent. Reflow our parent.
                 unsafe_flow = unsafe_parent
@@ -296,12 +295,11 @@ trait ParallelPreorderFlowTraversal : PreorderFlowTraversal {
                            top_down_func: ChunkedFlowTraversalFunction,
                            bottom_up_func: FlowTraversalFunction) {
         let mut discovered_child_flows = Vec::new();
-        for mut unsafe_flow in *unsafe_flows.0 {
+        for unsafe_flow in *unsafe_flows.0 {
             let mut had_children = false;
             unsafe {
                 // Get a real flow.
-                let flow: &mut FlowRef = mem::transmute(&mut unsafe_flow);
-                let flow = flow_ref::deref_mut(flow);
+                let flow: &mut Flow = mem::transmute(unsafe_flow);
 
                 if self.should_record_thread_ids() {
                     flow::mut_base(flow).thread_id = proxy.worker_index();