Auto merge of #16721 - Manishearth:vector-iter, r=emilio

Set vector properties via iterator in stylo

https://bugzilla.mozilla.org/show_bug.cgi?id=1360882

Avoids transient allocations

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components/style/properties/data.py

@@ -140,7 +140,7 @@ class Longhand(object):
                  need_clone=False, need_index=False, gecko_ffi_name=None, depend_on_viewport_size=False,
                  allowed_in_keyframe_block=True, complex_color=False, cast_type='u8',
                  has_uncacheable_values=False, logical=False, alias=None, extra_prefixes=None, boxed=False,
-                 flags=None, allowed_in_page_rule=False, allow_quirks=False):
+                 flags=None, allowed_in_page_rule=False, allow_quirks=False, vector=False):
         self.name = name
         if not spec:
             raise TypeError("Spec should be specified for %s" % name)
@@ -167,6 +167,7 @@ class Longhand(object):
         self.flags = flags.split() if flags else []
         self.allowed_in_page_rule = arg_to_bool(allowed_in_page_rule)
         self.allow_quirks = allow_quirks
+        self.is_vector = vector
 
         # https://drafts.csswg.org/css-animations/#keyframes
         # > The <declaration-list> inside of <keyframe-block> accepts any CSS property

components/style/properties/gecko.mako.rs

@@ -1697,14 +1697,18 @@ fn static_assert() {
 
 <%def name="impl_animation_or_transition_time_value(type, ident, gecko_ffi_name)">
     #[allow(non_snake_case)]
-    pub fn set_${type}_${ident}(&mut self, v: longhands::${type}_${ident}::computed_value::T) {
+    pub fn set_${type}_${ident}<I>(&mut self, v: I)
-        debug_assert!(!v.0.is_empty());
+        where I: IntoIterator<Item = longhands::${type}_${ident}::computed_value::single_value::T>,
-        let input_len = v.0.len();
+              I::IntoIter: ExactSizeIterator + Clone
+    {
+        let v = v.into_iter();
+        debug_assert!(v.len() != 0);
+        let input_len = v.len();
         unsafe { self.gecko.m${type.capitalize()}s.ensure_len(input_len) };
 
         self.gecko.m${type.capitalize()}${gecko_ffi_name}Count = input_len as u32;
-        for (i, gecko) in self.gecko.m${type.capitalize()}s.iter_mut().enumerate() {
+        for (gecko, servo) in self.gecko.m${type.capitalize()}s.iter_mut().zip(v.cycle()) {
-            gecko.m${gecko_ffi_name} = v.0[i % input_len].seconds() * 1000.;
+            gecko.m${gecko_ffi_name} = servo.seconds() * 1000.;
         }
     }
     #[allow(non_snake_case)]
@@ -1718,14 +1722,18 @@ fn static_assert() {
 </%def>
 
 <%def name="impl_animation_or_transition_timing_function(type)">
-    pub fn set_${type}_timing_function(&mut self, v: longhands::${type}_timing_function::computed_value::T) {
+    pub fn set_${type}_timing_function<I>(&mut self, v: I)
-        debug_assert!(!v.0.is_empty());
+        where I: IntoIterator<Item = longhands::${type}_timing_function::computed_value::single_value::T>,
-        let input_len = v.0.len();
+              I::IntoIter: ExactSizeIterator + Clone
+    {
+        let v = v.into_iter();
+        debug_assert!(v.len() != 0);
+        let input_len = v.len();
         unsafe { self.gecko.m${type.capitalize()}s.ensure_len(input_len) };
 
         self.gecko.m${type.capitalize()}TimingFunctionCount = input_len as u32;
-        for (i, gecko) in self.gecko.m${type.capitalize()}s.iter_mut().enumerate() {
+        for (gecko, servo) in self.gecko.m${type.capitalize()}s.iter_mut().zip(v.cycle()) {
-            gecko.mTimingFunction = v.0[i % input_len].into();
+            gecko.mTimingFunction = servo.into();
         }
     }
     ${impl_animation_or_transition_count(type, 'timing_function', 'TimingFunction')}
@@ -1766,18 +1774,23 @@ fn static_assert() {
 
 <%def name="impl_animation_keyword(ident, gecko_ffi_name, keyword, cast_type='u8')">
     #[allow(non_snake_case)]
-    pub fn set_animation_${ident}(&mut self, v: longhands::animation_${ident}::computed_value::T) {
+    pub fn set_animation_${ident}<I>(&mut self, v: I)
+        where I: IntoIterator<Item = longhands::animation_${ident}::computed_value::single_value::T>,
+              I::IntoIter: ExactSizeIterator + Clone
+    {
         use properties::longhands::animation_${ident}::single_value::computed_value::T as Keyword;
         use gecko_bindings::structs;
 
-        debug_assert!(!v.0.is_empty());
+        let v = v.into_iter();
-        let input_len = v.0.len();
+
+        debug_assert!(v.len() != 0);
+        let input_len = v.len();
         unsafe { self.gecko.mAnimations.ensure_len(input_len) };
 
         self.gecko.mAnimation${gecko_ffi_name}Count = input_len as u32;
 
-        for (i, gecko) in self.gecko.mAnimations.iter_mut().enumerate() {
+        for (gecko, servo) in self.gecko.mAnimations.iter_mut().zip(v.cycle()) {
-            let result = match v.0[i % input_len] {
+            let result = match servo {
                 % for value in keyword.gecko_values():
                     Keyword::${to_rust_ident(value)} =>
                         structs::${keyword.gecko_constant(value)} ${keyword.maybe_cast(cast_type)},
@@ -1970,11 +1983,16 @@ fn static_assert() {
 
     ${impl_coord_copy('scroll_snap_points_y', 'mScrollSnapPointsY')}
 
-    pub fn set_scroll_snap_coordinate(&mut self, v: longhands::scroll_snap_coordinate::computed_value::T) {
+    pub fn set_scroll_snap_coordinate<I>(&mut self, v: I)
-        unsafe { self.gecko.mScrollSnapCoordinate.set_len_pod(v.0.len() as u32); }
+        where I: IntoIterator<Item = longhands::scroll_snap_coordinate::computed_value::single_value::T>,
+              I::IntoIter: ExactSizeIterator
+    {
+        let v = v.into_iter();
+
+        unsafe { self.gecko.mScrollSnapCoordinate.set_len_pod(v.len() as u32); }
         for (gecko, servo) in self.gecko.mScrollSnapCoordinate
                                .iter_mut()
-                               .zip(v.0.iter()) {
+                               .zip(v) {
             gecko.mXPosition = servo.horizontal.0.into();
             gecko.mYPosition = servo.vertical.0.into();
         }
@@ -2165,13 +2183,18 @@ fn static_assert() {
         self.gecko.mTransitions[index].mDuration.max(0.0) + self.gecko.mTransitions[index].mDelay
     }
 
-    pub fn set_transition_property(&mut self, v: longhands::transition_property::computed_value::T) {
+    pub fn set_transition_property<I>(&mut self, v: I)
+        where I: IntoIterator<Item = longhands::transition_property::computed_value::single_value::T>,
+              I::IntoIter: ExactSizeIterator
+    {
         use gecko_bindings::structs::nsCSSPropertyID::eCSSPropertyExtra_no_properties;
 
-        if !v.0.is_empty() {
+        let v = v.into_iter();
-            unsafe { self.gecko.mTransitions.ensure_len(v.0.len()) };
+
-            self.gecko.mTransitionPropertyCount = v.0.len() as u32;
+        if v.len() != 0 {
-            for (servo, gecko) in v.0.into_iter().zip(self.gecko.mTransitions.iter_mut()) {
+            unsafe { self.gecko.mTransitions.ensure_len(v.len()) };
+            self.gecko.mTransitionPropertyCount = v.len() as u32;
+            for (servo, gecko) in v.zip(self.gecko.mTransitions.iter_mut()) {
                 match servo {
                     TransitionProperty::Unsupported(ref atom) => unsafe {
                         Gecko_StyleTransition_SetUnsupportedProperty(gecko, atom.as_ptr())
@@ -2246,12 +2269,17 @@ fn static_assert() {
         unsafe { bindings::Gecko_StyleAnimationsEquals(&self.gecko.mAnimations, &other.gecko.mAnimations) }
     }
 
-    pub fn set_animation_name(&mut self, v: longhands::animation_name::computed_value::T) {
+    pub fn set_animation_name<I>(&mut self, v: I)
-        debug_assert!(!v.0.is_empty());
+        where I: IntoIterator<Item = longhands::animation_name::computed_value::single_value::T>,
-        unsafe { self.gecko.mAnimations.ensure_len(v.0.len()) };
+              I::IntoIter: ExactSizeIterator
+    {
 
-        self.gecko.mAnimationNameCount = v.0.len() as u32;
+        let v = v.into_iter();
-        for (servo, gecko) in v.0.into_iter().zip(self.gecko.mAnimations.iter_mut()) {
+        debug_assert!(v.len() != 0);
+        unsafe { self.gecko.mAnimations.ensure_len(v.len()) };
+
+        self.gecko.mAnimationNameCount = v.len() as u32;
+        for (servo, gecko) in v.zip(self.gecko.mAnimations.iter_mut()) {
             // TODO This is inefficient. We should fix this in bug 1329169.
             gecko.mName.assign(match servo.0 {
                 Some(ref name) => name.as_atom().as_slice(),
@@ -2294,17 +2322,22 @@ fn static_assert() {
     ${impl_animation_keyword('play_state', 'PlayState',
                              data.longhands_by_name["animation-play-state"].keyword)}
 
-    pub fn set_animation_iteration_count(&mut self, v: longhands::animation_iteration_count::computed_value::T) {
+    pub fn set_animation_iteration_count<I>(&mut self, v: I)
+        where I: IntoIterator<Item = longhands::animation_iteration_count::computed_value::single_value::T>,
+              I::IntoIter: ExactSizeIterator + Clone
+    {
         use std::f32;
         use properties::longhands::animation_iteration_count::single_value::SpecifiedValue as AnimationIterationCount;
 
-        debug_assert!(!v.0.is_empty());
+        let v = v.into_iter();
-        let input_len = v.0.len();
+
+        debug_assert!(v.len() != 0);
+        let input_len = v.len();
         unsafe { self.gecko.mAnimations.ensure_len(input_len) };
 
         self.gecko.mAnimationIterationCountCount = input_len as u32;
-        for (i, gecko) in self.gecko.mAnimations.iter_mut().enumerate() {
+        for (gecko, servo) in self.gecko.mAnimations.iter_mut().zip(v.cycle()) {
-            match v.0[i % input_len] {
+            match servo {
                 AnimationIterationCount::Number(n) => gecko.mIterationCount = n,
                 AnimationIterationCount::Infinite => gecko.mIterationCount = f32::INFINITY,
             }
@@ -2558,18 +2591,21 @@ fn static_assert() {
             other.gecko.${image_layers_field}.${field_name}Count;
     }
 
-    pub fn set_${shorthand}_${name}(&mut self,
+
-                                    v: longhands::${shorthand}_${name}::computed_value::T) {
+    pub fn set_${shorthand}_${name}<I>(&mut self, v: I)
+        where I: IntoIterator<Item=longhands::${shorthand}_${name}::computed_value::single_value::T>,
+              I::IntoIter: ExactSizeIterator
+    {
         use gecko_bindings::structs::nsStyleImageLayers_LayerType as LayerType;
+        let v = v.into_iter();
 
         unsafe {
-          Gecko_EnsureImageLayersLength(&mut self.gecko.${image_layers_field}, v.0.len(),
+          Gecko_EnsureImageLayersLength(&mut self.gecko.${image_layers_field}, v.len(),
                                         LayerType::${shorthand.title()});
         }
 
-        self.gecko.${image_layers_field}.${field_name}Count = v.0.len() as u32;
+        self.gecko.${image_layers_field}.${field_name}Count = v.len() as u32;
-        for (servo, geckolayer) in v.0.into_iter()
+        for (servo, geckolayer) in v.zip(self.gecko.${image_layers_field}.mLayers.iter_mut()) {
-                                    .zip(self.gecko.${image_layers_field}.mLayers.iter_mut()) {
             geckolayer.${field_name} = {
                 ${caller.body()}
             };
@@ -2675,17 +2711,23 @@ fn static_assert() {
         )
     }
 
-    pub fn set_${shorthand}_position_${orientation[0]}(&mut self,
+    pub fn set_${shorthand}_position_${orientation[0]}<I>(&mut self,
-                                     v: longhands::${shorthand}_position_${orientation[0]}::computed_value::T) {
+                                     v: I)
+        where I: IntoIterator<Item = longhands::${shorthand}_position_${orientation[0]}
+                                              ::computed_value::single_value::T>,
+              I::IntoIter: ExactSizeIterator
+    {
         use gecko_bindings::structs::nsStyleImageLayers_LayerType as LayerType;
 
+        let v = v.into_iter();
+
         unsafe {
-            Gecko_EnsureImageLayersLength(&mut self.gecko.${image_layers_field}, v.0.len(),
+            Gecko_EnsureImageLayersLength(&mut self.gecko.${image_layers_field}, v.len(),
                                         LayerType::${shorthand.capitalize()});
         }
 
-        self.gecko.${image_layers_field}.mPosition${orientation[0].upper()}Count = v.0.len() as u32;
+        self.gecko.${image_layers_field}.mPosition${orientation[0].upper()}Count = v.len() as u32;
-        for (servo, geckolayer) in v.0.into_iter().zip(self.gecko.${image_layers_field}
+        for (servo, geckolayer) in v.zip(self.gecko.${image_layers_field}
                                                            .mLayers.iter_mut()) {
             geckolayer.mPosition.m${orientation[0].upper()}Position = servo.0.into();
         }
@@ -2772,24 +2814,27 @@ fn static_assert() {
     }
 
     #[allow(unused_variables)]
-    pub fn set_${shorthand}_image(&mut self,
+    pub fn set_${shorthand}_image<I>(&mut self, images: I, cacheable: &mut bool)
-                                  images: longhands::${shorthand}_image::computed_value::T,
+        where I: IntoIterator<Item = longhands::${shorthand}_image::computed_value::single_value::T>,
-                                  cacheable: &mut bool) {
+              I::IntoIter: ExactSizeIterator
+    {
         use gecko_bindings::structs::nsStyleImageLayers_LayerType as LayerType;
 
+        let images = images.into_iter();
+
         unsafe {
             // Prevent leaking of the last elements we did set
             for image in &mut self.gecko.${image_layers_field}.mLayers {
                 Gecko_SetNullImageValue(&mut image.mImage)
             }
             // XXXManishearth clear mSourceURI for masks
-            Gecko_EnsureImageLayersLength(&mut self.gecko.${image_layers_field}, images.0.len(),
+            Gecko_EnsureImageLayersLength(&mut self.gecko.${image_layers_field}, images.len(),
                                           LayerType::${shorthand.title()});
         }
 
-        self.gecko.${image_layers_field}.mImageCount = images.0.len() as u32;
+        self.gecko.${image_layers_field}.mImageCount = images.len() as u32;
 
-        for (image, geckoimage) in images.0.into_iter().zip(self.gecko.${image_layers_field}
+        for (image, geckoimage) in images.zip(self.gecko.${image_layers_field}
                                                   .mLayers.iter_mut()) {
             if let Some(image) = image.0 {
                 geckoimage.mImage.set(image, cacheable)
@@ -2986,12 +3031,15 @@ fn static_assert() {
 
 <%self:impl_trait style_struct_name="Effects"
                   skip_longhands="box-shadow clip filter">
-    pub fn set_box_shadow(&mut self, v: longhands::box_shadow::computed_value::T) {
+    pub fn set_box_shadow<I>(&mut self, v: I)
+        where I: IntoIterator<Item = longhands::box_shadow::computed_value::single_value::T>,
+              I::IntoIter: ExactSizeIterator
+    {
+        let v = v.into_iter();
 
-        self.gecko.mBoxShadow.replace_with_new(v.0.len() as u32);
+        self.gecko.mBoxShadow.replace_with_new(v.len() as u32);
 
-        for (servo, gecko_shadow) in v.0.into_iter()
+        for (servo, gecko_shadow) in v.zip(self.gecko.mBoxShadow.iter_mut()) {
-                                      .zip(self.gecko.mBoxShadow.iter_mut()) {
 
             gecko_shadow.mXOffset = servo.offset_x.0;
             gecko_shadow.mYOffset = servo.offset_y.0;
@@ -3790,12 +3838,16 @@ clip-path
 
     ${impl_simple_copy('paint_order', 'mPaintOrder')}
 
-    pub fn set_stroke_dasharray(&mut self, v: longhands::stroke_dasharray::computed_value::T) {
+    pub fn set_stroke_dasharray<I>(&mut self, v: I)
+        where I: IntoIterator<Item = longhands::stroke_dasharray::computed_value::single_value::T>,
+              I::IntoIter: ExactSizeIterator
+    {
+        let v = v.into_iter();
         unsafe {
-            bindings::Gecko_nsStyleSVG_SetDashArrayLength(&mut self.gecko, v.0.len() as u32);
+            bindings::Gecko_nsStyleSVG_SetDashArrayLength(&mut self.gecko, v.len() as u32);
         }
 
-        for (mut gecko, servo) in self.gecko.mStrokeDasharray.iter_mut().zip(v.0.into_iter()) {
+        for (mut gecko, servo) in self.gecko.mStrokeDasharray.iter_mut().zip(v) {
             match servo {
                 Either::First(number) => gecko.set_value(CoordDataValue::Factor(number)),
                 Either::Second(lop) => gecko.set(lop),

components/style/properties/helpers.mako.rs

@@ -74,7 +74,7 @@
 </%doc>
 <%def name="vector_longhand(name, gecko_only=False, allow_empty=False,
             delegate_animate=False, space_separated_allowed=False, **kwargs)">
-    <%call expr="longhand(name, **kwargs)">
+    <%call expr="longhand(name, vector=True, **kwargs)">
         % if not gecko_only:
             use smallvec::SmallVec;
             use std::fmt;
@@ -102,7 +102,9 @@
             pub mod computed_value {
                 pub use super::single_value::computed_value as single_value;
                 pub use self::single_value::T as SingleComputedValue;
-                use smallvec::SmallVec;
+                use smallvec::{IntoIter, SmallVec};
+                use values::computed::ComputedVecIter;
+
                 /// The computed value, effectively a list of single values.
                 #[derive(Debug, Clone, PartialEq)]
                 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
@@ -129,6 +131,16 @@
                         }
                     }
                 % endif
+
+                pub type Iter<'a, 'cx, 'cx_a> = ComputedVecIter<'a, 'cx, 'cx_a, super::single_value::SpecifiedValue>;
+
+                impl IntoIterator for T {
+                    type Item = single_value::T;
+                    type IntoIter = IntoIter<[single_value::T; 1]>;
+                    fn into_iter(self) -> Self::IntoIter {
+                        self.0.into_iter()
+                    }
+                }
             }
 
             impl ToCss for computed_value::T {
@@ -212,12 +224,19 @@
 
             pub use self::single_value::SpecifiedValue as SingleSpecifiedValue;
 
+            impl SpecifiedValue {
+                pub fn compute_iter<'a, 'cx, 'cx_a>(&'a self, context: &'cx Context<'cx_a>)
+                    -> computed_value::Iter<'a, 'cx, 'cx_a> {
+                    computed_value::Iter::new(context, &self.0)
+                }
+            }
+
             impl ToComputedValue for SpecifiedValue {
                 type ComputedValue = computed_value::T;
 
                 #[inline]
                 fn to_computed_value(&self, context: &Context) -> computed_value::T {
-                    computed_value::T(self.0.iter().map(|x| x.to_computed_value(context)).collect())
+                    computed_value::T(self.compute_iter(context).collect())
                 }
                 #[inline]
                 fn from_computed_value(computed: &computed_value::T) -> Self {
@@ -294,7 +313,10 @@
                         % if property.logical:
                             let wm = context.style.writing_mode;
                         % endif
-                        <% maybe_wm = ", wm" if property.logical else "" %>
+                        <%
+                            maybe_wm = ", wm" if property.logical else ""
+                            maybe_cacheable = ", cacheable" if property.has_uncacheable_values == "True" else ""
+                        %>
                         match *value {
                             DeclaredValue::Value(ref specified_value) => {
                                 % if property.ident in SYSTEM_FONT_LONGHANDS and product == "gecko":
@@ -302,6 +324,21 @@
                                         longhands::system_font::resolve_system_font(sf, context);
                                     }
                                 % endif
+                                % if property.is_vector:
+                                    // In the case of a vector property we want to pass down
+                                    // an iterator so that this can be computed without allocation
+                                    //
+                                    // However, computing requires a context, but the style struct
+                                    // being mutated is on the context. We temporarily remove it,
+                                    // mutate it, and then put it back. Vector longhands cannot
+                                    // touch their own style struct whilst computing, else this will panic.
+                                    let mut s = context.mutate_style().take_${data.current_style_struct.name_lower}();
+                                    {
+                                        let iter = specified_value.compute_iter(context);
+                                        s.set_${property.ident}(iter ${maybe_cacheable});
+                                    }
+                                    context.mutate_style().put_${data.current_style_struct.name_lower}(s);
+                                % else:
                                     let computed = specified_value.to_computed_value(context);
                                      % if property.ident == "font_size":
                                          longhands::font_size::cascade_specified_font_size(context,
@@ -309,12 +346,8 @@
                                                                                            computed,
                                                                                            inherited_style.get_font());
                                     % else:
-                                    % if property.has_uncacheable_values == "True":
                                         context.mutate_style().mutate_${data.current_style_struct.name_lower}()
-                                                          .set_${property.ident}(computed, cacheable ${maybe_wm});
+                                               .set_${property.ident}(computed ${maybe_cacheable} ${maybe_wm});
-                                    % else:
-                                    context.mutate_style().mutate_${data.current_style_struct.name_lower}()
-                                                          .set_${property.ident}(computed ${maybe_wm});
                                     % endif
                                 % endif
                             }

components/style/properties/properties.mako.rs

@@ -14,7 +14,7 @@ use std::borrow::Cow;
 use std::collections::HashSet;
 use std::fmt;
 use std::ops::Deref;
-use stylearc::Arc;
+use stylearc::{Arc, UniqueArc};
 
 use app_units::Au;
 #[cfg(feature = "servo")] use cssparser::{Color as CSSParserColor, RGBA};
@@ -1508,6 +1508,19 @@ pub mod style_structs {
             % for longhand in style_struct.longhands:
                 % if longhand.logical:
                     ${helpers.logical_setter(name=longhand.name)}
+                % else:
+                    % if longhand.is_vector:
+                        /// Set ${longhand.name}.
+                        #[allow(non_snake_case)]
+                        #[inline]
+                        pub fn set_${longhand.ident}<I>(&mut self, v: I)
+                            where I: IntoIterator<Item = longhands::${longhand.ident}
+                                                                  ::computed_value::single_value::T>,
+                                  I::IntoIter: ExactSizeIterator
+                        {
+                            self.${longhand.ident} = longhands::${longhand.ident}::computed_value
+                                                              ::T(v.into_iter().collect());
+                        }
                     % else:
                         /// Set ${longhand.name}.
                         #[allow(non_snake_case)]
@@ -1515,6 +1528,7 @@ pub mod style_structs {
                         pub fn set_${longhand.ident}(&mut self, v: longhands::${longhand.ident}::computed_value::T) {
                             self.${longhand.ident} = v;
                         }
+                    % endif
                     /// Set ${longhand.name} from other struct.
                     #[allow(non_snake_case)]
                     #[inline]
@@ -2057,7 +2071,9 @@ pub enum StyleStructRef<'a, T: 'a> {
     /// A borrowed struct from the parent, for example, for inheriting style.
     Borrowed(&'a Arc<T>),
     /// An owned struct, that we've already mutated.
-    Owned(Arc<T>),
+    Owned(UniqueArc<T>),
+    /// Temporarily vacated, will panic if accessed
+    Vacated,
 }
 
 impl<'a, T: 'a> StyleStructRef<'a, T>
@@ -2067,21 +2083,45 @@ impl<'a, T: 'a> StyleStructRef<'a, T>
     /// borrowed value, or returning the owned one.
     pub fn mutate(&mut self) -> &mut T {
         if let StyleStructRef::Borrowed(v) = *self {
-            *self = StyleStructRef::Owned(Arc::new((**v).clone()));
+            *self = StyleStructRef::Owned(UniqueArc::new((**v).clone()));
         }
 
         match *self {
-            StyleStructRef::Owned(ref mut v) => Arc::get_mut(v).unwrap(),
+            StyleStructRef::Owned(ref mut v) => v,
             StyleStructRef::Borrowed(..) => unreachable!(),
+            StyleStructRef::Vacated => panic!("Accessed vacated style struct")
         }
     }
 
+    /// Extract a unique Arc from this struct, vacating it.
+    ///
+    /// The vacated state is a transient one, please put the Arc back
+    /// when done via `put()`. This function is to be used to separate
+    /// the struct being mutated from the computed context
+    pub fn take(&mut self) -> UniqueArc<T> {
+        use std::mem::replace;
+        let inner = replace(self, StyleStructRef::Vacated);
+
+        match inner {
+            StyleStructRef::Owned(arc) => arc,
+            StyleStructRef::Borrowed(arc) => UniqueArc::new((**arc).clone()),
+            StyleStructRef::Vacated => panic!("Accessed vacated style struct"),
+        }
+    }
+
+    /// Replace vacated ref with an arc
+    pub fn put(&mut self, arc: UniqueArc<T>) {
+        debug_assert!(matches!(*self, StyleStructRef::Vacated));
+        *self = StyleStructRef::Owned(arc);
+    }
+
     /// Get a mutable reference to the owned struct, or `None` if the struct
     /// hasn't been mutated.
     pub fn get_if_mutated(&mut self) -> Option<<&mut T> {
         match *self {
-            StyleStructRef::Owned(ref mut v) => Some(Arc::get_mut(v).unwrap()),
+            StyleStructRef::Owned(ref mut v) => Some(v),
             StyleStructRef::Borrowed(..) => None,
+            StyleStructRef::Vacated => panic!("Accessed vacated style struct")
         }
     }
 
@@ -2089,8 +2129,9 @@ impl<'a, T: 'a> StyleStructRef<'a, T>
     /// appropriate.
     pub fn build(self) -> Arc<T> {
         match self {
-            StyleStructRef::Owned(v) => v,
+            StyleStructRef::Owned(v) => v.shareable(),
             StyleStructRef::Borrowed(v) => v.clone(),
+            StyleStructRef::Vacated => panic!("Accessed vacated style struct")
         }
     }
 }
@@ -2102,6 +2143,7 @@ impl<'a, T: 'a> Deref for StyleStructRef<'a, T> {
         match *self {
             StyleStructRef::Owned(ref v) => &**v,
             StyleStructRef::Borrowed(v) => &**v,
+            StyleStructRef::Vacated => panic!("Accessed vacated style struct")
         }
     }
 }
@@ -2183,6 +2225,16 @@ impl<'a> StyleBuilder<'a> {
             self.${style_struct.ident}.mutate()
         }
 
+        /// Gets a mutable view of the current `${style_struct.name}` style.
+        pub fn take_${style_struct.name_lower}(&mut self) -> UniqueArc<style_structs::${style_struct.name}> {
+            self.${style_struct.ident}.take()
+        }
+
+        /// Gets a mutable view of the current `${style_struct.name}` style.
+        pub fn put_${style_struct.name_lower}(&mut self, s: UniqueArc<style_structs::${style_struct.name}>) {
+            self.${style_struct.ident}.put(s)
+        }
+
         /// Gets a mutable view of the current `${style_struct.name}` style,
         /// only if it's been mutated before.
         pub fn get_${style_struct.name_lower}_if_mutated(&mut self)

components/style/stylearc.rs

@@ -31,7 +31,7 @@ use std::convert::From;
 use std::fmt;
 use std::hash::{Hash, Hasher};
 use std::mem;
-use std::ops::Deref;
+use std::ops::{Deref, DerefMut};
 use std::sync::atomic;
 use std::sync::atomic::Ordering::{Acquire, Relaxed, Release};
 
@@ -71,6 +71,40 @@ pub struct Arc<T: ?Sized> {
     ptr: *mut ArcInner<T>,
 }
 
+/// An Arc that is known to be uniquely owned
+///
+/// This lets us build arcs that we can mutate before
+/// freezing, without needing to change the allocation
+pub struct UniqueArc<T: ?Sized>(Arc<T>);
+
+impl<T> UniqueArc<T> {
+    #[inline]
+    /// Construct a new UniqueArc
+    pub fn new(data: T) -> Self {
+        UniqueArc(Arc::new(data))
+    }
+
+    #[inline]
+    /// Convert to a shareable Arc<T> once we're done using it
+    pub fn shareable(self) -> Arc<T> {
+        self.0
+    }
+}
+
+impl<T> Deref for UniqueArc<T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        &*self.0
+    }
+}
+
+impl<T> DerefMut for UniqueArc<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // We know this to be uniquely owned
+        unsafe { &mut (*self.0.ptr).data }
+    }
+}
+
 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
 

components/style/values/computed/mod.rs

@@ -100,6 +100,42 @@ impl<'a> Context<'a> {
     pub fn mutate_style(&mut self) -> &mut StyleBuilder<'a> { &mut self.style }
 }
 
+/// An iterator over a slice of computed values
+#[derive(Clone)]
+pub struct ComputedVecIter<'a, 'cx, 'cx_a: 'cx, S: ToComputedValue + 'a> {
+    cx: &'cx Context<'cx_a>,
+    values: &'a [S],
+}
+
+impl<'a, 'cx, 'cx_a: 'cx, S: ToComputedValue + 'a> ComputedVecIter<'a, 'cx, 'cx_a, S> {
+    /// Construct an iterator from a slice of specified values and a context
+    pub fn new(cx: &'cx Context<'cx_a>, values: &'a [S]) -> Self {
+        ComputedVecIter {
+            cx: cx,
+            values: values,
+        }
+    }
+}
+
+impl<'a, 'cx, 'cx_a: 'cx, S: ToComputedValue + 'a> ExactSizeIterator for ComputedVecIter<'a, 'cx, 'cx_a, S> {
+    fn len(&self) -> usize {
+        self.values.len()
+    }
+}
+
+impl<'a, 'cx, 'cx_a: 'cx, S: ToComputedValue + 'a> Iterator for ComputedVecIter<'a, 'cx, 'cx_a, S> {
+    type Item = S::ComputedValue;
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some((next, rest)) = self.values.split_first() {
+            let ret = next.to_computed_value(self.cx);
+            self.values = rest;
+            Some(ret)
+        } else {
+            None
+        }
+    }
+}
+
 /// A trait to represent the conversion between computed and specified values.
 pub trait ToComputedValue {
     /// The computed value type we're going to be converted to.