Explicitly register rayon threads, rather than assuming that as the default.

MozReview-Commit-ID: E4kUyy8HjmV

components/layout_thread/lib.rs

@@ -461,7 +461,8 @@ impl LayoutThread {
             ScaleFactor::new(opts::get().device_pixels_per_px.unwrap_or(1.0)));
 
         let configuration =
-            rayon::Configuration::new().num_threads(layout_threads);
+            rayon::Configuration::new().num_threads(layout_threads)
+                                       .start_handler(|_| thread_state::initialize_layout_worker_thread());
         let parallel_traversal = if layout_threads > 1 {
             Some(rayon::ThreadPool::new(configuration).expect("ThreadPool creation failed"))
         } else {

components/style/gecko/global_style_data.rs

@@ -15,6 +15,7 @@ use shared_lock::SharedRwLock;
 use std::cmp;
 use std::env;
 use std::ffi::CString;
+use thread_state;
 
 /// Global style data
 pub struct GlobalStyleData {
@@ -39,6 +40,7 @@ fn thread_name(index: usize) -> String {
 }
 
 fn thread_startup(index: usize) {
+    thread_state::initialize_layout_worker_thread();
     unsafe {
         Gecko_SetJemallocThreadLocalArena(true);
     }

components/style/thread_state.rs

@@ -40,9 +40,6 @@ macro_rules! thread_types ( ( $( $fun:ident = $flag:ident ; )* ) => (
             }
         )*
     }
-
-    static TYPES: &'static [ThreadState] =
-        &[ $( $flag ),* ];
 ));
 
 thread_types! {
@@ -60,21 +57,22 @@ pub fn initialize(x: ThreadState) {
         }
         *k.borrow_mut() = Some(x);
     });
-    get(); // check the assertion below
+}
+
+/// Initializes the current thread as a layout worker thread.
+pub fn initialize_layout_worker_thread() {
+    initialize(LAYOUT | IN_WORKER);
 }
 
 /// Gets the current thread state.
 pub fn get() -> ThreadState {
     let state = STATE.with(|ref k| {
         match *k.borrow() {
-            // This is one of the layout threads, that use rayon.
-            None => LAYOUT | IN_WORKER,
+            None => ThreadState::empty(), // Unknown thread.
             Some(s) => s,
         }
     });
 
-    // Exactly one of the thread type flags should be set.
-    debug_assert_eq!(1, TYPES.iter().filter(|&&ty| state.contains(ty)).count());
     state
 }