Auto merge of #18499 - heycam:rule-cache, r=emilio

Rule cache

<!-- Please describe your changes on the following line: -->
This adds a TLS-based cache reset styles structs keyed off rule nodes.  Reviewed in https://bugzilla.mozilla.org/show_bug.cgi?id=1367635 by me and Emilio.

---
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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/style/values/computed/length.rs

@@ -866,6 +866,10 @@ impl ToComputedValue for specified::MozLength {
                 MozLength::LengthOrPercentageOrAuto(lopoa.to_computed_value(context))
             }
             specified::MozLength::ExtremumLength(ref ext) => {
+                debug_assert!(context.for_non_inherited_property.is_some(),
+                              "should check whether we're a non-inherited property");
+                context.rule_cache_conditions.borrow_mut()
+                    .set_writing_mode_dependency(context.builder.writing_mode);
                 MozLength::ExtremumLength(ext.clone())
             }
         }

components/style/values/computed/mod.rs

@@ -11,10 +11,12 @@ use font_metrics::FontMetricsProvider;
 use media_queries::Device;
 #[cfg(feature = "gecko")]
 use properties;
-use properties::{ComputedValues, StyleBuilder};
+use properties::{ComputedValues, LonghandId, StyleBuilder};
+use rule_cache::RuleCacheConditions;
 #[cfg(feature = "servo")]
 use servo_url::ServoUrl;
 use std::{f32, fmt};
+use std::cell::RefCell;
 #[cfg(feature = "servo")]
 use std::sync::Arc;
 use style_traits::ToCss;
@@ -116,6 +118,17 @@ pub struct Context<'a> {
     /// This is used to allow certain properties to generate out-of-range
     /// values, which SMIL allows.
     pub for_smil_animation: bool,
+
+    /// The property we are computing a value for, if it is a non-inherited
+    /// property.  None if we are computed a value for an inherited property
+    /// or not computing for a property at all (e.g. in a media query
+    /// evaluation).
+    pub for_non_inherited_property: Option<LonghandId>,
+
+    /// The conditions to cache a rule node on the rule cache.
+    ///
+    /// FIXME(emilio): Drop the refcell.
+    pub rule_cache_conditions: RefCell<&'a mut RuleCacheConditions>,
 }
 
 impl<'a> Context<'a> {

components/style/values/specified/color.rs

@@ -249,18 +249,36 @@ fn convert_nscolor_to_computedcolor(color: nscolor) -> ComputedColor {
 impl ToComputedValue for Color {
     type ComputedValue = ComputedColor;
 
-    fn to_computed_value(&self, _context: &Context) -> ComputedColor {
+    fn to_computed_value(&self, context: &Context) -> ComputedColor {
         match *self {
-            Color::CurrentColor => ComputedColor::currentcolor(),
+            Color::CurrentColor => {
+                if let Some(longhand) = context.for_non_inherited_property {
+                    if longhand.stores_complex_colors_lossily() {
+                        context.rule_cache_conditions.borrow_mut()
+                            .set_uncacheable();
+                    }
+                }
+                ComputedColor::currentcolor()
+            }
             Color::Numeric { ref parsed, .. } => ComputedColor::rgba(*parsed),
-            Color::Complex(ref complex) => *complex,
+            Color::Complex(ref complex) => {
+                if complex.foreground_ratio != 0 {
+                    if let Some(longhand) = context.for_non_inherited_property {
+                        if longhand.stores_complex_colors_lossily() {
+                            context.rule_cache_conditions.borrow_mut()
+                                .set_uncacheable();
+                        }
+                    }
+                }
+                *complex
+            }
             #[cfg(feature = "gecko")]
             Color::System(system) =>
-                convert_nscolor_to_computedcolor(system.to_computed_value(_context)),
+                convert_nscolor_to_computedcolor(system.to_computed_value(context)),
             #[cfg(feature = "gecko")]
             Color::Special(special) => {
                 use self::gecko::SpecialColorKeyword as Keyword;
-                let pres_context = _context.device().pres_context();
+                let pres_context = context.device().pres_context();
                 convert_nscolor_to_computedcolor(match special {
                     Keyword::MozDefaultColor => pres_context.mDefaultColor,
                     Keyword::MozDefaultBackgroundColor => pres_context.mBackgroundColor,
@@ -274,7 +292,7 @@ impl ToComputedValue for Color {
                 use dom::TElement;
                 use gecko::wrapper::GeckoElement;
                 use gecko_bindings::bindings::Gecko_GetBody;
-                let pres_context = _context.device().pres_context();
+                let pres_context = context.device().pres_context();
                 let body = unsafe { Gecko_GetBody(pres_context) }.map(GeckoElement);
                 let data = body.as_ref().and_then(|wrap| wrap.borrow_data());
                 if let Some(data) = data {

components/style/values/specified/length.rs

@@ -132,9 +132,18 @@ impl FontRelativeLength {
 
         match *self {
             FontRelativeLength::Em(length) => {
+                if !matches!(base_size, FontBaseSize::InheritedStyle) {
+                    context.rule_cache_conditions.borrow_mut()
+                        .set_font_size_dependency(
+                            reference_font_size.into()
+                        );
+                }
                 (reference_font_size, length)
             },
             FontRelativeLength::Ex(length) => {
+                if context.for_non_inherited_property.is_some() {
+                    context.rule_cache_conditions.borrow_mut().set_uncacheable();
+                }
                 let reference_size = match query_font_metrics(context, reference_font_size) {
                     FontMetricsQueryResult::Available(metrics) => {
                         metrics.x_height
@@ -152,6 +161,9 @@ impl FontRelativeLength {
                 (reference_size, length)
             },
             FontRelativeLength::Ch(length) => {
+                if context.for_non_inherited_property.is_some() {
+                    context.rule_cache_conditions.borrow_mut().set_uncacheable();
+                }
                 let reference_size = match query_font_metrics(context, reference_font_size) {
                     FontMetricsQueryResult::Available(metrics) => {
                         metrics.zero_advance_measure