Use Arc<PropertyDeclarationBlock> everwhere it’s appropriate.

ports/geckolib/glue.rs

@@ -356,7 +356,7 @@ pub extern "C" fn Servo_StyleSet_Drop(data: *mut RawServoStyleSet) -> () {
 }
 
 pub struct GeckoDeclarationBlock {
-    pub declarations: Option<PropertyDeclarationBlock>,
+    pub declarations: Option<Arc<PropertyDeclarationBlock>>,
     // XXX The following two fields are made atomic to work around the
     // ownership system so that they can be changed inside a shared
     // instance. It wouldn't provide safety as Rust usually promises,
@@ -377,7 +377,7 @@ pub extern "C" fn Servo_ParseStyleAttribute(bytes: *const u8, length: u32,
                                             -> ServoDeclarationBlockStrong {
     let value = unsafe { from_utf8_unchecked(slice::from_raw_parts(bytes, length as usize)) };
     GeckoDeclarationBlock::from_arc(Arc::new(GeckoDeclarationBlock {
-        declarations: GeckoElement::parse_style_attribute(value),
+        declarations: GeckoElement::parse_style_attribute(value).map(Arc::new),
         cache: AtomicPtr::new(cache),
         immutable: AtomicBool::new(false),
     }))

ports/geckolib/wrapper.rs

@@ -35,7 +35,6 @@ use snapshot::GeckoElementSnapshot;
 use snapshot_helpers;
 use std::fmt;
 use std::marker::PhantomData;
-use std::mem::transmute;
 use std::ops::BitOr;
 use std::ptr;
 use std::sync::Arc;
@@ -460,8 +459,6 @@ lazy_static! {
     };
 }
 
-static NO_STYLE_ATTRIBUTE: Option<PropertyDeclarationBlock> = None;
-
 impl<'le> TElement for GeckoElement<'le> {
     type ConcreteNode = GeckoNode<'le>;
     type ConcreteDocument = GeckoDocument<'le>;
@@ -470,14 +467,16 @@ impl<'le> TElement for GeckoElement<'le> {
         unsafe { GeckoNode::from_raw(self.element as *mut RawGeckoNode) }
     }
 
-    fn style_attribute(&self) -> &Option<PropertyDeclarationBlock> {
+    fn style_attribute(&self) -> Option<&Arc<PropertyDeclarationBlock>> {
         let declarations = unsafe { Gecko_GetServoDeclarationBlock(self.element) };
         if declarations.is_null() {
-            &NO_STYLE_ATTRIBUTE
+            None
         } else {
-            GeckoDeclarationBlock::with(declarations, |declarations| {
-                unsafe { transmute(&declarations.declarations) }
-            })
+            let opt_ptr = GeckoDeclarationBlock::with(declarations, |declarations| {
+                // Use a raw poointer to extend the lifetime
+                declarations.declarations.as_ref().map(|r| r as *const Arc<_>)
+            });
+            opt_ptr.map(|ptr| unsafe { &*ptr })
         }
     }