Auto merge of #8958 - jkachmar:separate-layout-msg, r=KiChjang

Separate script and layout messages, issue #8843

Separated layout-specific messages to the constellation out from the `ScriptMsg` enum into a `LayoutMsg` enum within `script_traits/script_msg.rs`, addresses [#8843](https://github.com/servo/servo/issues/8843).

I initially tried to move `LayoutMsg` into `layout_traits/lib.rs`, but this introduced a cyclic dependency: `layout_traits` depends on `script_traits` for the `LayoutTaskFactory` implementation, and `script_traits/script_task.rs` now depends on `LayoutMsg` for new layout channels in `InitialScriptState` and `ScriptTask`.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8958)
<!-- Reviewable:end -->

3 files changed, 16 insertions, 5 deletions

diff --git a/components/script/dom/bindings/trace.rs b/components/script/dom/bindings/trace.rs
index cbe870f073c..496d70add18 100644
--- a/components/script/dom/bindings/trace.rs
+++ b/components/script/dom/bindings/trace.rs
@@ -63,7 +63,7 @@ use net_traits::storage_task::StorageType;
 use profile_traits::mem::ProfilerChan as MemProfilerChan;
 use profile_traits::time::ProfilerChan as TimeProfilerChan;
 use script_task::ScriptChan;
-use script_traits::{ScriptMsg, TimerEventId, TimerSource, UntrustedNodeAddress};
+use script_traits::{LayoutMsg, ScriptMsg, TimerEventId, TimerSource, UntrustedNodeAddress};
 use selectors::parser::PseudoElement;
 use selectors::states::*;
 use serde::{Deserialize, Serialize};
@@ -306,6 +306,13 @@ impl JSTraceable for ConstellationChan<ScriptMsg> {
     }
 }
 
+impl JSTraceable for ConstellationChan<LayoutMsg> {
+    #[inline]
+    fn trace(&self, _trc: *mut JSTracer) {
+        // Do nothing
+    }
+}
+
 impl JSTraceable for Box<ScriptChan + Send> {
     #[inline]
     fn trace(&self, _trc: *mut JSTracer) {
diff --git a/components/script/layout_interface.rs b/components/script/layout_interface.rs
index a533fd6418d..4570192f6fb 100644
--- a/components/script/layout_interface.rs
+++ b/components/script/layout_interface.rs
@@ -18,8 +18,8 @@ use msg::constellation_msg::{ConstellationChan, Failure, PipelineId};
 use msg::constellation_msg::{WindowSizeData};
 use net_traits::image_cache_task::ImageCacheTask;
 use profile_traits::mem::ReportsChan;
-use script_traits::{ConstellationControlMsg, LayoutControlMsg, OpaqueScriptLayoutChannel};
-use script_traits::{ScriptMsg as ConstellationMsg, UntrustedNodeAddress};
+use script_traits::{ConstellationControlMsg, LayoutControlMsg, LayoutMsg as ConstellationMsg};
+use script_traits::{OpaqueScriptLayoutChannel, UntrustedNodeAddress};
 use selectors::parser::PseudoElement;
 use std::any::Any;
 use std::sync::Arc;
diff --git a/components/script/script_task.rs b/components/script/script_task.rs
index 953bf3dacde..b07b5086f45 100644
--- a/components/script/script_task.rs
+++ b/components/script/script_task.rs
@@ -80,7 +80,7 @@ use profile_traits::time::{self, ProfilerCategory, profile};
 use script_traits::CompositorEvent::{KeyEvent, MouseButtonEvent, MouseMoveEvent, ResizeEvent};
 use script_traits::CompositorEvent::{TouchEvent};
 use script_traits::{CompositorEvent, ConstellationControlMsg, EventResult, InitialScriptState, NewLayoutInfo};
-use script_traits::{OpaqueScriptLayoutChannel, ScriptMsg as ConstellationMsg};
+use script_traits::{LayoutMsg, OpaqueScriptLayoutChannel, ScriptMsg as ConstellationMsg};
 use script_traits::{ScriptTaskFactory, ScriptToCompositorMsg, TimerEvent, TimerEventRequest, TimerSource};
 use script_traits::{TouchEventType, TouchId};
 use std::any::Any;
@@ -406,6 +406,9 @@ pub struct ScriptTask {
     /// For communicating load url messages to the constellation
     constellation_chan: ConstellationChan<ConstellationMsg>,
 
+    /// For communicating layout messages to the constellation
+    layout_to_constellation_chan: ConstellationChan<LayoutMsg>,
+
     /// A handle to the compositor for communicating ready state messages.
     compositor: DOMRefCell<IpcSender<ScriptToCompositorMsg>>,
 
@@ -669,6 +672,7 @@ impl ScriptTask {
             control_chan: state.control_chan,
             control_port: control_port,
             constellation_chan: state.constellation_chan,
+            layout_to_constellation_chan: state.layout_to_constellation_chan,
             compositor: DOMRefCell::new(state.compositor),
             time_profiler_chan: state.time_profiler_chan,
             mem_profiler_chan: state.mem_profiler_chan,
@@ -1195,7 +1199,7 @@ impl ScriptTask {
             is_parent: false,
             layout_pair: layout_pair,
             pipeline_port: pipeline_port,
-            constellation_chan: self.constellation_chan.clone(),
+            constellation_chan: self.layout_to_constellation_chan.clone(),
             failure: failure,
             paint_chan: paint_chan,
             script_chan: self.control_chan.clone(),