Opt-in rather than opt-out to unsafe blocks in layout.

components/layout/css/matching.rs

@@ -4,6 +4,8 @@
 
 //! High-level interface to CSS selector matching.
 
+#![allow(unsafe_blocks)]
+
 use css::node_style::StyledNode;
 use incremental::{self, RestyleDamage};
 use util::{LayoutDataAccess, LayoutDataWrapper};

components/layout/css/node_style.rs

@@ -23,6 +23,7 @@ pub trait StyledNode {
 
 impl<'ln> StyledNode for ThreadSafeLayoutNode<'ln> {
     #[inline]
+    #[allow(unsafe_blocks)]
     fn style<'a>(&'a self) -> &'a Arc<ComputedValues> {
         unsafe {
             let layout_data_ref = self.borrow_layout_data();