auto merge of #5086 : glennw/servo/reap-more-stuff, r=jdm

Also introduce a clear() function to layout data which will be used to clear items such as compositor layouts.

Clear the layout data when a node becomes display:none.

components/layout/construct.rs

@@ -1329,6 +1329,13 @@ impl<'ln> NodeUtils for ThreadSafeLayoutNode<'ln> {
         let mut layout_data_ref = self.mutate_layout_data();
         let layout_data = layout_data_ref.as_mut().expect("no layout data");
 
+        match result {
+            ConstructionResult::None => {
+                layout_data.clear();
+            }
+            _ => {}
+        }
+
         let dst = self.get_construction_result(layout_data);
 
         *dst = result;

components/layout/layout_task.rs

@@ -35,7 +35,7 @@ use gfx::paint_task::Msg as PaintMsg;
 use layout_traits::{LayoutControlMsg, LayoutTaskFactory};
 use log;
 use script::dom::bindings::js::LayoutJS;
-use script::dom::node::{LayoutDataRef, Node, NodeTypeId};
+use script::dom::node::{LayoutData, Node, NodeTypeId};
 use script::dom::element::ElementTypeId;
 use script::dom::htmlelement::HTMLElementTypeId;
 use script::layout_interface::{ContentBoxResponse, ContentBoxesResponse};
@@ -937,11 +937,10 @@ impl LayoutTask {
     }
 
     /// Handles a message to destroy layout data. Layout data must be destroyed on *this* task
-    /// because it contains local managed pointers.
-    unsafe fn handle_reap_layout_data(layout_data: LayoutDataRef) {
-        let mut layout_data_ref = layout_data.borrow_mut();
-        let _: Option<LayoutDataWrapper> = mem::transmute(
-            mem::replace(&mut *layout_data_ref, None));
+    /// because the struct type is transmuted to a different type on the script side.
+    unsafe fn handle_reap_layout_data(layout_data: LayoutData) {
+        let layout_data_wrapper: LayoutDataWrapper = mem::transmute(layout_data);
+        layout_data_wrapper.clear();
     }
 
     /// Returns profiling information which is passed to the time profiler.

components/layout/util.rs

@@ -78,6 +78,12 @@ pub struct LayoutDataWrapper {
     pub data: Box<PrivateLayoutData>,
 }
 
+impl LayoutDataWrapper {
+    pub fn clear(&self) {
+        // TODO: Clear items related to this node, e.g. compositor layers
+    }
+}
+
 #[allow(dead_code)]
 fn static_assertion(x: Option<LayoutDataWrapper>) {
     unsafe {