gfx: Make display lists serializable using serde.

This commit introduces the `serde` dependency, which we will use to
serialize messages going between processes in multiprocess Servo.

This also adds a new debugging flag, `-Z print-display-list-json`,
allowing the output of display list serialization to be visualized.
This will be useful for our experiments with alternate rasterizers.

components/layout/Cargo.toml

@@ -71,3 +71,6 @@ smallvec = "0.1"
 string_cache = "0.1"
 string_cache_plugin = "0.1"
 euclid = "0.1"
+serde = "*"
+serde_macros = "*"
+

components/layout/display_list_builder.rs

@@ -829,7 +829,9 @@ impl FragmentDisplayListBuilding for Fragment {
 
         let line_display_item = box LineDisplayItem {
             base: BaseDisplayItem::new(baseline,
-                                       DisplayItemMetadata::new(self.node, style, Cursor::DefaultCursor),
+                                       DisplayItemMetadata::new(self.node,
+                                                                style,
+                                                                Cursor::DefaultCursor),
                                        (*clip).clone()),
             color: color::rgb(0, 200, 0),
             style: border_style::T::dashed,

components/layout/layout_task.rs

@@ -58,6 +58,7 @@ use script::layout_interface::{NewLayoutTaskInfo, Msg, Reflow, ReflowGoal, Reflo
 use script::layout_interface::{ScriptLayoutChan, ScriptReflow, TrustedNodeAddress};
 use script_traits::{ConstellationControlMsg, LayoutControlMsg, OpaqueScriptLayoutChannel};
 use script_traits::{ScriptControlChan, StylesheetLoadResponder};
+use serde::json;
 use std::borrow::ToOwned;
 use std::cell::Cell;
 use std::collections::HashMap;
@@ -907,6 +908,9 @@ impl LayoutTask {
                     println!("#### start printing display list.");
                     stacking_context.print("#".to_owned());
                 }
+                if opts::get().dump_display_list_json {
+                    println!("{}", json::to_string_pretty(&stacking_context).unwrap());
+                }
 
                 rw_data.stacking_context = Some(stacking_context.clone());
 

components/layout/lib.rs

@@ -5,6 +5,7 @@
 #![feature(append)]
 #![feature(arc_unique)]
 #![feature(box_syntax)]
+#![feature(custom_derive)]
 #![feature(filling_drop)]
 #![feature(hashmap_hasher)]
 #![feature(heap_api)]
@@ -36,7 +37,6 @@ extern crate profile_traits;
 #[macro_use]
 extern crate util;
 
-extern crate rustc_serialize;
 extern crate azure;
 extern crate canvas_traits;
 extern crate clock_ticks;
@@ -50,9 +50,11 @@ extern crate ipc_channel;
 extern crate layout_traits;
 extern crate libc;
 extern crate msg;
+extern crate rustc_serialize;
 extern crate script;
 extern crate script_traits;
 extern crate selectors;
+extern crate serde;
 extern crate smallvec;
 extern crate string_cache;
 extern crate style;