Auto merge of #16663 - emilio:style-builder, r=bholley

style: Add a StyleBuilder struct to avoid refcount and atomic CAS during the cascade.

This should fix most of the complaints that caused
https://bugzilla.mozilla.org/show_bug.cgi?id=1360889 to be open, and also fix a
bunch of other FIXMEs across the style system.

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components/style/gecko/media_queries.rs

@@ -16,7 +16,7 @@ use gecko_bindings::structs::{nsMediaFeature_ValueType, nsMediaFeature_RangeType
 use gecko_bindings::structs::RawGeckoPresContextOwned;
 use media_queries::MediaType;
 use parser::ParserContext;
-use properties::ComputedValues;
+use properties::{ComputedValues, StyleBuilder};
 use std::fmt::{self, Write};
 use std::sync::Arc;
 use str::starts_with_ignore_ascii_case;
@@ -552,6 +552,7 @@ impl Expression {
 
         let default_values = device.default_computed_values();
 
+
         let provider = get_metrics_provider_for_product();
 
         // http://dev.w3.org/csswg/mediaqueries3/#units
@@ -561,10 +562,9 @@ impl Expression {
             device: device,
             inherited_style: default_values,
             layout_parent_style: default_values,
-            // This cloning business is kind of dumb.... It's because Context
-            // insists on having an actual ComputedValues inside itself.
-            style: default_values.clone(),
+            style: StyleBuilder::for_derived_style(default_values),
             font_metrics_provider: &provider,
+            cached_system_font: None,
             in_media_query: true,
             // TODO: pass the correct value here.
             quirks_mode: quirks_mode,

components/style/properties/gecko.mako.rs

@@ -102,32 +102,13 @@ pub struct ComputedValues {
 }
 
 impl ComputedValues {
-    /// Inherits style from the parent element, accounting for the default
-    /// computed values that need to be provided as well.
-    pub fn inherit_from(parent: &Self, default: &Self) -> Self {
-        ComputedValues {
-            custom_properties: parent.custom_properties.clone(),
-            writing_mode: parent.writing_mode,
-            root_font_size: parent.root_font_size,
-            font_size_keyword: parent.font_size_keyword,
-            cached_system_font: None,
-            % for style_struct in data.style_structs:
-            % if style_struct.inherited:
-            ${style_struct.ident}: parent.${style_struct.ident}.clone(),
-            % else:
-            ${style_struct.ident}: default.${style_struct.ident}.clone(),
-            % endif
-            % endfor
-        }
-    }
-
     pub fn new(custom_properties: Option<Arc<ComputedValuesMap>>,
-           writing_mode: WritingMode,
-           root_font_size: Au,
-           font_size_keyword: Option<(longhands::font_size::KeywordSize, f32)>,
-            % for style_struct in data.style_structs:
-           ${style_struct.ident}: Arc<style_structs::${style_struct.name}>,
-            % endfor
+               writing_mode: WritingMode,
+               root_font_size: Au,
+               font_size_keyword: Option<(longhands::font_size::KeywordSize, f32)>,
+               % for style_struct in data.style_structs:
+               ${style_struct.ident}: Arc<style_structs::${style_struct.name}>,
+               % endfor
     ) -> Self {
         ComputedValues {
             custom_properties: custom_properties,
@@ -168,6 +149,11 @@ impl ComputedValues {
     pub fn get_${style_struct.name_lower}(&self) -> &style_structs::${style_struct.name} {
         &self.${style_struct.ident}
     }
+
+    pub fn ${style_struct.name_lower}_arc(&self) -> &Arc<style_structs::${style_struct.name}> {
+        &self.${style_struct.ident}
+    }
+
     #[inline]
     pub fn mutate_${style_struct.name_lower}(&mut self) -> &mut style_structs::${style_struct.name} {
         Arc::make_mut(&mut self.${style_struct.ident})
@@ -175,7 +161,7 @@ impl ComputedValues {
     % endfor
 
     pub fn custom_properties(&self) -> Option<Arc<ComputedValuesMap>> {
-        self.custom_properties.as_ref().map(|x| x.clone())
+        self.custom_properties.clone()
     }
 
     #[allow(non_snake_case)]
@@ -183,12 +169,6 @@ impl ComputedValues {
         !self.get_box().gecko.mBinding.mPtr.mRawPtr.is_null()
     }
 
-    #[allow(non_snake_case)]
-    pub fn in_top_layer(&self) -> bool {
-        matches!(self.get_box().clone__moz_top_layer(),
-                 longhands::_moz_top_layer::SpecifiedValue::top)
-    }
-
     // FIXME(bholley): Implement this properly.
     #[inline]
     pub fn is_multicol(&self) -> bool { false }

components/style/properties/helpers.mako.rs

@@ -478,7 +478,7 @@
                     SpecifiedValue::Keyword(v) => v,
                     SpecifiedValue::System(_) => {
                         % if product == "gecko":
-                            _cx.style.cached_system_font.as_ref().unwrap().${to_rust_ident(name)}
+                            _cx.cached_system_font.as_ref().unwrap().${to_rust_ident(name)}
                         % else:
                             unreachable!()
                         % endif

components/style/properties/longhand/font.mako.rs

@@ -50,7 +50,7 @@
                 SpecifiedValue::Value(v) => v,
                 SpecifiedValue::System(_) => {
                     <%self:nongecko_unreachable>
-                        _context.style.cached_system_font.as_ref().unwrap().${name}
+                        _context.cached_system_font.as_ref().unwrap().${name}
                     </%self:nongecko_unreachable>
                 }
             }
@@ -270,7 +270,7 @@
                 SpecifiedValue::Values(ref v) => computed_value::T(v.clone()),
                 SpecifiedValue::System(_) => {
                     <%self:nongecko_unreachable>
-                        _cx.style.cached_system_font.as_ref().unwrap().font_family.clone()
+                        _cx.cached_system_font.as_ref().unwrap().font_family.clone()
                     </%self:nongecko_unreachable>
                 }
             }
@@ -532,7 +532,7 @@ ${helpers.single_keyword_system("font-variant-caps",
                 },
                 SpecifiedValue::System(_) => {
                     <%self:nongecko_unreachable>
-                        context.style.cached_system_font.as_ref().unwrap().font_weight.clone()
+                        context.cached_system_font.as_ref().unwrap().font_weight.clone()
                     </%self:nongecko_unreachable>
                 }
             }
@@ -809,7 +809,7 @@ ${helpers.single_keyword_system("font-variant-caps",
 
                 SpecifiedValue::System(_) => {
                     <%self:nongecko_unreachable>
-                        context.style.cached_system_font.as_ref().unwrap().font_size
+                        context.cached_system_font.as_ref().unwrap().font_size
                     </%self:nongecko_unreachable>
                 }
             }
@@ -982,7 +982,7 @@ ${helpers.single_keyword_system("font-variant-caps",
                 SpecifiedValue::Number(ref n) => computed_value::T::Number(n.to_computed_value(context)),
                 SpecifiedValue::System(_) => {
                     <%self:nongecko_unreachable>
-                        context.style.cached_system_font.as_ref().unwrap().font_size_adjust
+                        context.cached_system_font.as_ref().unwrap().font_size_adjust
                     </%self:nongecko_unreachable>
                 }
             }
@@ -1966,7 +1966,7 @@ ${helpers.single_keyword_system("font-variant-position",
                 }
                 SpecifiedValue::System(_) => {
                     <%self:nongecko_unreachable>
-                        _context.style.cached_system_font.as_ref().unwrap().font_language_override
+                        _context.cached_system_font.as_ref().unwrap().font_language_override
                     </%self:nongecko_unreachable>
                 }
             }
@@ -2365,11 +2365,11 @@ ${helpers.single_keyword("-moz-math-variant",
         /// Must be called before attempting to compute a system font
         /// specified value
         pub fn resolve_system_font(system: SystemFont, context: &mut Context) {
-            if context.style.cached_system_font.is_none() {
+            if context.cached_system_font.is_none() {
                 let computed = system.to_computed_value(context);
-                context.style.cached_system_font = Some(computed);
+                context.cached_system_font = Some(computed);
             }
-            debug_assert!(system == context.style.cached_system_font.as_ref().unwrap().system_font)
+            debug_assert!(system == context.cached_system_font.as_ref().unwrap().system_font)
         }
 
         #[derive(Clone, Debug)]

components/style/properties/properties.mako.rs

@@ -13,6 +13,7 @@
 use std::borrow::Cow;
 use std::collections::HashSet;
 use std::fmt;
+use std::ops::Deref;
 use std::sync::Arc;
 
 use app_units::Au;
@@ -1725,6 +1726,12 @@ impl ComputedValues {
             &self.${style_struct.ident}
         }
 
+        /// Gets an immutable reference to the refcounted value that wraps
+        /// `${style_struct.name}`.
+        pub fn ${style_struct.name_lower}_arc(&self) -> &Arc<style_structs::${style_struct.name}> {
+            &self.${style_struct.ident}
+        }
+
         /// Get a mutable reference to the ${style_struct.name} struct.
         #[inline]
         pub fn mutate_${style_struct.name_lower}(&mut self) -> &mut style_structs::${style_struct.name} {
@@ -1737,18 +1744,13 @@ impl ComputedValues {
     /// Cloning the Arc here is fine because it only happens in the case where
     /// we have custom properties, and those are both rare and expensive.
     fn custom_properties(&self) -> Option<Arc<::custom_properties::ComputedValuesMap>> {
-        self.custom_properties.as_ref().map(|x| x.clone())
+        self.custom_properties.clone()
     }
 
     /// Whether this style has a -moz-binding value. This is always false for
     /// Servo for obvious reasons.
     pub fn has_moz_binding(&self) -> bool { false }
 
-    /// Whether this style has a top-layer style. That's implemented in Gecko
-    /// via the -moz-top-layer property, but servo doesn't have any concept of a
-    /// top layer (yet, it's needed for fullscreen).
-    pub fn in_top_layer(&self) -> bool { false }
-
     /// Returns whether this style's display value is equal to contents.
     ///
     /// Since this isn't supported in Servo, this is always false for Servo.
@@ -1967,22 +1969,6 @@ impl ComputedValues {
     }
 }
 
-impl ComputedValues {
-    /// Returns whether this computed style represents a floated object.
-    pub fn floated(&self) -> bool {
-        self.get_box().clone_float() != longhands::float::computed_value::T::none
-    }
-
-    /// Returns whether this computed style represents an out of flow-positioned
-    /// object.
-    pub fn out_of_flow_positioned(&self) -> bool {
-        use properties::longhands::position::computed_value::T as position;
-        matches!(self.get_box().clone_position(),
-                 position::absolute | position::fixed)
-    }
-}
-
-
 /// Return a WritingMode bitflags from the relevant CSS properties.
 pub fn get_writing_mode(inheritedbox_style: &style_structs::InheritedBox) -> WritingMode {
     use logical_geometry;
@@ -2034,6 +2020,176 @@ pub fn get_writing_mode(inheritedbox_style: &style_structs::InheritedBox) -> Wri
     flags
 }
 
+/// A reference to a style struct of the parent, or our own style struct.
+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(T),
+}
+
+impl<'a, T: 'a> StyleStructRef<'a, T>
+    where T: Clone,
+{
+    /// Ensure a mutable reference of this value exists, either cloning the
+    /// borrowed value, or returning the owned one.
+    pub fn mutate(&mut self) -> &mut T {
+        if let StyleStructRef::Borrowed(v) = *self {
+            *self = StyleStructRef::Owned((**v).clone());
+        }
+
+        match *self {
+            StyleStructRef::Owned(ref mut v) => v,
+            StyleStructRef::Borrowed(..) => unreachable!(),
+        }
+    }
+
+    /// Returns an `Arc` to the internal struct, constructing one if
+    /// appropriate.
+    pub fn build(self) -> Arc<T> {
+        match self {
+            StyleStructRef::Owned(v) => Arc::new(v),
+            StyleStructRef::Borrowed(v) => v.clone(),
+        }
+    }
+}
+
+impl<'a, T: 'a> Deref for StyleStructRef<'a, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        match *self {
+            StyleStructRef::Owned(ref v) => v,
+            StyleStructRef::Borrowed(v) => &**v,
+        }
+    }
+}
+
+/// A type used to compute a struct with minimal overhead.
+///
+/// This allows holding references to the parent/default computed values without
+/// actually cloning them, until we either build the style, or mutate the
+/// inherited value.
+pub struct StyleBuilder<'a> {
+    custom_properties: Option<Arc<::custom_properties::ComputedValuesMap>>,
+    /// The writing mode flags.
+    ///
+    /// TODO(emilio): Make private.
+    pub writing_mode: WritingMode,
+    /// The font size of the root element.
+    pub root_font_size: Au,
+    /// The keyword behind the current font-size property, if any.
+    pub font_size_keyword: Option<(longhands::font_size::KeywordSize, f32)>,
+    % for style_struct in data.active_style_structs():
+        ${style_struct.ident}: StyleStructRef<'a, style_structs::${style_struct.name}>,
+    % endfor
+}
+
+impl<'a> StyleBuilder<'a> {
+    /// Trivially construct a `StyleBuilder`.
+    pub fn new(
+        custom_properties: Option<Arc<::custom_properties::ComputedValuesMap>>,
+        writing_mode: WritingMode,
+        root_font_size: Au,
+        font_size_keyword: Option<(longhands::font_size::KeywordSize, f32)>,
+        % for style_struct in data.active_style_structs():
+            ${style_struct.ident}: &'a Arc<style_structs::${style_struct.name}>,
+        % endfor
+    ) -> Self {
+        StyleBuilder {
+            custom_properties: custom_properties,
+            writing_mode: writing_mode,
+            root_font_size: root_font_size,
+            font_size_keyword: font_size_keyword,
+        % for style_struct in data.active_style_structs():
+            ${style_struct.ident}: StyleStructRef::Borrowed(${style_struct.ident}),
+        % endfor
+        }
+    }
+
+    /// Creates a StyleBuilder holding only references to the structs of `s`, in
+    /// order to create a derived style.
+    pub fn for_derived_style(s: &'a ComputedValues) -> Self {
+        Self::for_inheritance(s, s)
+    }
+
+    /// Inherits style from the parent element, accounting for the default
+    /// computed values that need to be provided as well.
+    pub fn for_inheritance(parent: &'a ComputedValues, default: &'a ComputedValues) -> Self {
+        Self::new(parent.custom_properties(),
+                  parent.writing_mode,
+                  parent.root_font_size,
+                  parent.font_size_keyword,
+                  % for style_struct in data.active_style_structs():
+                  % if style_struct.inherited:
+                  parent.${style_struct.name_lower}_arc(),
+                  % else:
+                  default.${style_struct.name_lower}_arc(),
+                  % endif
+                  % endfor
+        )
+    }
+
+
+    % for style_struct in data.active_style_structs():
+        /// Gets an immutable view of the current `${style_struct.name}` style.
+        pub fn get_${style_struct.name_lower}(&self) -> &style_structs::${style_struct.name} {
+            &self.${style_struct.ident}
+        }
+
+        /// Gets a mutable view of the current `${style_struct.name}` style.
+        pub fn mutate_${style_struct.name_lower}(&mut self) -> &mut style_structs::${style_struct.name} {
+            self.${style_struct.ident}.mutate()
+        }
+    % endfor
+
+    /// Returns whether this computed style represents a floated object.
+    pub fn floated(&self) -> bool {
+        self.get_box().clone_float() != longhands::float::computed_value::T::none
+    }
+
+    /// Returns whether this computed style represents an out of flow-positioned
+    /// object.
+    pub fn out_of_flow_positioned(&self) -> bool {
+        use properties::longhands::position::computed_value::T as position;
+        matches!(self.get_box().clone_position(),
+                 position::absolute | position::fixed)
+    }
+
+    /// Whether this style has a top-layer style. That's implemented in Gecko
+    /// via the -moz-top-layer property, but servo doesn't have any concept of a
+    /// top layer (yet, it's needed for fullscreen).
+    #[cfg(feature = "servo")]
+    pub fn in_top_layer(&self) -> bool { false }
+
+    /// Whether this style has a top-layer style.
+    #[cfg(feature = "gecko")]
+    pub fn in_top_layer(&self) -> bool {
+        matches!(self.get_box().clone__moz_top_layer(),
+                 longhands::_moz_top_layer::computed_value::T::top)
+    }
+
+
+    /// Turns this `StyleBuilder` into a proper `ComputedValues` instance.
+    pub fn build(self) -> ComputedValues {
+        ComputedValues::new(self.custom_properties,
+                            self.writing_mode,
+                            self.root_font_size,
+                            self.font_size_keyword,
+                            % for style_struct in data.active_style_structs():
+                            self.${style_struct.ident}.build(),
+                            % endfor
+        )
+    }
+
+    /// Get the custom properties map if necessary.
+    ///
+    /// Cloning the Arc here is fine because it only happens in the case where
+    /// we have custom properties, and those are both rare and expensive.
+    fn custom_properties(&self) -> Option<Arc<::custom_properties::ComputedValuesMap>> {
+        self.custom_properties.clone()
+    }
+}
 
 #[cfg(feature = "servo")]
 pub use self::lazy_static_module::INITIAL_SERVO_VALUES;
@@ -2202,28 +2358,28 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
         ::custom_properties::finish_cascade(
             custom_properties, &inherited_custom_properties);
 
-    let starting_style = if !flags.contains(INHERIT_ALL) {
-        ComputedValues::new(custom_properties,
-                            WritingMode::empty(),
-                            inherited_style.root_font_size,
-                            inherited_style.font_size_keyword,
-                            % for style_struct in data.active_style_structs():
-                                % if style_struct.inherited:
-                                    inherited_style.clone_${style_struct.name_lower}(),
-                                % else:
-                                    default_style.clone_${style_struct.name_lower}(),
-                                % endif
-                            % endfor
-                            )
+    let builder = if !flags.contains(INHERIT_ALL) {
+        StyleBuilder::new(custom_properties,
+                          WritingMode::empty(),
+                          inherited_style.root_font_size,
+                          inherited_style.font_size_keyword,
+                          % for style_struct in data.active_style_structs():
+                              % if style_struct.inherited:
+                                  inherited_style.${style_struct.name_lower}_arc(),
+                              % else:
+                                  default_style.${style_struct.name_lower}_arc(),
+                              % endif
+                          % endfor
+                          )
     } else {
-        ComputedValues::new(custom_properties,
-                            WritingMode::empty(),
-                            inherited_style.root_font_size,
-                            inherited_style.font_size_keyword,
-                            % for style_struct in data.active_style_structs():
-                                inherited_style.clone_${style_struct.name_lower}(),
-                            % endfor
-                            )
+        StyleBuilder::new(custom_properties,
+                          WritingMode::empty(),
+                          inherited_style.root_font_size,
+                          inherited_style.font_size_keyword,
+                          % for style_struct in data.active_style_structs():
+                              inherited_style.${style_struct.name_lower}_arc(),
+                          % endfor
+                          )
     };
 
     let mut context = computed::Context {
@@ -2231,8 +2387,9 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
         device: device,
         inherited_style: inherited_style,
         layout_parent_style: layout_parent_style,
-        style: starting_style,
+        style: builder,
         font_metrics_provider: font_metrics_provider,
+        cached_system_font: None,
         in_media_query: false,
         quirks_mode: quirks_mode,
     };
@@ -2382,9 +2539,11 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
 
     let mut style = context.style;
 
-    StyleAdjuster::new(&mut style, is_root_element)
-        .adjust(context.layout_parent_style,
-                flags.contains(SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP));
+    {
+        StyleAdjuster::new(&mut style, is_root_element)
+            .adjust(context.layout_parent_style,
+                    flags.contains(SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP));
+    }
 
     % if product == "gecko":
         // FIXME(emilio): This is effectively creating a new nsStyleBackground
@@ -2408,12 +2567,12 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
         }
     % endif
 
-    style
+    style.build()
 }
 
 
 /// See StyleAdjuster::adjust_for_border_width.
-pub fn adjust_border_width(style: &mut ComputedValues) {
+pub fn adjust_border_width(style: &mut StyleBuilder) {
     % for side in ["top", "right", "bottom", "left"]:
         // Like calling to_computed_value, which wouldn't type check.
         if style.get_border().clone_border_${side}_style().none_or_hidden() &&

components/style/servo/media_queries.rs

@@ -11,7 +11,7 @@ use euclid::{Size2D, TypedSize2D};
 use font_metrics::ServoMetricsProvider;
 use media_queries::MediaType;
 use parser::ParserContext;
-use properties::ComputedValues;
+use properties::{ComputedValues, StyleBuilder};
 use std::fmt;
 use style_traits::{CSSPixel, ToCss};
 use style_traits::viewport::ViewportConstraints;
@@ -185,14 +185,13 @@ impl Range<specified::Length> {
             device: device,
             inherited_style: default_values,
             layout_parent_style: default_values,
-            // This cloning business is kind of dumb.... It's because Context
-            // insists on having an actual ComputedValues inside itself.
-            style: default_values.clone(),
+            style: StyleBuilder::for_derived_style(default_values),
             // Servo doesn't support font metrics
             // A real provider will be needed here once we do; since
             // ch units can exist in media queries.
             font_metrics_provider: &ServoMetricsProvider,
             in_media_query: true,
+            cached_system_font: None,
             quirks_mode: quirks_mode,
         };
 

components/style/style_adjuster.rs

@@ -6,7 +6,7 @@
 //! for it to adhere to the CSS spec.
 
 use app_units::Au;
-use properties::{self, ComputedValues};
+use properties::{self, ComputedValues, StyleBuilder};
 use properties::longhands::display::computed_value::T as display;
 use properties::longhands::float::computed_value::T as float;
 use properties::longhands::overflow_x::computed_value::T as overflow;
@@ -14,14 +14,14 @@ use properties::longhands::position::computed_value::T as position;
 
 
 /// An unsized struct that implements all the adjustment methods.
-pub struct StyleAdjuster<'a> {
-    style: &'a mut ComputedValues,
+pub struct StyleAdjuster<'a, 'b: 'a> {
+    style: &'a mut StyleBuilder<'b>,
     is_root_element: bool,
 }
 
-impl<'a> StyleAdjuster<'a> {
+impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
     /// Trivially constructs a new StyleAdjuster.
-    pub fn new(style: &'a mut ComputedValues, is_root_element: bool) -> Self {
+    pub fn new(style: &'a mut StyleBuilder<'b>, is_root_element: bool) -> Self {
         StyleAdjuster {
             style: style,
             is_root_element: is_root_element,
@@ -264,7 +264,7 @@ impl<'a> StyleAdjuster<'a> {
     ///
     /// When comparing to Gecko, this is similar to the work done by
     /// `nsStyleContext::ApplyStyleFixups`.
-    pub fn adjust(mut self,
+    pub fn adjust(&mut self,
                   layout_parent_style: &ComputedValues,
                   skip_root_and_element_display_fixup: bool) {
         self.adjust_for_top_layer();

components/style/values/computed/mod.rs

@@ -8,7 +8,9 @@ use context::QuirksMode;
 use euclid::size::Size2D;
 use font_metrics::FontMetricsProvider;
 use media_queries::Device;
-use properties::ComputedValues;
+#[cfg(feature = "gecko")]
+use properties;
+use properties::{ComputedValues, StyleBuilder};
 use std::fmt;
 use style_traits::ToCss;
 use super::{CSSFloat, CSSInteger, RGBA};
@@ -53,10 +55,23 @@ pub struct Context<'a> {
     /// isn't `contents`.
     pub layout_parent_style: &'a ComputedValues,
 
-    /// Values access through this need to be in the properties "computed
+    /// Values accessed through this need to be in the properties "computed
     /// early": color, text-decoration, font-size, display, position, float,
     /// border-*-style, outline-style, font-family, writing-mode...
-    pub style: ComputedValues,
+    pub style: StyleBuilder<'a>,
+
+    /// A cached computed system font value, for use by gecko.
+    ///
+    /// See properties/longhands/font.mako.rs
+    #[cfg(feature = "gecko")]
+    pub cached_system_font: Option<properties::longhands::system_font::ComputedSystemFont>,
+
+    /// A dummy option for servo so initializing a computed::Context isn't
+    /// painful.
+    ///
+    /// TODO(emilio): Make constructors for Context, and drop this.
+    #[cfg(feature = "servo")]
+    pub cached_system_font: Option<()>,
 
     /// A font metrics provider, used to access font metrics to implement
     /// font-relative units.
@@ -78,9 +93,9 @@ impl<'a> Context<'a> {
     pub fn inherited_style(&self) -> &ComputedValues { &self.inherited_style }
     /// The current style. Note that only "eager" properties should be accessed
     /// from here, see the comment in the member.
-    pub fn style(&self) -> &ComputedValues { &self.style }
+    pub fn style(&self) -> &StyleBuilder { &self.style }
     /// A mutable reference to the current style.
-    pub fn mutate_style(&mut self) -> &mut ComputedValues { &mut self.style }
+    pub fn mutate_style(&mut self) -> &mut StyleBuilder<'a> { &mut self.style }
 }
 
 /// A trait to represent the conversion between computed and specified values.

components/style/viewport.rs

@@ -17,6 +17,7 @@ use euclid::size::TypedSize2D;
 use font_metrics::get_metrics_provider_for_product;
 use media_queries::Device;
 use parser::{Parse, ParserContext, log_css_error};
+use properties::StyleBuilder;
 use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
 use std::ascii::AsciiExt;
 use std::borrow::Cow;
@@ -678,14 +679,17 @@ impl MaybeNew for ViewportConstraints {
 
         let provider = get_metrics_provider_for_product();
 
+        let default_values = device.default_computed_values();
+
         // TODO(emilio): Stop cloning `ComputedValues` around!
         let context = Context {
             is_root_element: false,
             device: device,
-            inherited_style: device.default_computed_values(),
-            layout_parent_style: device.default_computed_values(),
-            style: device.default_computed_values().clone(),
+            inherited_style: default_values,
+            layout_parent_style: default_values,
+            style: StyleBuilder::for_derived_style(default_values),
             font_metrics_provider: &provider,
+            cached_system_font: None,
             in_media_query: false,
             quirks_mode: quirks_mode,
         };

ports/geckolib/glue.rs

@@ -72,7 +72,7 @@ use style::keyframes::KeyframesStepValue;
 use style::media_queries::{MediaList, parse_media_query_list};
 use style::parallel;
 use style::parser::{LengthParsingMode, ParserContext};
-use style::properties::{CascadeFlags, ComputedValues, Importance, ParsedDeclaration};
+use style::properties::{CascadeFlags, ComputedValues, Importance, ParsedDeclaration, StyleBuilder};
 use style::properties::{PropertyDeclarationBlock, PropertyId};
 use style::properties::SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP;
 use style::properties::animated_properties::{AnimationValue, ComputeDistance, Interpolate, TransitionProperty};
@@ -1002,14 +1002,14 @@ pub extern "C" fn Servo_ComputedValues_Inherit(
     let for_text = target == structs::InheritTarget::Text;
     let style = if let Some(reference) = maybe_arc.as_ref() {
         let mut style =
-            ComputedValues::inherit_from(reference,
-                                         &data.default_computed_values());
+            StyleBuilder::for_inheritance(reference,
+                                          &data.default_computed_values());
         if for_text {
             StyleAdjuster::new(&mut style, /* is_root = */ false)
                 .adjust_for_text();
         }
 
-        Arc::new(style)
+        Arc::new(style.build())
     } else {
         debug_assert!(!for_text);
         data.default_computed_values().clone()
@@ -2002,8 +2002,9 @@ pub extern "C" fn Servo_GetComputedKeyframeValues(keyframes: RawGeckoKeyframeLis
         device: &data.stylist.device,
         inherited_style: parent_style.unwrap_or(default_values),
         layout_parent_style: parent_style.unwrap_or(default_values),
-        style: (**style).clone(),
+        style: StyleBuilder::for_derived_style(&style),
         font_metrics_provider: &metrics,
+        cached_system_font: None,
         in_media_query: false,
         quirks_mode: QuirksMode::NoQuirks,
     };

tests/unit/style/parsing/image.rs

@@ -8,7 +8,7 @@ use std::f32::consts::PI;
 use style::context::QuirksMode;
 use style::font_metrics::ServoMetricsProvider;
 use style::media_queries::{Device, MediaType};
-use style::properties::ComputedValues;
+use style::properties::{ComputedValues, StyleBuilder};
 use style::values::computed;
 use style::values::computed::{Angle, Context, ToComputedValue};
 use style::values::specified;
@@ -49,7 +49,8 @@ fn test_linear_gradient() {
         device: &device,
         inherited_style: initial_style,
         layout_parent_style: initial_style,
-        style: initial_style.clone(),
+        style: StyleBuilder::for_derived_style(&initial_style),
+        cached_system_font: None,
         font_metrics_provider: &ServoMetricsProvider,
         in_media_query: false,
         quirks_mode: QuirksMode::NoQuirks,