Introduce and use Scoped TLS.

It turns out that it's problematic to embed ThreadLocalStyleContext within
LayoutContext, because parameterizing the former on TElement (which we do
in the next patch) infects all the traversal stuff with the trait parameters,
which we don't really want.

In general, it probably makes sense to use separate scoped TLS types for
the separate DOM and Flow tree passes, so we can add a different ScopedTLS
type for the Flow pass if we ever need it.

We also reorder the |scope| and |shared| parameters in parallel.rs, because
it aligns more with the order in style/parallel.rs. I did this when I was
adding a TLS parameter to all these functions, which I realized we don't need
for now.

16 files changed, 213 insertions, 206 deletions

diff --git a/components/layout/context.rs b/components/layout/context.rs
index 9497d736b47..cc20fa4ef0c 100644
--- a/components/layout/context.rs
+++ b/components/layout/context.rs
@@ -19,7 +19,6 @@ use net_traits::image_cache_thread::{ImageOrMetadataAvailable, UsePlaceholder};
 use parking_lot::RwLock;
 use servo_config::opts;
 use servo_url::ServoUrl;
-use std::borrow::Borrow;
 use std::cell::{RefCell, RefMut};
 use std::collections::HashMap;
 use std::hash::BuildHasherDefault;
@@ -27,59 +26,64 @@ use std::rc::Rc;
 use std::sync::{Arc, Mutex};
 use style::context::{SharedStyleContext, ThreadLocalStyleContext};
 
-pub struct ThreadLocalLayoutContext {
+/// TLS data scoped to the traversal.
+pub struct ScopedThreadLocalLayoutContext {
     pub style_context: ThreadLocalStyleContext,
+}
+
+impl ScopedThreadLocalLayoutContext {
+    pub fn new(shared: &SharedLayoutContext) -> Self {
+        ScopedThreadLocalLayoutContext {
+            style_context: ThreadLocalStyleContext::new(&shared.style_context),
+        }
+    }
+}
+
+/// TLS data that persists across traversals.
+pub struct PersistentThreadLocalLayoutContext {
+    // FontContext uses Rc all over the place and so isn't Send, which means we
+    // can't use ScopedTLS for it. There's also no reason to scope it to the
+    // traversal, and performance is probably better if we don't.
     pub font_context: RefCell<FontContext>,
 }
 
-impl ThreadLocalLayoutContext {
+impl PersistentThreadLocalLayoutContext {
     pub fn new(shared: &SharedLayoutContext) -> Rc<Self> {
         let font_cache_thread = shared.font_cache_thread.lock().unwrap().clone();
-        let local_style_data = shared.style_context.local_context_creation_data.lock().unwrap();
-
-        Rc::new(ThreadLocalLayoutContext {
-            style_context: ThreadLocalStyleContext::new(&local_style_data),
+        Rc::new(PersistentThreadLocalLayoutContext {
             font_context: RefCell::new(FontContext::new(font_cache_thread)),
         })
     }
 }
 
-impl Borrow<ThreadLocalStyleContext> for ThreadLocalLayoutContext {
-    fn borrow(&self) -> &ThreadLocalStyleContext {
-        &self.style_context
-    }
-}
-
-impl HeapSizeOf for ThreadLocalLayoutContext {
-    // FIXME(njn): measure other fields eventually.
+impl HeapSizeOf for PersistentThreadLocalLayoutContext {
     fn heap_size_of_children(&self) -> usize {
         self.font_context.heap_size_of_children()
     }
 }
 
-thread_local!(static LOCAL_CONTEXT_KEY: RefCell<Option<Rc<ThreadLocalLayoutContext>>> = RefCell::new(None));
+thread_local!(static LOCAL_CONTEXT_KEY: RefCell<Option<Rc<PersistentThreadLocalLayoutContext>>> = RefCell::new(None));
 
-pub fn heap_size_of_local_context() -> usize {
-    LOCAL_CONTEXT_KEY.with(|r| {
-        r.borrow().clone().map_or(0, |context| context.heap_size_of_children())
-    })
-}
-
-// Keep this implementation in sync with the one in ports/geckolib/traversal.rs.
-pub fn create_or_get_local_context(shared: &SharedLayoutContext)
-                                   -> Rc<ThreadLocalLayoutContext> {
+fn create_or_get_persistent_context(shared: &SharedLayoutContext)
+                                    -> Rc<PersistentThreadLocalLayoutContext> {
     LOCAL_CONTEXT_KEY.with(|r| {
         let mut r = r.borrow_mut();
         if let Some(context) = r.clone() {
             context
         } else {
-            let context = ThreadLocalLayoutContext::new(shared);
+            let context = PersistentThreadLocalLayoutContext::new(shared);
             *r = Some(context.clone());
             context
         }
     })
 }
 
+pub fn heap_size_of_persistent_local_context() -> usize {
+    LOCAL_CONTEXT_KEY.with(|r| {
+        r.borrow().clone().map_or(0, |context| context.heap_size_of_children())
+    })
+}
+
 /// Layout information shared among all workers. This must be thread-safe.
 pub struct SharedLayoutContext {
     /// Bits shared by the layout and style system.
@@ -100,24 +104,17 @@ pub struct SharedLayoutContext {
                                                   BuildHasherDefault<FnvHasher>>>>,
 }
 
-impl Borrow<SharedStyleContext> for SharedLayoutContext {
-    fn borrow(&self) -> &SharedStyleContext {
-        &self.style_context
-    }
-}
-
 pub struct LayoutContext<'a> {
     pub shared: &'a SharedLayoutContext,
-    pub thread_local: &'a ThreadLocalLayoutContext,
+    pub persistent: Rc<PersistentThreadLocalLayoutContext>,
 }
 
 impl<'a> LayoutContext<'a> {
-    pub fn new(shared: &'a SharedLayoutContext,
-               thread_local: &'a ThreadLocalLayoutContext) -> Self
+    pub fn new(shared: &'a SharedLayoutContext) -> Self
     {
         LayoutContext {
             shared: shared,
-            thread_local: thread_local,
+            persistent: create_or_get_persistent_context(shared),
         }
     }
 }
@@ -138,7 +135,7 @@ impl<'a> LayoutContext<'a> {
 
     #[inline(always)]
     pub fn font_context(&self) -> RefMut<FontContext> {
-        self.thread_local.font_context.borrow_mut()
+        self.persistent.font_context.borrow_mut()
     }
 }
 
diff --git a/components/layout/parallel.rs b/components/layout/parallel.rs
index 505afa7897f..a51f585eb3c 100644
--- a/components/layout/parallel.rs
+++ b/components/layout/parallel.rs
@@ -8,8 +8,7 @@
 
 #![allow(unsafe_code)]
 
-use context::{LayoutContext, SharedLayoutContext, ThreadLocalLayoutContext};
-use context::create_or_get_local_context;
+use context::{LayoutContext, SharedLayoutContext};
 use flow::{self, Flow, MutableFlowUtils, PostorderFlowTraversal, PreorderFlowTraversal};
 use flow_ref::FlowRef;
 use profile_traits::time::{self, TimerMetadata, profile};
@@ -51,7 +50,7 @@ pub fn borrowed_flow_to_unsafe_flow(flow: &Flow) -> UnsafeFlow {
 }
 
 pub type ChunkedFlowTraversalFunction<'scope> =
-    extern "Rust" fn(Box<[UnsafeFlow]>, &'scope SharedLayoutContext, &rayon::Scope<'scope>);
+    extern "Rust" fn(Box<[UnsafeFlow]>, &rayon::Scope<'scope>, &'scope SharedLayoutContext);
 
 pub type FlowTraversalFunction = extern "Rust" fn(UnsafeFlow, &LayoutContext);
 
@@ -133,23 +132,22 @@ trait ParallelPostorderFlowTraversal : PostorderFlowTraversal {
 trait ParallelPreorderFlowTraversal : PreorderFlowTraversal {
     fn run_parallel<'scope>(&self,
                             unsafe_flows: &[UnsafeFlow],
-                            layout_context: &'scope SharedLayoutContext,
-                            scope: &rayon::Scope<'scope>);
+                            scope: &rayon::Scope<'scope>,
+                            shared: &'scope SharedLayoutContext);
 
     fn should_record_thread_ids(&self) -> bool;
 
     #[inline(always)]
     fn run_parallel_helper<'scope>(&self,
                                    unsafe_flows: &[UnsafeFlow],
-                                   shared: &'scope SharedLayoutContext,
                                    scope: &rayon::Scope<'scope>,
+                                   shared: &'scope SharedLayoutContext,
                                    top_down_func: ChunkedFlowTraversalFunction<'scope>,
                                    bottom_up_func: FlowTraversalFunction)
     {
-        let tlc = create_or_get_local_context(shared);
-        let context = LayoutContext::new(&shared, &*tlc);
-
         let mut discovered_child_flows = vec![];
+        let context = LayoutContext::new(&shared);
+
         for unsafe_flow in unsafe_flows {
             let mut had_children = false;
             unsafe {
@@ -187,7 +185,7 @@ trait ParallelPreorderFlowTraversal : PreorderFlowTraversal {
             let nodes = chunk.iter().cloned().collect::<Vec<_>>().into_boxed_slice();
 
             scope.spawn(move |scope| {
-                top_down_func(nodes, shared, scope);
+                top_down_func(nodes, scope, shared);
             });
         }
     }
@@ -196,12 +194,12 @@ trait ParallelPreorderFlowTraversal : PreorderFlowTraversal {
 impl<'a> ParallelPreorderFlowTraversal for AssignISizes<'a> {
     fn run_parallel<'scope>(&self,
                                   unsafe_flows: &[UnsafeFlow],
-                                  layout_context: &'scope SharedLayoutContext,
-                                  scope: &rayon::Scope<'scope>)
+                                  scope: &rayon::Scope<'scope>,
+                                  shared: &'scope SharedLayoutContext)
     {
         self.run_parallel_helper(unsafe_flows,
-                                 layout_context,
                                  scope,
+                                 shared,
                                  assign_inline_sizes,
                                  assign_block_sizes_and_store_overflow)
     }
@@ -214,12 +212,12 @@ impl<'a> ParallelPreorderFlowTraversal for AssignISizes<'a> {
 impl<'a> ParallelPostorderFlowTraversal for AssignBSizes<'a> {}
 
 fn assign_inline_sizes<'scope>(unsafe_flows: Box<[UnsafeFlow]>,
-                               shared_layout_context: &'scope SharedLayoutContext,
-                               scope: &rayon::Scope<'scope>) {
+                               scope: &rayon::Scope<'scope>,
+                               shared: &'scope SharedLayoutContext) {
     let assign_inline_sizes_traversal = AssignISizes {
-        shared_context: &shared_layout_context.style_context,
+        shared_context: &shared.style_context,
     };
-    assign_inline_sizes_traversal.run_parallel(&unsafe_flows, shared_layout_context, scope)
+    assign_inline_sizes_traversal.run_parallel(&unsafe_flows, scope, shared)
 }
 
 fn assign_block_sizes_and_store_overflow(
@@ -238,8 +236,7 @@ pub fn traverse_flow_tree_preorder(
         shared: &SharedLayoutContext,
         queue: &rayon::ThreadPool) {
     if opts::get().bubble_inline_sizes_separately {
-        let tlc = ThreadLocalLayoutContext::new(shared);
-        let context = LayoutContext::new(shared, &*tlc);
+        let context = LayoutContext::new(shared);
         let bubble_inline_sizes = BubbleISizes { layout_context: &context };
         root.traverse_postorder(&bubble_inline_sizes);
     }
@@ -250,7 +247,7 @@ pub fn traverse_flow_tree_preorder(
         rayon::scope(move |scope| {
             profile(time::ProfilerCategory::LayoutParallelWarmup,
                     profiler_metadata, time_profiler_chan, move || {
-                assign_inline_sizes(nodes, &shared, scope);
+                assign_inline_sizes(nodes, scope, &shared);
             });
         });
     });
diff --git a/components/layout/query.rs b/components/layout/query.rs
index b6eb7cfeb39..0a44a9b1b49 100644
--- a/components/layout/query.rs
+++ b/components/layout/query.rs
@@ -6,8 +6,7 @@
 
 use app_units::Au;
 use construct::ConstructionResult;
-use context::SharedLayoutContext;
-use context::create_or_get_local_context;
+use context::{ScopedThreadLocalLayoutContext, SharedLayoutContext};
 use euclid::point::Point2D;
 use euclid::rect::Rect;
 use euclid::size::Size2D;
@@ -646,10 +645,10 @@ pub fn process_resolved_style_request<'a, N>(shared: &SharedLayoutContext,
     // we'd need a mechanism to prevent detect when it's stale (since we don't
     // traverse display:none subtrees during restyle).
     let display_none_root = if element.get_data().is_none() {
-        let tlc = create_or_get_local_context(shared);
+        let mut tlc = ScopedThreadLocalLayoutContext::new(shared);
         let context = StyleContext {
             shared: &shared.style_context,
-            thread_local: &tlc.style_context,
+            thread_local: &mut tlc.style_context,
         };
 
         Some(style_element_in_display_none_subtree(&context, element,
diff --git a/components/layout/sequential.rs b/components/layout/sequential.rs
index e5f62fe1cea..b8c62aaafb0 100644
--- a/components/layout/sequential.rs
+++ b/components/layout/sequential.rs
@@ -5,7 +5,7 @@
 //! Implements sequential traversals over the DOM and flow trees.
 
 use app_units::Au;
-use context::{LayoutContext, SharedLayoutContext, ThreadLocalLayoutContext};
+use context::{LayoutContext, SharedLayoutContext};
 use display_list_builder::DisplayListBuildState;
 use euclid::point::Point2D;
 use floats::SpeculatedFloatPlacement;
@@ -34,8 +34,7 @@ pub fn resolve_generated_content(root: &mut Flow, shared: &SharedLayoutContext)
         }
     }
 
-    let tlc = ThreadLocalLayoutContext::new(shared);
-    let layout_context = LayoutContext::new(shared, &*tlc);
+    let layout_context = LayoutContext::new(shared);
     let mut traversal = ResolveGeneratedContent::new(&layout_context);
     doit(root, 0, &mut traversal)
 }
@@ -58,8 +57,7 @@ pub fn traverse_flow_tree_preorder(root: &mut Flow,
         }
     }
 
-    let tlc = ThreadLocalLayoutContext::new(shared);
-    let layout_context = LayoutContext::new(shared, &*tlc);
+    let layout_context = LayoutContext::new(shared);
 
     if opts::get().bubble_inline_sizes_separately {
         let bubble_inline_sizes = BubbleISizes { layout_context: &layout_context };
diff --git a/components/layout/traversal.rs b/components/layout/traversal.rs
index 43ef8e14fda..c94dccab2ff 100644
--- a/components/layout/traversal.rs
+++ b/components/layout/traversal.rs
@@ -5,15 +5,13 @@
 //! Traversals over the DOM and flow trees, running the layout computations.
 
 use construct::FlowConstructor;
-use context::{LayoutContext, SharedLayoutContext, ThreadLocalLayoutContext};
-use context::create_or_get_local_context;
+use context::{LayoutContext, ScopedThreadLocalLayoutContext, SharedLayoutContext};
 use display_list_builder::DisplayListBuildState;
 use flow::{self, PreorderFlowTraversal};
 use flow::{CAN_BE_FRAGMENTED, Flow, ImmutableFlowUtils, PostorderFlowTraversal};
 use gfx::display_list::OpaqueNode;
 use script_layout_interface::wrapper_traits::{LayoutNode, ThreadSafeLayoutNode};
 use servo_config::opts;
-use std::rc::Rc;
 use style::atomic_refcell::AtomicRefCell;
 use style::context::{SharedStyleContext, StyleContext};
 use style::data::ElementData;
@@ -56,9 +54,10 @@ impl<N> DomTraversal<N> for RecalcStyleAndConstructFlows
     where N: LayoutNode + TNode,
           N::ConcreteElement: TElement
 {
-    type ThreadLocalContext = ThreadLocalLayoutContext;
+    type ThreadLocalContext = ScopedThreadLocalLayoutContext;
 
-    fn process_preorder(&self, node: N, traversal_data: &mut PerLevelTraversalData) {
+    fn process_preorder(&self, traversal_data: &mut PerLevelTraversalData,
+                        thread_local: &mut Self::ThreadLocalContext, node: N) {
         // FIXME(pcwalton): Stop allocating here. Ideally this should just be
         // done by the HTML parser.
         node.initialize_data();
@@ -66,19 +65,17 @@ impl<N> DomTraversal<N> for RecalcStyleAndConstructFlows
         if !node.is_text_node() {
             let el = node.as_element().unwrap();
             let mut data = el.mutate_data().unwrap();
-            let tlc = create_or_get_local_context(&self.shared);
-            let context = StyleContext {
+            let mut context = StyleContext {
                 shared: &self.shared.style_context,
-                thread_local: &tlc.style_context,
+                thread_local: &mut thread_local.style_context,
             };
-            recalc_style_at(self, traversal_data, &context, el, &mut data);
+            recalc_style_at(self, traversal_data, &mut context, el, &mut data);
         }
     }
 
-    fn process_postorder(&self, node: N) {
-        let tlc = create_or_get_local_context(&self.shared);
-        let context = LayoutContext::new(&self.shared, &*tlc);
-        construct_flows_at(&context, self.root, node);
+    fn process_postorder(&self, thread_local: &mut Self::ThreadLocalContext, node: N) {
+        let context = LayoutContext::new(&self.shared);
+        construct_flows_at(&context, thread_local, self.root, node);
     }
 
     fn text_node_needs_traversal(node: N) -> bool {
@@ -103,8 +100,8 @@ impl<N> DomTraversal<N> for RecalcStyleAndConstructFlows
         &self.shared.style_context
     }
 
-    fn create_or_get_thread_local_context(&self) -> Rc<ThreadLocalLayoutContext> {
-        create_or_get_local_context(&self.shared)
+    fn create_thread_local_context(&self) -> Self::ThreadLocalContext {
+        ScopedThreadLocalLayoutContext::new(&self.shared)
     }
 }
 
@@ -117,7 +114,9 @@ pub trait PostorderNodeMutTraversal<ConcreteThreadSafeLayoutNode: ThreadSafeLayo
 /// The flow construction traversal, which builds flows for styled nodes.
 #[inline]
 #[allow(unsafe_code)]
-fn construct_flows_at<'a, N>(context: &LayoutContext<'a>, root: OpaqueNode, node: N)
+fn construct_flows_at<'a, N>(context: &LayoutContext<'a>,
+                             _thread_local: &ScopedThreadLocalLayoutContext,
+                             root: OpaqueNode, node: N)
     where N: LayoutNode,
 {
     debug!("construct_flows_at: {:?}", node);
diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index d2bdf3a8f35..524b325dcec 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -63,7 +63,8 @@ use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
 use ipc_channel::router::ROUTER;
 use layout::animation;
 use layout::construct::ConstructionResult;
-use layout::context::{LayoutContext, SharedLayoutContext, ThreadLocalLayoutContext, heap_size_of_local_context};
+use layout::context::{LayoutContext, SharedLayoutContext};
+use layout::context::heap_size_of_persistent_local_context;
 use layout::display_list_builder::ToGfxColor;
 use layout::flow::{self, Flow, ImmutableFlowUtils, MutableFlowUtils, MutableOwnedFlowUtils};
 use layout::flow_ref::FlowRef;
@@ -723,11 +724,11 @@ impl LayoutThread {
             size: stylist.heap_size_of_children(),
         });
 
-        // The LayoutThread has a context in TLS...
+        // The LayoutThread has data in Persistent TLS...
         reports.push(Report {
             path: path![formatted_url, "layout-thread", "local-context"],
             kind: ReportKind::ExplicitJemallocHeapSize,
-            size: heap_size_of_local_context(),
+            size: heap_size_of_persistent_local_context(),
         });
 
         reports_chan.send(reports);
@@ -1447,8 +1448,7 @@ impl LayoutThread {
                     self.profiler_metadata(),
                     self.time_profiler_chan.clone(),
                     || {
-                let tlc = ThreadLocalLayoutContext::new(&shared);
-                let context = LayoutContext::new(&shared, &*tlc);
+                let context = LayoutContext::new(&shared);
                 sequential::store_overflow(&context,
                                            FlowRef::deref_mut(&mut root_flow) as &mut Flow);
             });
diff --git a/components/style/context.rs b/components/style/context.rs
index ffe9668bd32..e8e2ee1cc13 100644
--- a/components/style/context.rs
+++ b/components/style/context.rs
@@ -84,10 +84,10 @@ pub struct ThreadLocalStyleContext {
 }
 
 impl ThreadLocalStyleContext {
-    pub fn new(local_context_creation_data: &ThreadLocalStyleContextCreationInfo) -> Self {
+    pub fn new(shared: &SharedStyleContext) -> Self {
         ThreadLocalStyleContext {
             style_sharing_candidate_cache: RefCell::new(StyleSharingCandidateCache::new()),
-            new_animations_sender: local_context_creation_data.new_animations_sender.clone(),
+            new_animations_sender: shared.local_context_creation_data.lock().unwrap().new_animations_sender.clone(),
         }
     }
 }
diff --git a/components/style/gecko/context.rs b/components/style/gecko/context.rs
deleted file mode 100644
index c7ba65dce61..00000000000
--- a/components/style/gecko/context.rs
+++ /dev/null
@@ -1,27 +0,0 @@
-/* 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/. */
-
-use context::{SharedStyleContext, ThreadLocalStyleContext};
-use std::cell::RefCell;
-use std::rc::Rc;
-
-thread_local!(static LOCAL_CONTEXT_KEY: RefCell<Option<Rc<ThreadLocalStyleContext>>> = RefCell::new(None));
-
-// Keep this implementation in sync with the one in components/layout/context.rs.
-pub fn create_or_get_local_context(shared: &SharedStyleContext) -> Rc<ThreadLocalStyleContext> {
-    LOCAL_CONTEXT_KEY.with(|r| {
-        let mut r = r.borrow_mut();
-        if let Some(context) = r.clone() {
-            context
-        } else {
-            let context = Rc::new(ThreadLocalStyleContext::new(&shared.local_context_creation_data.lock().unwrap()));
-            *r = Some(context.clone());
-            context
-        }
-    })
-}
-
-pub fn clear_local_context() {
-    LOCAL_CONTEXT_KEY.with(|r| *r.borrow_mut() = None);
-}
diff --git a/components/style/gecko/mod.rs b/components/style/gecko/mod.rs
index 2e1e653f88e..7f227f154dc 100644
--- a/components/style/gecko/mod.rs
+++ b/components/style/gecko/mod.rs
@@ -3,7 +3,6 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 
-pub mod context;
 pub mod data;
 pub mod restyle_damage;
 pub mod snapshot;
diff --git a/components/style/gecko/traversal.rs b/components/style/gecko/traversal.rs
index 005da9620d0..8d7ff1c9731 100644
--- a/components/style/gecko/traversal.rs
+++ b/components/style/gecko/traversal.rs
@@ -6,9 +6,8 @@ use atomic_refcell::AtomicRefCell;
 use context::{SharedStyleContext, StyleContext, ThreadLocalStyleContext};
 use data::ElementData;
 use dom::{NodeInfo, TNode};
-use gecko::context::create_or_get_local_context;
 use gecko::wrapper::{GeckoElement, GeckoNode};
-use std::rc::Rc; use traversal::{DomTraversal, PerLevelTraversalData, recalc_style_at};
+use traversal::{DomTraversal, PerLevelTraversalData, recalc_style_at};
 
 pub struct RecalcStyleOnly {
     shared: SharedStyleContext,
@@ -25,20 +24,22 @@ impl RecalcStyleOnly {
 impl<'ln> DomTraversal<GeckoNode<'ln>> for RecalcStyleOnly {
     type ThreadLocalContext = ThreadLocalStyleContext;
 
-    fn process_preorder(&self, node: GeckoNode<'ln>, traversal_data: &mut PerLevelTraversalData) {
+    fn process_preorder(&self, traversal_data: &mut PerLevelTraversalData,
+                        thread_local: &mut ThreadLocalStyleContext,
+                        node: GeckoNode<'ln>)
+    {
         if node.is_element() {
             let el = node.as_element().unwrap();
             let mut data = unsafe { el.ensure_data() }.borrow_mut();
-            let tlc = self.create_or_get_thread_local_context();
-            let context = StyleContext {
+            let mut context = StyleContext {
                 shared: &self.shared,
-                thread_local: &*tlc,
+                thread_local: thread_local,
             };
-            recalc_style_at(self, traversal_data, &context, el, &mut data);
+            recalc_style_at(self, traversal_data, &mut context, el, &mut data);
         }
     }
 
-    fn process_postorder(&self, _: GeckoNode<'ln>) {
+    fn process_postorder(&self, _: &mut ThreadLocalStyleContext, _: GeckoNode<'ln>) {
         unreachable!();
     }
 
@@ -57,7 +58,7 @@ impl<'ln> DomTraversal<GeckoNode<'ln>> for RecalcStyleOnly {
         &self.shared
     }
 
-    fn create_or_get_thread_local_context(&self) -> Rc<ThreadLocalStyleContext> {
-        create_or_get_local_context(&self.shared)
+    fn create_thread_local_context(&self) -> ThreadLocalStyleContext {
+        ThreadLocalStyleContext::new(&self.shared)
     }
 }
diff --git a/components/style/lib.rs b/components/style/lib.rs
index 64b40b9e96e..3481a03c4fe 100644
--- a/components/style/lib.rs
+++ b/components/style/lib.rs
@@ -115,6 +115,7 @@ pub mod parallel;
 pub mod parser;
 pub mod restyle_hints;
 pub mod rule_tree;
+pub mod scoped_tls;
 pub mod selector_parser;
 pub mod stylist;
 #[cfg(feature = "servo")] #[allow(unsafe_code)] pub mod servo;
diff --git a/components/style/parallel.rs b/components/style/parallel.rs
index 5fa27f8ac96..80652bfa6b8 100644
--- a/components/style/parallel.rs
+++ b/components/style/parallel.rs
@@ -8,8 +8,8 @@
 
 use dom::{OpaqueNode, TElement, TNode, UnsafeNode};
 use rayon;
+use scoped_tls::ScopedTLS;
 use servo_config::opts;
-use std::borrow::Borrow;
 use std::sync::atomic::Ordering;
 use traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
 use traversal::{STYLE_SHARING_CACHE_HITS, STYLE_SHARING_CACHE_MISSES};
@@ -45,14 +45,15 @@ pub fn traverse_dom<N, D>(traversal: &D,
         (vec![root.as_node().to_unsafe()], known_root_dom_depth)
     };
 
-    let data = PerLevelTraversalData {
+    let traversal_data = PerLevelTraversalData {
         current_dom_depth: depth,
     };
-
+    let tls = ScopedTLS::<D::ThreadLocalContext>::new(queue);
     let root = root.as_node().opaque();
+
     queue.install(|| {
         rayon::scope(|scope| {
-            traverse_nodes(nodes, root, data, scope, traversal);
+            traverse_nodes(nodes, root, traversal_data, scope, traversal, &tls);
         });
     });
 
@@ -71,57 +72,60 @@ pub fn traverse_dom<N, D>(traversal: &D,
 #[allow(unsafe_code)]
 fn top_down_dom<'a, 'scope, N, D>(unsafe_nodes: &'a [UnsafeNode],
                                   root: OpaqueNode,
-                                  mut data: PerLevelTraversalData,
+                                  mut traversal_data: PerLevelTraversalData,
                                   scope: &'a rayon::Scope<'scope>,
-                                  traversal: &'scope D)
+                                  traversal: &'scope D,
+                                  tls: &'scope ScopedTLS<'scope, D::ThreadLocalContext>)
     where N: TNode,
           D: DomTraversal<N>,
 {
     let mut discovered_child_nodes = vec![];
-    for unsafe_node in unsafe_nodes {
-        // Get a real layout node.
-        let node = unsafe { N::from_unsafe(&unsafe_node) };
-
-        // Perform the appropriate traversal.
-        let mut children_to_process = 0isize;
-        traversal.process_preorder(node, &mut data);
-        if let Some(el) = node.as_element() {
-            D::traverse_children(el, |kid| {
-                children_to_process += 1;
-                discovered_child_nodes.push(kid.to_unsafe())
-            });
-        }
+    {
+        // Scope the borrow of the TLS so that the borrow is dropped before
+        // potentially traversing a child on this thread.
+        let mut tlc = tls.ensure(|| traversal.create_thread_local_context());
+
+        for unsafe_node in unsafe_nodes {
+            // Get a real layout node.
+            let node = unsafe { N::from_unsafe(&unsafe_node) };
+
+            // Perform the appropriate traversal.
+            let mut children_to_process = 0isize;
+            traversal.process_preorder(&mut traversal_data, &mut *tlc, node);
+            if let Some(el) = node.as_element() {
+                D::traverse_children(el, |kid| {
+                    children_to_process += 1;
+                    discovered_child_nodes.push(kid.to_unsafe())
+                });
+            }
 
-        // Reset the count of children if we need to do a bottom-up traversal
-        // after the top up.
-        if D::needs_postorder_traversal() {
-            if children_to_process == 0 {
-                // If there were no more children, start walking back up.
-                bottom_up_dom(root, *unsafe_node, traversal)
-            } else {
-                // Otherwise record the number of children to process when the
-                // time comes.
-                node.as_element().unwrap().store_children_to_process(children_to_process);
+            // Reset the count of children if we need to do a bottom-up traversal
+            // after the top up.
+            if D::needs_postorder_traversal() {
+                if children_to_process == 0 {
+                    // If there were no more children, start walking back up.
+                    bottom_up_dom(traversal, &mut *tlc, root, *unsafe_node)
+                } else {
+                    // Otherwise record the number of children to process when the
+                    // time comes.
+                    node.as_element().unwrap().store_children_to_process(children_to_process);
+                }
             }
         }
     }
 
-    // NB: In parallel traversal mode we have to purge the LRU cache in order to
-    // be able to access it without races.
-    let tlc = traversal.create_or_get_thread_local_context();
-    (*tlc).borrow().style_sharing_candidate_cache.borrow_mut().clear();
-
-    if let Some(ref mut depth) = data.current_dom_depth {
+    if let Some(ref mut depth) = traversal_data.current_dom_depth {
         *depth += 1;
     }
 
-    traverse_nodes(discovered_child_nodes, root, data, scope, traversal);
+    traverse_nodes(discovered_child_nodes, root, traversal_data, scope, traversal, tls);
 }
 
 fn traverse_nodes<'a, 'scope, N, D>(nodes: Vec<UnsafeNode>, root: OpaqueNode,
-                                    data: PerLevelTraversalData,
+                                    traversal_data: PerLevelTraversalData,
                                     scope: &'a rayon::Scope<'scope>,
-                                    traversal: &'scope D)
+                                    traversal: &'scope D,
+                                    tls: &'scope ScopedTLS<'scope, D::ThreadLocalContext>)
     where N: TNode,
           D: DomTraversal<N>,
 {
@@ -133,17 +137,17 @@ fn traverse_nodes<'a, 'scope, N, D>(nodes: Vec<UnsafeNode>, root: OpaqueNode,
     // we're only pushing one work unit.
     if nodes.len() <= CHUNK_SIZE {
         let nodes = nodes.into_boxed_slice();
-        top_down_dom(&nodes, root, data, scope, traversal);
+        top_down_dom(&nodes, root, traversal_data, scope, traversal, tls);
         return;
     }
 
     // General case.
     for chunk in nodes.chunks(CHUNK_SIZE) {
         let nodes = chunk.iter().cloned().collect::<Vec<_>>().into_boxed_slice();
-        let data = data.clone();
+        let traversal_data = traversal_data.clone();
         scope.spawn(move |scope| {
             let nodes = nodes;
-            top_down_dom(&nodes, root, data, scope, traversal)
+            top_down_dom(&nodes, root, traversal_data, scope, traversal, tls)
         })
     }
 }
@@ -160,9 +164,10 @@ fn traverse_nodes<'a, 'scope, N, D>(nodes: Vec<UnsafeNode>, root: OpaqueNode,
 /// The only communication between siblings is that they both
 /// fetch-and-subtract the parent's children count.
 #[allow(unsafe_code)]
-fn bottom_up_dom<N, D>(root: OpaqueNode,
-                       unsafe_node: UnsafeNode,
-                       traversal: &D)
+fn bottom_up_dom<N, D>(traversal: &D,
+                       thread_local: &mut D::ThreadLocalContext,
+                       root: OpaqueNode,
+                       unsafe_node: UnsafeNode)
     where N: TNode,
           D: DomTraversal<N>
 {
@@ -170,7 +175,7 @@ fn bottom_up_dom<N, D>(root: OpaqueNode,
     let mut node = unsafe { N::from_unsafe(&unsafe_node) };
     loop {
         // Perform the appropriate operation.
-        traversal.process_postorder(node);
+        traversal.process_postorder(thread_local, node);
 
         if node.opaque() == root {
             break;
diff --git a/components/style/scoped_tls.rs b/components/style/scoped_tls.rs
new file mode 100644
index 00000000000..1f695aaa4c9
--- /dev/null
+++ b/components/style/scoped_tls.rs
@@ -0,0 +1,51 @@
+/* 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/. */
+
+#![allow(unsafe_code)]
+
+use rayon;
+use std::cell::{Ref, RefCell, RefMut};
+
+/// Stack-scoped thread-local storage for rayon thread pools.
+
+pub struct ScopedTLS<'a, T: Send> {
+    pool: &'a rayon::ThreadPool,
+    slots: Box<[RefCell<Option<T>>]>,
+}
+
+unsafe impl<'a, T: Send> Sync for ScopedTLS<'a, T> {}
+
+impl<'a, T: Send> ScopedTLS<'a, T> {
+    pub fn new(p: &'a rayon::ThreadPool) -> Self {
+        let count = p.num_threads();
+        let mut v = Vec::with_capacity(count);
+        for _ in 0..count {
+            v.push(RefCell::new(None));
+        }
+
+        ScopedTLS {
+            pool: p,
+            slots: v.into_boxed_slice(),
+        }
+    }
+
+    pub fn borrow(&self) -> Ref<Option<T>> {
+        let idx = self.pool.current_thread_index().unwrap();
+        self.slots[idx].borrow()
+    }
+
+    pub fn borrow_mut(&self) -> RefMut<Option<T>> {
+        let idx = self.pool.current_thread_index().unwrap();
+        self.slots[idx].borrow_mut()
+    }
+
+    pub fn ensure<F: FnOnce() -> T>(&self, f: F) -> RefMut<T> {
+        let mut opt = self.borrow_mut();
+        if opt.is_none() {
+            *opt = Some(f());
+        }
+
+        RefMut::map(opt, |x| x.as_mut().unwrap())
+    }
+}
diff --git a/components/style/sequential.rs b/components/style/sequential.rs
index 1b8d602a0f4..b47d95a8517 100644
--- a/components/style/sequential.rs
+++ b/components/style/sequential.rs
@@ -5,7 +5,6 @@
 //! Implements sequential traversal over the DOM tree.
 
 use dom::{TElement, TNode};
-use std::borrow::Borrow;
 use traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
 
 pub fn traverse_dom<N, D>(traversal: &D,
@@ -16,43 +15,41 @@ pub fn traverse_dom<N, D>(traversal: &D,
 {
     debug_assert!(token.should_traverse());
 
-    fn doit<N, D>(traversal: &D, node: N, data: &mut PerLevelTraversalData)
+    fn doit<N, D>(traversal: &D, traversal_data: &mut PerLevelTraversalData,
+                  thread_local: &mut D::ThreadLocalContext, node: N)
         where N: TNode,
               D: DomTraversal<N>
     {
-        traversal.process_preorder(node, data);
+        traversal.process_preorder(traversal_data, thread_local, node);
         if let Some(el) = node.as_element() {
-            if let Some(ref mut depth) = data.current_dom_depth {
+            if let Some(ref mut depth) = traversal_data.current_dom_depth {
                 *depth += 1;
             }
 
-            D::traverse_children(el, |kid| doit(traversal, kid, data));
+            D::traverse_children(el, |kid| doit(traversal, traversal_data, thread_local, kid));
 
-            if let Some(ref mut depth) = data.current_dom_depth {
+            if let Some(ref mut depth) = traversal_data.current_dom_depth {
                 *depth -= 1;
             }
         }
 
         if D::needs_postorder_traversal() {
-            traversal.process_postorder(node);
+            traversal.process_postorder(thread_local, node);
         }
     }
 
-    let mut data = PerLevelTraversalData {
+    let mut traversal_data = PerLevelTraversalData {
         current_dom_depth: None,
     };
 
+    let mut tlc = traversal.create_thread_local_context();
     if token.traverse_unstyled_children_only() {
         for kid in root.as_node().children() {
             if kid.as_element().map_or(false, |el| el.get_data().is_none()) {
-                doit(traversal, kid, &mut data);
+                doit(traversal, &mut traversal_data, &mut tlc, kid);
             }
         }
     } else {
-        doit(traversal, root.as_node(), &mut data);
+        doit(traversal, &mut traversal_data, &mut tlc, root.as_node());
     }
-
-    // Clear the local LRU cache since we store stateful elements inside.
-    let tlc = traversal.create_or_get_thread_local_context();
-    (*tlc).borrow().style_sharing_candidate_cache.borrow_mut().clear();
 }
diff --git a/components/style/traversal.rs b/components/style/traversal.rs
index 4a44619cef4..177c6a59ddc 100644
--- a/components/style/traversal.rs
+++ b/components/style/traversal.rs
@@ -6,7 +6,7 @@
 
 use atomic_refcell::{AtomicRefCell, AtomicRefMut};
 use bloom::StyleBloom;
-use context::{SharedStyleContext, StyleContext, ThreadLocalStyleContext};
+use context::{SharedStyleContext, StyleContext};
 use data::{ElementData, StoredRestyleHint};
 use dom::{OpaqueNode, TElement, TNode};
 use matching::{MatchMethods, StyleSharingResult};
@@ -15,10 +15,8 @@ use selector_parser::RestyleDamage;
 use selectors::Element;
 use selectors::matching::StyleRelations;
 use servo_config::opts;
-use std::borrow::Borrow;
 use std::cell::RefCell;
 use std::mem;
-use std::rc::Rc;
 use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
 use stylist::Stylist;
 
@@ -128,15 +126,16 @@ impl LogBehavior {
 }
 
 pub trait DomTraversal<N: TNode> : Sync {
-    type ThreadLocalContext: Borrow<ThreadLocalStyleContext>;
+    type ThreadLocalContext: Send;
 
     /// Process `node` on the way down, before its children have been processed.
-    fn process_preorder(&self, node: N, data: &mut PerLevelTraversalData);
+    fn process_preorder(&self, data: &mut PerLevelTraversalData,
+                        thread_local: &mut Self::ThreadLocalContext, node: N);
 
     /// Process `node` on the way up, after its children have been processed.
     ///
     /// This is only executed if `needs_postorder_traversal` returns true.
-    fn process_postorder(&self, node: N);
+    fn process_postorder(&self, thread_local: &mut Self::ThreadLocalContext, node: N);
 
     /// Boolean that specifies whether a bottom up traversal should be
     /// performed.
@@ -321,7 +320,7 @@ pub trait DomTraversal<N: TNode> : Sync {
 
     fn shared_context(&self) -> &SharedStyleContext;
 
-    fn create_or_get_thread_local_context(&self) -> Rc<Self::ThreadLocalContext>;
+    fn create_thread_local_context(&self) -> Self::ThreadLocalContext;
 }
 
 /// Determines the amount of relations where we're going to share style.
diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index 4d47ec460a4..470fa15dcfa 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -17,11 +17,11 @@ use std::ptr;
 use std::sync::{Arc, Mutex};
 use style::arc_ptr_eq;
 use style::atomic_refcell::AtomicRefMut;
-use style::context::{ThreadLocalStyleContextCreationInfo, QuirksMode, ReflowGoal, SharedStyleContext, StyleContext};
+use style::context::{QuirksMode, ReflowGoal, SharedStyleContext, StyleContext};
+use style::context::{ThreadLocalStyleContext, ThreadLocalStyleContextCreationInfo};
 use style::data::{ElementData, RestyleData};
 use style::dom::{ShowSubtreeData, TElement, TNode};
 use style::error_reporting::StdoutErrorReporter;
-use style::gecko::context::clear_local_context;
 use style::gecko::data::{NUM_THREADS, PerDocumentStyleData, PerDocumentStyleDataImpl};
 use style::gecko::restyle_damage::GeckoRestyleDamage;
 use style::gecko::selector_parser::{SelectorImpl, PseudoElement};
@@ -88,12 +88,6 @@ pub extern "C" fn Servo_Initialize() -> () {
 pub extern "C" fn Servo_Shutdown() -> () {
     // Destroy our default computed values.
     unsafe { ComputedValues::shutdown(); }
-
-    // In general, ThreadLocalStyleContexts will get destroyed when the worker thread
-    // is joined and the TLS is dropped. However, under some configurations we
-    // may do sequential style computation on the main thread, so we need to be
-    // sure to clear the main thread TLS entry as well.
-    clear_local_context();
 }
 
 fn create_shared_context(mut per_doc_data: &mut AtomicRefMut<PerDocumentStyleDataImpl>) -> SharedStyleContext {
@@ -875,22 +869,19 @@ pub extern "C" fn Servo_ResolveStyle(element: RawGeckoElementBorrowed,
             let mut per_doc_data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
             let shared_style_context = create_shared_context(&mut per_doc_data);
             let traversal = RecalcStyleOnly::new(shared_style_context);
-            let tlc = traversal.create_or_get_thread_local_context();
 
             let mut traversal_data = PerLevelTraversalData {
                 current_dom_depth: None,
             };
 
+            let mut tlc = ThreadLocalStyleContext::new(traversal.shared_context());
             let context = StyleContext {
                 shared: traversal.shared_context(),
-                thread_local: &*tlc,
+                thread_local: &mut tlc,
             };
 
             recalc_style_at(&traversal, &mut traversal_data, &context, element, &mut data);
 
-            // We don't want to keep any cached style around after this one-off style resolution.
-            tlc.style_sharing_candidate_cache.borrow_mut().clear();
-
             // The element was either unstyled or needed restyle. If it was unstyled, it may have
             // additional unstyled children that subsequent traversals won't find now that the style
             // on this element is up-to-date. Mark dirty descendants in that case.