stylo: Make Servo Arc types use ptr to T instead of ptr to ArcInner<T>

components/servo_arc/lib.rs

@@ -194,6 +194,30 @@ impl<T> Arc<T> {
             p: NonZeroPtrMut::new(ptr as *mut ArcInner<T>),
         }
     }
+
+    /// Produce a pointer to the data that can be converted back
+    /// to an arc
+    pub fn borrow_arc<'a>(&'a self) -> ArcBorrow<'a, T> {
+        ArcBorrow(&**self)
+    }
+    /// Temporarily converts |self| into a bonafide RawOffsetArc and exposes it to the
+    /// provided callback. The refcount is not modified.
+    #[inline(always)]
+    pub fn with_raw_offset_arc<F, U>(&self, f: F) -> U
+        where F: FnOnce(&RawOffsetArc<T>) -> U
+    {
+        // Synthesize transient Arc, which never touches the refcount of the ArcInner.
+        let transient = unsafe { NoDrop::new(Arc::into_raw_offset(ptr::read(self))) };
+
+        // Expose the transient Arc to the callback, which may clone it if it wants.
+        let result = f(&transient);
+
+        // Forget the transient Arc to leave the refcount untouched.
+        mem::forget(transient);
+
+        // Forward the result.
+        result
+    }
 }
 
 impl<T: ?Sized> Arc<T> {
@@ -717,6 +741,7 @@ impl<H: Eq + 'static, T: Eq + 'static> Eq for ThinArc<H, T> {}
 /// This means that this is a direct pointer to
 /// its contained data (and can be read from by both C++ and Rust),
 /// but we can also convert it to a "regular" Arc<T> by removing the offset
+#[derive(Eq)]
 pub struct RawOffsetArc<T: 'static> {
     ptr: NonZeroPtrMut<T>,
 }
@@ -733,7 +758,7 @@ impl<T: 'static> Deref for RawOffsetArc<T> {
 
 impl<T: 'static> Clone for RawOffsetArc<T> {
     fn clone(&self) -> Self {
-        RawOffsetArc::with_arc(self, |a| Arc::into_raw_offset(a.clone()))
+        Arc::into_raw_offset(self.clone_arc())
     }
 }
 
@@ -750,6 +775,16 @@ impl<T: fmt::Debug + 'static> fmt::Debug for RawOffsetArc<T> {
     }
 }
 
+impl<T: PartialEq> PartialEq for RawOffsetArc<T> {
+    fn eq(&self, other: &RawOffsetArc<T>) -> bool {
+        *(*self) == *(*other)
+    }
+
+    fn ne(&self, other: &RawOffsetArc<T>) -> bool {
+        *(*self) != *(*other)
+    }
+}
+
 impl<T: 'static> RawOffsetArc<T> {
     /// Temporarily converts |self| into a bonafide Arc and exposes it to the
     /// provided callback. The refcount is not modified.
@@ -789,6 +824,17 @@ impl<T: 'static> RawOffsetArc<T> {
             &mut *ret
         }
     }
+
+    /// Clone it as an Arc
+    pub fn clone_arc(&self) -> Arc<T> {
+        RawOffsetArc::with_arc(self, |a| a.clone())
+    }
+
+    /// Produce a pointer to the data that can be converted back
+    /// to an arc
+    pub fn borrow_arc<'a>(&'a self) -> ArcBorrow<'a, T> {
+        ArcBorrow(&**self)
+    }
 }
 
 impl<T: 'static> Arc<T> {
@@ -811,6 +857,62 @@ impl<T: 'static> Arc<T> {
     }
 }
 
+/// A "borrowed Arc". This is a pointer to
+/// a T that is known to have been allocated within an
+/// Arc.
+///
+/// This is equivalent in guarantees to `&Arc<T>`, however it is
+/// a bit more flexible. To obtain an `&Arc<T>` you must have
+/// an Arc<T> instance somewhere pinned down until we're done with it.
+///
+/// However, Gecko hands us refcounted things as pointers to T directly,
+/// so we have to conjure up a temporary Arc on the stack each time. The
+/// same happens for when the object is managed by a RawOffsetArc.
+///
+/// ArcBorrow lets us deal with borrows of known-refcounted objects
+/// without needing to worry about how they're actually stored.
+#[derive(PartialEq, Eq)]
+pub struct ArcBorrow<'a, T: 'a>(&'a T);
+
+impl<'a, T> Copy for ArcBorrow<'a, T> {}
+impl<'a, T> Clone for ArcBorrow<'a, T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<'a, T> ArcBorrow<'a, T> {
+    pub fn clone_arc(&self) -> Arc<T> {
+        let arc = unsafe { Arc::from_raw(self.0) };
+        // addref it!
+        mem::forget(arc.clone());
+        arc
+    }
+
+    pub fn with_arc<F, U>(&self, f: F) -> U where F: FnOnce(&Arc<T>) -> U, T: 'static {
+        // Synthesize transient Arc, which never touches the refcount.
+        let transient = unsafe { NoDrop::new(Arc::from_raw(self.0)) };
+
+        // Expose the transient Arc to the callback, which may clone it if it wants.
+        let result = f(&transient);
+
+        // Forget the transient Arc to leave the refcount untouched.
+        // XXXManishearth this can be removed when unions stabilize,
+        // since then NoDrop becomes zero overhead
+        mem::forget(transient);
+
+        // Forward the result.
+        result
+    }
+}
+
+impl<'a, T> Deref for ArcBorrow<'a, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        &*self.0
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::clone::Clone;

components/style/dom.rs

@@ -30,7 +30,7 @@ use std::fmt;
 use std::fmt::Debug;
 use std::hash::Hash;
 use std::ops::Deref;
-use stylearc::Arc;
+use stylearc::{Arc, ArcBorrow};
 use stylist::Stylist;
 use thread_state;
 
@@ -360,7 +360,7 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
     }
 
     /// Get this element's style attribute.
-    fn style_attribute(&self) -> Option<&Arc<Locked<PropertyDeclarationBlock>>>;
+    fn style_attribute(&self) -> Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>;
 
     /// Unset the style attribute's dirty bit.
     /// Servo doesn't need to manage ditry bit for style attribute.
@@ -368,7 +368,7 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
     }
 
     /// Get this element's SMIL override declarations.
-    fn get_smil_override(&self) -> Option<&Arc<Locked<PropertyDeclarationBlock>>> {
+    fn get_smil_override(&self) -> Option<ArcBorrow<Locked<PropertyDeclarationBlock>>> {
         None
     }
 

components/style/gecko/restyle_damage.rs

@@ -56,7 +56,7 @@ impl GeckoRestyleDamage {
         let mut any_style_changed: bool = false;
         let hint = unsafe {
             bindings::Gecko_CalcStyleDifference(context,
-                                                new_style.as_borrowed_opt().unwrap(),
+                                                new_style.as_borrowed(),
                                                 &mut any_style_changed)
         };
         let change = if any_style_changed { StyleChange::Changed } else { StyleChange::Unchanged };

components/style/gecko/wrapper.rs

@@ -64,7 +64,7 @@ use gecko_bindings::structs::EffectCompositor_CascadeLevel as CascadeLevel;
 use gecko_bindings::structs::NODE_IS_IN_NATIVE_ANONYMOUS_SUBTREE;
 use gecko_bindings::structs::NODE_IS_NATIVE_ANONYMOUS;
 use gecko_bindings::structs::nsIDocument_DocumentTheme as DocumentTheme;
-use gecko_bindings::sugar::ownership::{HasArcFFI, HasSimpleFFI};
+use gecko_bindings::sugar::ownership::{FFIArcHelpers, HasArcFFI, HasSimpleFFI};
 use logical_geometry::WritingMode;
 use media_queries::Device;
 use properties::{ComputedValues, parse_style_attribute};
@@ -88,7 +88,7 @@ use std::mem;
 use std::ops::DerefMut;
 use std::ptr;
 use string_cache::{Atom, Namespace, WeakAtom, WeakNamespace};
-use stylearc::Arc;
+use stylearc::{Arc, ArcBorrow, RawOffsetArc};
 use stylesheets::UrlExtraData;
 use stylist::Stylist;
 
@@ -860,13 +860,15 @@ impl<'le> TElement for GeckoElement<'le> {
             device.pres_context().mDocument.raw::<structs::nsIDocument>()
     }
 
-    fn style_attribute(&self) -> Option<&Arc<Locked<PropertyDeclarationBlock>>> {
+    fn style_attribute(&self) -> Option<ArcBorrow<Locked<PropertyDeclarationBlock>>> {
         if !self.may_have_style_attribute() {
             return None;
         }
 
         let declarations = unsafe { Gecko_GetStyleAttrDeclarationBlock(self.0) };
-        declarations.map_or(None, |s| s.as_arc_opt())
+        let declarations: Option<&RawOffsetArc<Locked<PropertyDeclarationBlock>>>
+            = declarations.and_then(|s| s.as_arc_opt());
+        declarations.map(|s| s.borrow_arc())
     }
 
     fn unset_dirty_style_attribute(&self) {
@@ -877,9 +879,11 @@ impl<'le> TElement for GeckoElement<'le> {
         unsafe { Gecko_UnsetDirtyStyleAttr(self.0) };
     }
 
-    fn get_smil_override(&self) -> Option<&Arc<Locked<PropertyDeclarationBlock>>> {
+    fn get_smil_override(&self) -> Option<ArcBorrow<Locked<PropertyDeclarationBlock>>> {
         let declarations = unsafe { Gecko_GetSMILOverrideDeclarationBlock(self.0) };
-        declarations.map(|s| s.as_arc_opt()).unwrap_or(None)
+        let declarations: Option<&RawOffsetArc<Locked<PropertyDeclarationBlock>>>
+            = declarations.and_then(|s| s.as_arc_opt());
+        declarations.map(|s| s.borrow_arc())
     }
 
     fn get_animation_rule_by_cascade(&self, cascade_level: ServoCascadeLevel)
@@ -1098,9 +1102,9 @@ impl<'le> TElement for GeckoElement<'le> {
         let computed_values =
             computed_data.as_ref().map(|d| d.styles.primary());
         let computed_values_opt =
-            computed_values.map(|v| *HasArcFFI::arc_as_borrowed(v));
+            computed_values.map(|v| v.as_borrowed());
         let before_change_values =
-            before_change_style.as_ref().map(|v| *HasArcFFI::arc_as_borrowed(v));
+            before_change_style.as_ref().map(|v| v.as_borrowed());
         unsafe {
             Gecko_UpdateAnimations(self.0,
                                    before_change_values,
@@ -1182,7 +1186,7 @@ impl<'le> TElement for GeckoElement<'le> {
             };
             let end_value = AnimationValue::arc_from_borrowed(&raw_end_value);
             debug_assert!(end_value.is_some());
-            map.insert(property, end_value.unwrap().clone());
+            map.insert(property, end_value.unwrap().clone_arc());
         }
         map
     }
@@ -1428,17 +1432,19 @@ impl<'le> PresentationalHintsSynthesizer for GeckoElement<'le> {
             }
         }
         let declarations = unsafe { Gecko_GetHTMLPresentationAttrDeclarationBlock(self.0) };
-        let declarations = declarations.and_then(|s| s.as_arc_opt());
+        let declarations: Option<&RawOffsetArc<Locked<PropertyDeclarationBlock>>>
+            = declarations.and_then(|s| s.as_arc_opt());
         if let Some(decl) = declarations {
             hints.push(
-                ApplicableDeclarationBlock::from_declarations(Clone::clone(decl), ServoCascadeLevel::PresHints)
+                ApplicableDeclarationBlock::from_declarations(decl.clone_arc(), ServoCascadeLevel::PresHints)
             );
         }
         let declarations = unsafe { Gecko_GetExtraContentStyleDeclarations(self.0) };
-        let declarations = declarations.and_then(|s| s.as_arc_opt());
+        let declarations: Option<&RawOffsetArc<Locked<PropertyDeclarationBlock>>>
+            = declarations.and_then(|s| s.as_arc_opt());
         if let Some(decl) = declarations {
             hints.push(
-                ApplicableDeclarationBlock::from_declarations(Clone::clone(decl), ServoCascadeLevel::PresHints)
+                ApplicableDeclarationBlock::from_declarations(decl.clone_arc(), ServoCascadeLevel::PresHints)
             );
         }
 
@@ -1458,20 +1464,22 @@ impl<'le> PresentationalHintsSynthesizer for GeckoElement<'le> {
                     Gecko_GetVisitedLinkAttrDeclarationBlock(self.0)
                 },
             };
-            let declarations = declarations.and_then(|s| s.as_arc_opt());
+            let declarations: Option<&RawOffsetArc<Locked<PropertyDeclarationBlock>>> =
+                declarations.and_then(|s| s.as_arc_opt());
             if let Some(decl) = declarations {
                 hints.push(
-                    ApplicableDeclarationBlock::from_declarations(Clone::clone(decl), ServoCascadeLevel::PresHints)
+                    ApplicableDeclarationBlock::from_declarations(decl.clone_arc(), ServoCascadeLevel::PresHints)
                 );
             }
 
             let active = self.get_state().intersects(NonTSPseudoClass::Active.state_flag());
             if active {
                 let declarations = unsafe { Gecko_GetActiveLinkAttrDeclarationBlock(self.0) };
-                let declarations = declarations.and_then(|s| s.as_arc_opt());
+                let declarations: Option<&RawOffsetArc<Locked<PropertyDeclarationBlock>>>
+                    = declarations.and_then(|s| s.as_arc_opt());
                 if let Some(decl) = declarations {
                     hints.push(
-                        ApplicableDeclarationBlock::from_declarations(Clone::clone(decl), ServoCascadeLevel::PresHints)
+                        ApplicableDeclarationBlock::from_declarations(decl.clone_arc(), ServoCascadeLevel::PresHints)
                     );
                 }
             }

components/style/gecko_bindings/sugar/ownership.rs

@@ -8,7 +8,7 @@ use std::marker::PhantomData;
 use std::mem::{forget, transmute};
 use std::ops::{Deref, DerefMut};
 use std::ptr;
-use stylearc::Arc;
+use stylearc::{Arc, RawOffsetArc};
 
 /// Indicates that a given Servo type has a corresponding Gecko FFI type.
 pub unsafe trait HasFFI : Sized + 'static {
@@ -88,7 +88,7 @@ pub unsafe trait HasArcFFI : HasFFI {
     /// (usually on the C++ side) without running the Arc destructor.
     unsafe fn release_opt(ptr: Option<&Self::FFIType>) {
         if let Some(arc) = Self::arc_from_borrowed(&ptr) {
-            let _: Arc<_> = ptr::read(arc as *const Arc<_>);
+            let _: RawOffsetArc<_> = ptr::read(arc as *const RawOffsetArc<_>);
         }
     }
 
@@ -102,16 +102,16 @@ pub unsafe trait HasArcFFI : HasFFI {
     /// know that a strong reference to the backing Arc is disappearing
     /// (usually on the C++ side) without running the Arc destructor.
     unsafe fn release(ptr: &Self::FFIType) {
-        let _: Arc<_> = ptr::read(Self::as_arc(&ptr) as *const Arc<_>);
+        let _: RawOffsetArc<_> = ptr::read(Self::as_arc(&ptr) as *const RawOffsetArc<_>);
     }
     #[inline]
     /// Converts a borrowed FFI reference to a borrowed Arc.
     ///
     /// &GeckoType -> &Arc<ServoType>
-    fn as_arc<'a>(ptr: &'a &Self::FFIType) -> &'a Arc<Self> {
+    fn as_arc<'a>(ptr: &'a &Self::FFIType) -> &'a RawOffsetArc<Self> {
         debug_assert!(!(ptr as *const _).is_null());
         unsafe {
-            transmute::<&&Self::FFIType, &Arc<Self>>(ptr)
+            transmute::<&&Self::FFIType, &RawOffsetArc<Self>>(ptr)
         }
     }
 
@@ -119,9 +119,9 @@ pub unsafe trait HasArcFFI : HasFFI {
     /// Converts a borrowed Arc to a borrowed FFI reference.
     ///
     /// &Arc<ServoType> -> &GeckoType
-    fn arc_as_borrowed<'a>(arc: &'a Arc<Self>) -> &'a &Self::FFIType {
+    fn arc_as_borrowed<'a>(arc: &'a RawOffsetArc<Self>) -> &'a &Self::FFIType {
         unsafe {
-            transmute::<&Arc<Self>, &&Self::FFIType>(arc)
+            transmute::<&RawOffsetArc<Self>, &&Self::FFIType>(arc)
         }
     }
 
@@ -129,10 +129,10 @@ pub unsafe trait HasArcFFI : HasFFI {
     /// Converts a borrowed nullable FFI reference to a borrowed Arc.
     ///
     /// &GeckoType -> &Arc<ServoType>
-    fn arc_from_borrowed<'a>(ptr: &'a Option<&Self::FFIType>) -> Option<&'a Arc<Self>> {
+    fn arc_from_borrowed<'a>(ptr: &'a Option<&Self::FFIType>) -> Option<&'a RawOffsetArc<Self>> {
         unsafe {
             if let Some(ref reference) = *ptr {
-                Some(transmute::<&&Self::FFIType, &Arc<_>>(reference))
+                Some(transmute::<&&Self::FFIType, &RawOffsetArc<_>>(reference))
             } else {
                 None
             }
@@ -165,7 +165,7 @@ impl<GeckoType> Strong<GeckoType> {
     /// Panics on null.
     ///
     /// Strong<GeckoType> -> Arc<ServoType>
-    pub fn into_arc<ServoType>(self) -> Arc<ServoType>
+    pub fn into_arc<ServoType>(self) -> RawOffsetArc<ServoType>
         where ServoType: HasArcFFI<FFIType = GeckoType>,
     {
         self.into_arc_opt().unwrap()
@@ -177,7 +177,7 @@ impl<GeckoType> Strong<GeckoType> {
     /// Returns None on null.
     ///
     /// Strong<GeckoType> -> Arc<ServoType>
-    pub fn into_arc_opt<ServoType>(self) -> Option<Arc<ServoType>>
+    pub fn into_arc_opt<ServoType>(self) -> Option<RawOffsetArc<ServoType>>
         where ServoType: HasArcFFI<FFIType = GeckoType>,
     {
         if self.is_null() {
@@ -194,7 +194,7 @@ impl<GeckoType> Strong<GeckoType> {
     /// Returns None on null.
     ///
     /// Strong<GeckoType> -> Arc<ServoType>
-    pub fn as_arc_opt<ServoType>(&self) -> Option<&Arc<ServoType>>
+    pub fn as_arc_opt<ServoType>(&self) -> Option<&RawOffsetArc<ServoType>>
         where ServoType: HasArcFFI<FFIType = GeckoType>,
     {
         if self.is_null() {
@@ -222,18 +222,15 @@ pub unsafe trait FFIArcHelpers {
     /// Arc<ServoType> -> Strong<GeckoType>
     fn into_strong(self) -> Strong<<Self::Inner as HasFFI>::FFIType>;
 
-    /// Produces a (nullable) borrowed FFI reference by borrowing an Arc.
+    /// Produces a borrowed FFI reference by borrowing an Arc.
     ///
-    /// &Arc<ServoType> -> Option<&GeckoType>
-    ///
-    /// FIXME(emilio): What's the point of the nullability? Arc should be
-    /// non-null, right?
+    /// &Arc<ServoType> -> &GeckoType
     ///
     /// Then the `arc_as_borrowed` method can go away.
-    fn as_borrowed_opt(&self) -> Option<&<Self::Inner as HasFFI>::FFIType>;
+    fn as_borrowed(&self) -> &<Self::Inner as HasFFI>::FFIType;
 }
 
-unsafe impl<T: HasArcFFI> FFIArcHelpers for Arc<T> {
+unsafe impl<T: HasArcFFI> FFIArcHelpers for RawOffsetArc<T> {
     type Inner = T;
 
     #[inline]
@@ -242,9 +239,22 @@ unsafe impl<T: HasArcFFI> FFIArcHelpers for Arc<T> {
     }
 
     #[inline]
-    fn as_borrowed_opt(&self) -> Option<&T::FFIType> {
-        let borrowedptr = self as *const Arc<T> as *const Option<&T::FFIType>;
-        unsafe { ptr::read(borrowedptr) }
+    fn as_borrowed(&self) -> &T::FFIType {
+        unsafe { &*(&**self as *const T as *const T::FFIType) }
+    }
+}
+
+unsafe impl<T: HasArcFFI> FFIArcHelpers for Arc<T> {
+    type Inner = T;
+
+    #[inline]
+    fn into_strong(self) -> Strong<T::FFIType> {
+        Arc::into_raw_offset(self).into_strong()
+    }
+
+    #[inline]
+    fn as_borrowed(&self) -> &T::FFIType {
+        unsafe { &*(&**self as *const T as *const T::FFIType) }
     }
 }
 

components/style/gecko_bindings/sugar/refptr.rs

@@ -211,7 +211,7 @@ impl<T> structs::RefPtr<T> {
     /// Sets the contents to an Arc<T>
     /// will leak existing contents
     pub fn set_arc_leaky<U>(&mut self, other: Arc<U>) where U: HasArcFFI<FFIType = T> {
-        *self = unsafe { mem::transmute(other) }; // Arc::into_raw is unstable :(
+        *self = unsafe { mem::transmute(Arc::into_raw_offset(other)) };
     }
 }
 

components/style/matching.rs

@@ -18,7 +18,7 @@ use properties::longhands::display::computed_value as display;
 use rule_tree::{CascadeLevel, StrongRuleNode};
 use selector_parser::{PseudoElement, RestyleDamage};
 use selectors::matching::ElementSelectorFlags;
-use stylearc::Arc;
+use stylearc::{Arc, ArcBorrow};
 
 /// Represents the result of comparing an element's old and new style.
 pub struct StyleDifference {
@@ -690,7 +690,7 @@ pub trait MatchMethods : TElement {
         };
 
         let replace_rule_node = |level: CascadeLevel,
-                                 pdb: Option<&Arc<Locked<PropertyDeclarationBlock>>>,
+                                 pdb: Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>,
                                  path: &mut StrongRuleNode| -> bool {
             let new_node = stylist.rule_tree()
                                   .update_rule_at_level(level, pdb, path, guards);
@@ -737,7 +737,7 @@ pub trait MatchMethods : TElement {
                                                    primary_rules: &mut StrongRuleNode| {
                 let animation_rule = self.get_animation_rule_by_cascade(level);
                 replace_rule_node(level,
-                                  animation_rule.as_ref(),
+                                  animation_rule.as_ref().map(|a| a.borrow_arc()),
                                   primary_rules);
             };
 

components/style/rule_tree/mod.rs

@@ -16,7 +16,7 @@ use std::io::{self, Write};
 use std::mem;
 use std::ptr;
 use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
-use stylearc::{Arc, NonZeroPtrMut};
+use stylearc::{Arc, ArcBorrow, NonZeroPtrMut};
 use stylesheets::StyleRule;
 use thread_state;
 
@@ -308,7 +308,7 @@ impl RuleTree {
     /// the old path is still valid.
     pub fn update_rule_at_level(&self,
                                 level: CascadeLevel,
-                                pdb: Option<&Arc<Locked<PropertyDeclarationBlock>>>,
+                                pdb: Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>,
                                 path: &StrongRuleNode,
                                 guards: &StylesheetGuards)
                                 -> Option<StrongRuleNode> {
@@ -347,7 +347,7 @@ impl RuleTree {
                 // so let's skip it for now.
                 let is_here_already = match &current.get().source {
                     &StyleSource::Declarations(ref already_here) => {
-                        Arc::ptr_eq(pdb, already_here)
+                        pdb.with_arc(|arc| Arc::ptr_eq(arc, already_here))
                     },
                     _ => unreachable!("Replacing non-declarations style?"),
                 };
@@ -371,13 +371,13 @@ impl RuleTree {
             if level.is_important() {
                 if pdb.read_with(level.guard(guards)).any_important() {
                     current = current.ensure_child(self.root.downgrade(),
-                                                   StyleSource::Declarations(pdb.clone()),
+                                                   StyleSource::Declarations(pdb.clone_arc()),
                                                    level);
                 }
             } else {
                 if pdb.read_with(level.guard(guards)).any_normal() {
                     current = current.ensure_child(self.root.downgrade(),
-                                                   StyleSource::Declarations(pdb.clone()),
+                                                   StyleSource::Declarations(pdb.clone_arc()),
                                                    level);
                 }
             }

components/style/sharing/checks.rs

@@ -43,7 +43,7 @@ pub fn have_same_style_attribute<E>(
     match (target.style_attribute(), candidate.style_attribute()) {
         (None, None) => true,
         (Some(_), None) | (None, Some(_)) => false,
-        (Some(a), Some(b)) => Arc::ptr_eq(a, b)
+        (Some(a), Some(b)) => &*a as *const _ == &*b as *const _
     }
 }
 

components/style/stylesheets/rule_list.rs

@@ -5,7 +5,7 @@
 //! A list of CSS rules.
 
 use shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked, SharedRwLock, SharedRwLockReadGuard};
-use stylearc::Arc;
+use stylearc::{Arc, RawOffsetArc};
 use stylesheets::{CssRule, RulesMutateError};
 use stylesheets::loader::StylesheetLoader;
 use stylesheets::memory::{MallocSizeOfFn, MallocSizeOfWithGuard};
@@ -109,7 +109,7 @@ pub trait CssRulesHelpers {
                    -> Result<CssRule, RulesMutateError>;
 }
 
-impl CssRulesHelpers for Arc<Locked<CssRules>> {
+impl CssRulesHelpers for RawOffsetArc<Locked<CssRules>> {
     fn insert_rule(&self,
                    lock: &SharedRwLock,
                    rule: &str,

components/style/stylist.rs

@@ -37,7 +37,7 @@ use std::fmt::Debug;
 #[cfg(feature = "servo")]
 use std::marker::PhantomData;
 use style_traits::viewport::ViewportConstraints;
-use stylearc::Arc;
+use stylearc::{Arc, ArcBorrow};
 #[cfg(feature = "gecko")]
 use stylesheets::{CounterStyleRule, FontFaceRule};
 use stylesheets::{CssRule, StyleRule};
@@ -1103,8 +1103,8 @@ impl Stylist {
                                         &self,
                                         element: &E,
                                         pseudo_element: Option<&PseudoElement>,
-                                        style_attribute: Option<&Arc<Locked<PropertyDeclarationBlock>>>,
-                                        smil_override: Option<&Arc<Locked<PropertyDeclarationBlock>>>,
+                                        style_attribute: Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>,
+                                        smil_override: Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>,
                                         animation_rules: AnimationRules,
                                         rule_inclusion: RuleInclusion,
                                         applicable_declarations: &mut V,
@@ -1208,7 +1208,7 @@ impl Stylist {
             if let Some(sa) = style_attribute {
                 Push::push(
                     applicable_declarations,
-                    ApplicableDeclarationBlock::from_declarations(sa.clone(),
+                    ApplicableDeclarationBlock::from_declarations(sa.clone_arc(),
                                                                   CascadeLevel::StyleAttributeNormal));
             }
 
@@ -1217,7 +1217,7 @@ impl Stylist {
             if let Some(so) = smil_override {
                 Push::push(
                     applicable_declarations,
-                    ApplicableDeclarationBlock::from_declarations(so.clone(),
+                    ApplicableDeclarationBlock::from_declarations(so.clone_arc(),
                                                                   CascadeLevel::SMILOverride));
             }
 

ports/geckolib/glue.rs

@@ -90,7 +90,7 @@ use style::invalidation::element::restyle_hints::{self, RestyleHint};
 use style::media_queries::{MediaList, parse_media_query_list};
 use style::parallel;
 use style::parser::ParserContext;
-use style::properties::{ComputedValues, Importance, SourcePropertyDeclaration};
+use style::properties::{ComputedValues, ComputedValuesInner, Importance, SourcePropertyDeclaration};
 use style::properties::{LonghandIdSet, PropertyDeclaration, PropertyDeclarationBlock, PropertyId, StyleBuilder};
 use style::properties::SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP;
 use style::properties::animated_properties::{Animatable, AnimatableLonghand, AnimationValue};
@@ -101,7 +101,7 @@ use style::sequential;
 use style::shared_lock::{SharedRwLockReadGuard, StylesheetGuards, ToCssWithGuard, Locked};
 use style::string_cache::Atom;
 use style::style_adjuster::StyleAdjuster;
-use style::stylearc::Arc;
+use style::stylearc::{Arc, RawOffsetArc};
 use style::stylesheets::{CssRule, CssRules, CssRuleType, CssRulesHelpers, DocumentRule};
 use style::stylesheets::{ImportRule, KeyframesRule, MallocSizeOfWithGuard, MediaRule};
 use style::stylesheets::{NamespaceRule, Origin, PageRule, StyleRule, SupportsRule};
@@ -429,7 +429,7 @@ pub extern "C" fn Servo_AnimationCompose(raw_value_map: RawServoAnimationValueMa
         let previous_composed_value = value_map.get(&property).cloned();
         previous_composed_value.or_else(|| {
             let raw_base_style = unsafe { Gecko_AnimationGetBaseStyle(base_values, css_property) };
-            AnimationValue::arc_from_borrowed(&raw_base_style).map(|v| v.as_ref()).cloned()
+            AnimationValue::arc_from_borrowed(&raw_base_style).map(|v| &**v).cloned()
         })
     } else {
         None
@@ -459,7 +459,7 @@ pub extern "C" fn Servo_AnimationCompose(raw_value_map: RawServoAnimationValueMa
 
     // Composite with underlying value.
     // A return value of None means, "Just use keyframe_value as-is."
-    let composite_endpoint = |keyframe_value: Option<&Arc<AnimationValue>>,
+    let composite_endpoint = |keyframe_value: Option<&RawOffsetArc<AnimationValue>>,
                               composite_op: CompositeOperation| -> Option<AnimationValue> {
         match keyframe_value {
             Some(keyframe_value) => {
@@ -498,7 +498,7 @@ pub extern "C" fn Servo_AnimationCompose(raw_value_map: RawServoAnimationValueMa
         let raw_last_value;
         let last_value = if !last_segment.mToValue.mServo.mRawPtr.is_null() {
             raw_last_value = unsafe { &*last_segment.mToValue.mServo.mRawPtr };
-            AnimationValue::as_arc(&raw_last_value).as_ref()
+            &*AnimationValue::as_arc(&raw_last_value)
         } else {
             debug_assert!(need_underlying_value,
                           "Should have detected we need an underlying value");
@@ -506,7 +506,7 @@ pub extern "C" fn Servo_AnimationCompose(raw_value_map: RawServoAnimationValueMa
         };
 
         // As with composite_endpoint, a return value of None means, "Use keyframe_value as-is."
-        let apply_iteration_composite = |keyframe_value: Option<&Arc<AnimationValue>>,
+        let apply_iteration_composite = |keyframe_value: Option<&RawOffsetArc<AnimationValue>>,
                                          composited_value: Option<AnimationValue>|
                                         -> Option<AnimationValue> {
             let count = computed_timing.mCurrentIteration;
@@ -1244,7 +1244,7 @@ pub extern "C" fn Servo_StyleRule_SetStyle(rule: RawServoStyleRuleBorrowed,
                                            declarations: RawServoDeclarationBlockBorrowed) {
     let declarations = Locked::<PropertyDeclarationBlock>::as_arc(&declarations);
     write_locked_arc(rule, |rule: &mut StyleRule| {
-        rule.block = declarations.clone();
+        rule.block = declarations.clone_arc();
     })
 }
 
@@ -1382,7 +1382,7 @@ pub extern "C" fn Servo_Keyframe_SetStyle(keyframe: RawServoKeyframeBorrowed,
                                           declarations: RawServoDeclarationBlockBorrowed) {
     let declarations = Locked::<PropertyDeclarationBlock>::as_arc(&declarations);
     write_locked_arc(keyframe, |keyframe: &mut Keyframe| {
-        keyframe.block = declarations.clone();
+        keyframe.block = declarations.clone_arc();
     })
 }
 
@@ -1480,7 +1480,7 @@ pub extern "C" fn Servo_PageRule_SetStyle(rule: RawServoPageRuleBorrowed,
                                            declarations: RawServoDeclarationBlockBorrowed) {
     let declarations = Locked::<PropertyDeclarationBlock>::as_arc(&declarations);
     write_locked_arc(rule, |rule: &mut PageRule| {
-        rule.block = declarations.clone();
+        rule.block = declarations.clone_arc();
     })
 }
 
@@ -1559,7 +1559,7 @@ pub extern "C" fn Servo_ResolvePseudoStyle(element: RawGeckoElementBorrowed,
         &pseudo,
         RuleInclusion::All,
         &data.styles,
-        ComputedValues::arc_from_borrowed(&inherited_style),
+        ComputedValues::arc_from_borrowed(&inherited_style).map(|x| &***x),
         &*doc_data,
         is_probe
     );
@@ -1584,7 +1584,7 @@ pub extern "C" fn Servo_SetExplicitStyle(element: RawGeckoElementBorrowed,
     // work for other things, we just haven't had a reason to do so.
     debug_assert!(element.get_data().is_none());
     let mut data = unsafe { element.ensure_data() };
-    data.styles.primary = Some(style.clone());
+    data.styles.primary = Some(style.clone_arc());
 }
 
 #[no_mangle]
@@ -1613,7 +1613,7 @@ fn get_pseudo_style(
     pseudo: &PseudoElement,
     rule_inclusion: RuleInclusion,
     styles: &ElementStyles,
-    inherited_styles: Option<&Arc<ComputedValues>>,
+    inherited_styles: Option<&ComputedValuesInner>,
     doc_data: &PerDocumentStyleDataImpl,
     is_probe: bool,
 ) -> Option<Arc<ComputedValues>> {
@@ -1642,8 +1642,8 @@ fn get_pseudo_style(
                 },
                 _ => {
                     debug_assert!(inherited_styles.is_none() ||
-                                  ptr::eq(&**inherited_styles.unwrap(),
-                                          &**styles.primary()));
+                                  ptr::eq(&*inherited_styles.unwrap(),
+                                          &***styles.primary()));
                     styles.pseudos.get(&pseudo).cloned()
                 },
             }
@@ -1651,8 +1651,8 @@ fn get_pseudo_style(
         PseudoElementCascadeType::Precomputed => unreachable!("No anonymous boxes"),
         PseudoElementCascadeType::Lazy => {
             debug_assert!(inherited_styles.is_none() ||
-                          ptr::eq(&**inherited_styles.unwrap(),
-                                  &**styles.primary()));
+                          ptr::eq(&*inherited_styles.unwrap(),
+                                  &***styles.primary()));
             let base = if pseudo.inherits_from_default_values() {
                 doc_data.default_computed_values()
             } else {
@@ -1761,12 +1761,14 @@ pub extern "C" fn Servo_ComputedValues_GetStyleRuleList(values: ServoComputedVal
         let mut result = vec![];
         for node in rule_node.self_and_ancestors() {
             if let &StyleSource::Style(ref rule) = node.style_source() {
-                result.push(Locked::<StyleRule>::arc_as_borrowed(&rule));
+                result.push(rule);
             }
         }
         unsafe { rules.set_len(result.len() as u32) };
-        for (&src, dest) in result.into_iter().zip(rules.iter_mut()) {
-            *dest = src;
+        for (ref src, ref mut dest) in result.into_iter().zip(rules.iter_mut()) {
+            src.with_raw_offset_arc(|arc| {
+                **dest = *Locked::<StyleRule>::arc_as_borrowed(arc);
+            })
         }
     }
 }
@@ -3274,7 +3276,7 @@ pub extern "C" fn Servo_StyleSet_ResolveForDeclarations(
 
     doc_data.stylist.compute_for_declarations(&guards,
                                               parent_style,
-                                              declarations.clone()).into_strong()
+                                              declarations.clone_arc()).into_strong()
 }
 
 #[no_mangle]