Auto merge of #17875 - bzbarsky:first-line-dual-inheritance, r=emilio

Add support for having two separate parent styles.  Fixes gecko bug 1382806.

<!-- Please describe your changes on the following line: -->

This is needed for ::first-line support.  See https://drafts.csswg.org/css-pseudo-4/#first-line-inheritance

This PR doesn't quite implement what the CSS spec draft says right now.  It implements what Gecko does, which is what an earlier draft said.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1382806

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because servo doesn't support ::first-line yet

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11 files changed, 109 insertions, 33 deletions

diff --git a/components/style/animation.rs b/components/style/animation.rs
index d90467c7eea..69b94edb580 100644
--- a/components/style/animation.rs
+++ b/components/style/animation.rs
@@ -499,6 +499,7 @@ fn compute_style_for_animation_step(context: &SharedStyleContext,
                                                iter,
                                                Some(previous_style),
                                                Some(previous_style),
+                                               Some(previous_style),
                                                /* cascade_info = */ None,
                                                /* visited_style = */ None,
                                                font_metrics_provider,
diff --git a/components/style/properties/helpers/animated_properties.mako.rs b/components/style/properties/helpers/animated_properties.mako.rs
index 4e3a79f5e77..17c10809afc 100644
--- a/components/style/properties/helpers/animated_properties.mako.rs
+++ b/components/style/properties/helpers/animated_properties.mako.rs
@@ -605,8 +605,8 @@ impl AnimationValue {
                                 CSSWideKeyword::Unset |
                             % endif
                             CSSWideKeyword::Inherit => {
-                                let inherit_struct = context.inherited_style()
-                                                            .get_${prop.style_struct.name_lower}();
+                                let inherit_struct = context.builder
+                                                            .get_parent_${prop.style_struct.name_lower}();
                                 inherit_struct.clone_${prop.ident}()
                             },
                         };
diff --git a/components/style/properties/longhand/color.mako.rs b/components/style/properties/longhand/color.mako.rs
index d1fb5429261..9ac3bbfcfbc 100644
--- a/components/style/properties/longhand/color.mako.rs
+++ b/components/style/properties/longhand/color.mako.rs
@@ -22,7 +22,7 @@
         #[inline]
         fn to_computed_value(&self, context: &Context) -> computed_value::T {
             self.0.to_computed_value(context)
-                .to_rgba(context.inherited_style().get_color().clone_color())
+                .to_rgba(context.builder.get_parent_color().clone_color())
         }
 
         #[inline]
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index ca8d227537c..f6b78a91749 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -548,9 +548,9 @@ ${helpers.single_keyword_system("font-variant-caps",
                 SpecifiedValue::Normal => computed_value::T::normal(),
                 SpecifiedValue::Bold => computed_value::T::bold(),
                 SpecifiedValue::Bolder =>
-                    context.inherited_style().get_font().clone_font_weight().bolder(),
+                    context.builder.get_parent_font().clone_font_weight().bolder(),
                 SpecifiedValue::Lighter =>
-                    context.inherited_style().get_font().clone_font_weight().lighter(),
+                    context.builder.get_parent_font().clone_font_weight().lighter(),
                 SpecifiedValue::System(_) => {
                     <%self:nongecko_unreachable>
                         context.cached_system_font.as_ref().unwrap().font_weight.clone()
@@ -940,7 +940,7 @@ ${helpers.single_keyword_system("font-variant-caps",
             // recomputed from the base size for the keyword and the relative size.
             //
             // See bug 1355707
-            if let Some((kw, fraction)) = context.builder.inherited_style().font_computation_data.font_size_keyword {
+            if let Some((kw, fraction)) = context.builder.inherited_font_computation_data().font_size_keyword {
                 context.builder.font_size_keyword = Some((kw, fraction * ratio));
             } else {
                 context.builder.font_size_keyword = None;
@@ -956,9 +956,9 @@ ${helpers.single_keyword_system("font-variant-caps",
             // if the language or generic changed, we need to recalculate
             // the font size from the stored font-size origin information.
             if context.builder.get_font().gecko().mLanguage.raw::<nsIAtom>() !=
-               context.builder.inherited_style().get_font().gecko().mLanguage.raw::<nsIAtom>() ||
+               context.builder.get_parent_font().gecko().mLanguage.raw::<nsIAtom>() ||
                context.builder.get_font().gecko().mGenericID !=
-               context.builder.inherited_style().get_font().gecko().mGenericID {
+               context.builder.get_parent_font().gecko().mGenericID {
                 if let Some((kw, ratio)) = context.builder.font_size_keyword {
                     computed = kw.to_computed_value(context).scale_by(ratio);
                 }
@@ -968,8 +968,7 @@ ${helpers.single_keyword_system("font-variant-caps",
         let device = context.builder.device;
         let mut font = context.builder.take_font();
         let parent_unconstrained = {
-            let parent_style = context.builder.inherited_style();
-            let parent_font = parent_style.get_font();
+            let parent_font = context.builder.get_parent_font();
             font.apply_font_size(computed, parent_font, device)
         };
         context.builder.put_font(font);
@@ -997,9 +996,8 @@ ${helpers.single_keyword_system("font-variant-caps",
         let device = context.builder.device;
         let mut font = context.builder.take_font();
         let used_kw = {
-            let parent_style = context.builder.inherited_style();
-            let parent_font = parent_style.get_font();
-            parent_kw = parent_style.font_computation_data.font_size_keyword;
+            let parent_font = context.builder.get_parent_font();
+            parent_kw = context.builder.inherited_font_computation_data().font_size_keyword;
 
             font.inherit_font_size_from(parent_font, kw_inherited_size, device)
         };
@@ -2279,8 +2277,8 @@ https://drafts.csswg.org/css-fonts-4/#low-level-font-variation-settings-control-
 
             let int = match *self {
                 SpecifiedValue::Auto => {
-                    let parent = cx.inherited_style().get_font().clone__moz_script_level() as i32;
-                    let display = cx.inherited_style().get_font().clone__moz_math_display();
+                    let parent = cx.builder.get_parent_font().clone__moz_script_level() as i32;
+                    let display = cx.builder.get_parent_font().clone__moz_math_display();
                     if display == DisplayValue::inline {
                         parent + 1
                     } else {
@@ -2288,7 +2286,7 @@ https://drafts.csswg.org/css-fonts-4/#low-level-font-variation-settings-control-
                     }
                 }
                 SpecifiedValue::Relative(rel) => {
-                    let parent = cx.inherited_style().get_font().clone__moz_script_level();
+                    let parent = cx.builder.get_parent_font().clone__moz_script_level();
                     parent as i32 + rel
                 }
                 SpecifiedValue::Absolute(abs) => abs,
diff --git a/components/style/properties/longhand/inherited_text.mako.rs b/components/style/properties/longhand/inherited_text.mako.rs
index 80f0624d55f..50df62db72a 100644
--- a/components/style/properties/longhand/inherited_text.mako.rs
+++ b/components/style/properties/longhand/inherited_text.mako.rs
@@ -230,8 +230,8 @@ ${helpers.single_keyword("text-align-last",
                         if context.is_root_element {
                             return get_initial_value();
                         }
-                        let parent = context.inherited_style().get_inheritedtext().clone_text_align();
-                        let ltr = context.inherited_style().writing_mode.is_bidi_ltr();
+                        let parent = context.builder.get_parent_inheritedtext().clone_text_align();
+                        let ltr = context.builder.inherited_writing_mode().is_bidi_ltr();
                         match (parent, ltr) {
                             (computed_value::T::start, true) => computed_value::T::left,
                             (computed_value::T::start, false) => computed_value::T::right,
@@ -241,7 +241,7 @@ ${helpers.single_keyword("text-align-last",
                         }
                     }
                     SpecifiedValue::MozCenterOrInherit => {
-                        let parent = context.inherited_style().get_inheritedtext().clone_text_align();
+                        let parent = context.builder.get_parent_inheritedtext().clone_text_align();
                         if parent == computed_value::T::start {
                             computed_value::T::center
                         } else {
@@ -340,7 +340,7 @@ ${helpers.predefined_type("word-spacing",
                 overline: None,
                 line_through: None,
             },
-            _ => context.inherited_style().get_inheritedtext().clone__servo_text_decorations_in_effect()
+            _ => context.builder.get_parent_inheritedtext().clone__servo_text_decorations_in_effect()
         };
 
         result.underline = maybe(context.style().get_text().has_underline()
diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 712b2c644ac..41e9b3e00ac 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -14,6 +14,7 @@ use servo_arc::{Arc, UniqueArc};
 use std::borrow::Cow;
 use std::collections::HashSet;
 use std::{fmt, mem, ops};
+#[cfg(feature = "gecko")] use std::ptr;
 
 use app_units::Au;
 #[cfg(feature = "servo")] use cssparser::RGBA;
@@ -2470,6 +2471,11 @@ pub struct StyleBuilder<'a> {
     /// `parent_style.unwrap_or(device.default_computed_values())`.
     inherited_style: &'a ComputedValues,
 
+    /// The style we're inheriting from for properties that don't inherit from
+    /// ::first-line.  This is the same as inherited_style, unless
+    /// inherited_style is a ::first-line style.
+    inherited_style_ignoring_first_line: &'a ComputedValues,
+
     /// The style we're getting reset structs from.
     reset_style: &'a ComputedValues,
 
@@ -2507,6 +2513,7 @@ impl<'a> StyleBuilder<'a> {
     fn new(
         device: &'a Device,
         parent_style: Option<<&'a ComputedValues>,
+        parent_style_ignoring_first_line: Option<<&'a ComputedValues>,
         pseudo: Option<<&'a PseudoElement>,
         cascade_flags: CascadeFlags,
         rules: Option<StrongRuleNode>,
@@ -2516,8 +2523,19 @@ impl<'a> StyleBuilder<'a> {
         flags: ComputedValueFlags,
         visited_style: Option<Arc<ComputedValues>>,
     ) -> Self {
+        debug_assert_eq!(parent_style.is_some(), parent_style_ignoring_first_line.is_some());
+        #[cfg(feature = "gecko")]
+        debug_assert!(parent_style.is_none() ||
+                      ptr::eq(parent_style.unwrap(),
+                              parent_style_ignoring_first_line.unwrap()) ||
+                      parent_style.unwrap().pseudo() == Some(PseudoElement::FirstLine));
         let reset_style = device.default_computed_values();
         let inherited_style = parent_style.unwrap_or(reset_style);
+        let inherited_style_ignoring_first_line = parent_style_ignoring_first_line.unwrap_or(reset_style);
+        // FIXME(bz): INHERIT_ALL seems like a fundamentally broken idea.  I'm
+        // 99% sure it should give incorrect behavior for table anonymous box
+        // backgrounds, for example.  This code doesn't attempt to make it play
+        // nice with inherited_style_ignoring_first_line.
         let reset_style = if cascade_flags.contains(INHERIT_ALL) {
             inherited_style
         } else {
@@ -2528,6 +2546,7 @@ impl<'a> StyleBuilder<'a> {
             device,
             parent_style,
             inherited_style,
+            inherited_style_ignoring_first_line,
             reset_style,
             pseudo,
             rules,
@@ -2556,10 +2575,15 @@ impl<'a> StyleBuilder<'a> {
     ) -> Self {
         let reset_style = device.default_computed_values();
         let inherited_style = parent_style.unwrap_or(reset_style);
+        #[cfg(feature = "gecko")]
+        debug_assert!(parent_style.is_none() ||
+                      parent_style.unwrap().pseudo() != Some(PseudoElement::FirstLine));
         StyleBuilder {
             device,
             parent_style,
             inherited_style,
+            // None of our callers pass in ::first-line parent styles.
+            inherited_style_ignoring_first_line: inherited_style,
             reset_style,
             pseudo,
             rules: None, // FIXME(emilio): Dubious...
@@ -2581,8 +2605,13 @@ impl<'a> StyleBuilder<'a> {
     /// Inherit `${property.ident}` from our parent style.
     #[allow(non_snake_case)]
     pub fn inherit_${property.ident}(&mut self) {
+        % if property.style_struct.inherited:
         let inherited_struct =
             self.inherited_style.get_${property.style_struct.name_lower}();
+        % else:
+        let inherited_struct =
+            self.inherited_style_ignoring_first_line.get_${property.style_struct.name_lower}();
+        % endif
         self.${property.style_struct.ident}.mutate()
             .copy_${property.ident}_from(
                 inherited_struct,
@@ -2639,6 +2668,7 @@ impl<'a> StyleBuilder<'a> {
         Self::new(
             device,
             Some(parent),
+            Some(parent),
             pseudo,
             CascadeFlags::empty(),
             /* rules = */ None,
@@ -2655,11 +2685,6 @@ impl<'a> StyleBuilder<'a> {
         self.visited_style.is_some()
     }
 
-    /// Returns the style we're inheriting from.
-    pub fn inherited_style(&self) -> &'a ComputedValues {
-        self.inherited_style
-    }
-
     /// Returns the style we're getting reset properties from.
     pub fn default_style(&self) -> &'a ComputedValues {
         self.reset_style
@@ -2752,6 +2777,42 @@ impl<'a> StyleBuilder<'a> {
     fn custom_properties(&self) -> Option<Arc<::custom_properties::CustomPropertiesMap>> {
         self.custom_properties.clone()
     }
+
+    /// Access to various information about our inherited styles.  We don't
+    /// expose an inherited ComputedValues directly, because in the
+    /// ::first-line case some of the inherited information needs to come from
+    /// one ComputedValues instance and some from a different one.
+
+    /// Inherited font bits.
+    pub fn inherited_font_computation_data(&self) -> &FontComputationData {
+        &self.inherited_style.font_computation_data
+    }
+
+    /// Inherited writing-mode.
+    pub fn inherited_writing_mode(&self) -> &WritingMode {
+        &self.inherited_style.writing_mode
+    }
+
+    /// Inherited style flags.
+    pub fn inherited_flags(&self) -> &ComputedValueFlags {
+        &self.inherited_style.flags
+    }
+
+    /// And access to inherited style structs.
+    % for style_struct in data.active_style_structs():
+        /// Gets our inherited `${style_struct.name}`.  We don't name these
+        /// accessors `inherited_${style_struct.name_lower}` because we already
+        /// have things like "box" vs "inherited_box" as struct names.  Do the
+        /// next-best thing and call them `parent_${style_struct.name_lower}`
+        /// instead.
+        pub fn get_parent_${style_struct.name_lower}(&self) -> &style_structs::${style_struct.name} {
+            % if style_struct.inherited:
+            self.inherited_style.get_${style_struct.name_lower}()
+            % else:
+            self.inherited_style_ignoring_first_line.get_${style_struct.name_lower}()
+            % endif
+        }
+    % endfor
 }
 
 #[cfg(feature = "servo")]
@@ -2867,6 +2928,7 @@ pub fn cascade(
     rule_node: &StrongRuleNode,
     guards: &StylesheetGuards,
     parent_style: Option<<&ComputedValues>,
+    parent_style_ignoring_first_line: Option<<&ComputedValues>,
     layout_parent_style: Option<<&ComputedValues>,
     visited_style: Option<Arc<ComputedValues>>,
     cascade_info: Option<<&mut CascadeInfo>,
@@ -2874,6 +2936,12 @@ pub fn cascade(
     flags: CascadeFlags,
     quirks_mode: QuirksMode
 ) -> Arc<ComputedValues> {
+    debug_assert_eq!(parent_style.is_some(), parent_style_ignoring_first_line.is_some());
+    #[cfg(feature = "gecko")]
+    debug_assert!(parent_style.is_none() ||
+                  ptr::eq(parent_style.unwrap(),
+                          parent_style_ignoring_first_line.unwrap()) ||
+                  parent_style.unwrap().pseudo() == Some(PseudoElement::FirstLine));
     let iter_declarations = || {
         rule_node.self_and_ancestors().flat_map(|node| {
             let cascade_level = node.cascade_level();
@@ -2919,6 +2987,7 @@ pub fn cascade(
         rule_node,
         iter_declarations,
         parent_style,
+        parent_style_ignoring_first_line,
         layout_parent_style,
         visited_style,
         cascade_info,
@@ -2937,6 +3006,7 @@ pub fn apply_declarations<'a, F, I>(
     rules: &StrongRuleNode,
     iter_declarations: F,
     parent_style: Option<<&ComputedValues>,
+    parent_style_ignoring_first_line: Option<<&ComputedValues>,
     layout_parent_style: Option<<&ComputedValues>,
     visited_style: Option<Arc<ComputedValues>>,
     mut cascade_info: Option<<&mut CascadeInfo>,
@@ -2949,6 +3019,12 @@ where
     I: Iterator<Item = (&'a PropertyDeclaration, CascadeLevel)>,
 {
     debug_assert!(layout_parent_style.is_none() || parent_style.is_some());
+    debug_assert_eq!(parent_style.is_some(), parent_style_ignoring_first_line.is_some());
+    #[cfg(feature = "gecko")]
+    debug_assert!(parent_style.is_none() ||
+                  ptr::eq(parent_style.unwrap(),
+                          parent_style_ignoring_first_line.unwrap()) ||
+                  parent_style.unwrap().pseudo() == Some(PseudoElement::FirstLine));
     let (inherited_style, layout_parent_style) = match parent_style {
         Some(parent_style) => {
             (parent_style,
@@ -2983,6 +3059,7 @@ where
         builder: StyleBuilder::new(
             device,
             parent_style,
+            parent_style_ignoring_first_line,
             pseudo,
             flags,
             Some(rules.clone()),
diff --git a/components/style/style_adjuster.rs b/components/style/style_adjuster.rs
index 8222634317e..022e5e54264 100644
--- a/components/style/style_adjuster.rs
+++ b/components/style/style_adjuster.rs
@@ -446,7 +446,7 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
         let relevant_link_visited = if flags.contains(IS_LINK) {
             flags.contains(IS_VISITED_LINK)
         } else {
-            self.style.inherited_style().flags.contains(IS_RELEVANT_LINK_VISITED)
+            self.style.inherited_flags().contains(IS_RELEVANT_LINK_VISITED)
         };
 
         if relevant_link_visited {
diff --git a/components/style/style_resolver.rs b/components/style/style_resolver.rs
index 71be0609a6e..cf512c4fbdd 100644
--- a/components/style/style_resolver.rs
+++ b/components/style/style_resolver.rs
@@ -479,6 +479,7 @@ where
                 rules.unwrap_or(self.context.shared.stylist.rule_tree().root()),
                 &self.context.shared.guards,
                 parent_style,
+                parent_style,
                 layout_parent_style,
                 style_if_visited,
                 Some(&mut cascade_info),
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index e5e61752be4..29633ae8509 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -637,6 +637,7 @@ impl Stylist {
                             guards,
                             parent,
                             parent,
+                            parent,
                             None,
                             None,
                             font_metrics,
@@ -753,6 +754,7 @@ impl Stylist {
                                     guards,
                                     Some(inherited_style),
                                     Some(inherited_style),
+                                    Some(inherited_style),
                                     None,
                                     None,
                                     font_metrics,
@@ -778,6 +780,7 @@ impl Stylist {
                                  guards,
                                  Some(parent_style),
                                  Some(parent_style),
+                                 Some(parent_style),
                                  visited_values,
                                  None,
                                  font_metrics,
@@ -1342,6 +1345,7 @@ impl Stylist {
                             guards,
                             Some(parent_style),
                             Some(parent_style),
+                            Some(parent_style),
                             None,
                             None,
                             &metrics,
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index 6a82b8cf8bb..8e01eda014c 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -119,11 +119,6 @@ impl<'a> Context<'a> {
         self.builder.device.au_viewport_size()
     }
 
-    /// The style we're inheriting from.
-    pub fn inherited_style(&self) -> &ComputedValues {
-        self.builder.inherited_style()
-    }
-
     /// The default computed style we're getting our reset style from.
     pub fn default_style(&self) -> &ComputedValues {
         self.builder.default_style()
@@ -411,7 +406,7 @@ impl ToComputedValue for specified::JustifyItems {
         // If the inherited value of `justify-items` includes the `legacy` keyword, `auto` computes
         // to the inherited value.
         if self.0 == align::ALIGN_AUTO {
-            let inherited = context.inherited_style().get_position().clone_justify_items();
+            let inherited = context.builder.get_parent_position().clone_justify_items();
             if inherited.0.contains(align::ALIGN_LEGACY) {
                 return inherited
             }
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index c3591bdf484..bf4b83b5293 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -93,7 +93,7 @@ impl FontBaseSize {
         match *self {
             FontBaseSize::Custom(size) => size,
             FontBaseSize::CurrentStyle => context.style().get_font().clone_font_size(),
-            FontBaseSize::InheritedStyle => context.inherited_style().get_font().clone_font_size(),
+            FontBaseSize::InheritedStyle => context.style().get_parent_font().clone_font_size(),
         }
     }
 }