style: Refactor and add infrastructure for font metrics in style.

This commit itself only moves things around and adds an extra parameter to the
`apply_declarations` function to eventually handle #14079 correctly.

Probably needs a more granular API to query fonts, á la nsFontMetrics, but
that's trivial to do once this is landed.

Then we should make the font provider mandatory, and implement the missing stylo
bits.

components/style/animation.rs

@@ -403,6 +403,7 @@ fn compute_style_for_animation_step(context: &SharedStyleContext,
                                                previous_style,
                                                /* cascade_info = */ None,
                                                context.error_reporter.clone(),
+                                               /* Metrics provider */ None,
                                                CascadeFlags::empty());
             computed
         }

components/style/font_metrics.rs

@@ -0,0 +1,35 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use Atom;
+use app_units::Au;
+use euclid::Size2D;
+use std::fmt;
+
+/// Represents the font metrics that style needs from a font to compute the
+/// value of certain CSS units like `ex`.
+#[derive(Debug, PartialEq, Clone)]
+pub struct FontMetrics {
+    pub x_height: Au,
+    pub zero_advance_measure: Size2D<Au>,
+}
+
+#[derive(Debug, PartialEq, Clone)]
+pub enum FontMetricsQueryResult {
+    Available(Option<FontMetrics>),
+    NotAvailable,
+}
+
+/// A trait used to represent something capable of providing us font metrics.
+pub trait FontMetricsProvider: Send + Sync + fmt::Debug {
+    /// Obtain the metrics for given font family.
+    ///
+    /// TODO: We could make this take the full list, I guess, and save a few
+    /// virtual calls.
+    ///
+    /// This is not too common in practice though.
+    fn query(&self, _font_name: &Atom) -> FontMetricsQueryResult {
+        FontMetricsQueryResult::NotAvailable
+    }
+}

components/style/gecko/conversions.rs

@@ -38,7 +38,7 @@ impl From<CalcLengthOrPercentage> for nsStyleCoord_CalcValue {
     fn from(other: CalcLengthOrPercentage) -> nsStyleCoord_CalcValue {
         let has_percentage = other.percentage.is_some();
         nsStyleCoord_CalcValue {
-            mLength: other.length.map_or(0, |l| l.0),
+            mLength: other.length.0,
             mPercent: other.percentage.unwrap_or(0.0),
             mHasPercent: has_percentage,
         }
@@ -53,7 +53,7 @@ impl From<nsStyleCoord_CalcValue> for CalcLengthOrPercentage {
             None
         };
         CalcLengthOrPercentage {
-            length: Some(Au(other.mLength)),
+            length: Au(other.mLength),
             percentage: percentage,
         }
     }

components/style/lib.rs

@@ -103,6 +103,7 @@ pub mod dom;
 pub mod element_state;
 pub mod error_reporting;
 pub mod font_face;
+pub mod font_metrics;
 #[cfg(feature = "gecko")] #[allow(unsafe_code)] pub mod gecko;
 #[cfg(feature = "gecko")] #[allow(unsafe_code)] pub mod gecko_bindings;
 pub mod keyframes;

components/style/media_queries.rs

@@ -10,10 +10,10 @@ use Atom;
 use app_units::Au;
 use cssparser::{Delimiter, Parser, Token};
 use euclid::size::{Size2D, TypedSize2D};
-use properties::longhands;
 use serialize_comma_separated_list;
 use std::fmt::{self, Write};
 use style_traits::{ToCss, ViewportPx};
+use values::computed::{self, ToComputedValue};
 use values::specified;
 
 
@@ -49,28 +49,11 @@ impl Range<specified::Length> {
     fn to_computed_range(&self, viewport_size: Size2D<Au>) -> Range<Au> {
         // http://dev.w3.org/csswg/mediaqueries3/#units
         // em units are relative to the initial font-size.
-        let initial_font_size = longhands::font_size::get_initial_value();
-        let compute_width = |&width| {
-            match width {
-                specified::Length::Absolute(value) => value,
-                specified::Length::FontRelative(value)
-                    => value.to_computed_value(initial_font_size, initial_font_size),
-                specified::Length::ViewportPercentage(value)
-                    => value.to_computed_value(viewport_size),
-                specified::Length::Calc(val, range)
-                    => range.clamp(
-                        val.compute_from_viewport_and_font_size(viewport_size,
-                                                                initial_font_size,
-                                                                initial_font_size)
-                           .length()),
-                specified::Length::ServoCharacterWidth(..)
-                    => unreachable!(),
-            }
-        };
+        let context = computed::Context::initial(viewport_size, false);
 
         match *self {
-            Range::Min(ref width) => Range::Min(compute_width(width)),
-            Range::Max(ref width) => Range::Max(compute_width(width)),
+            Range::Min(ref width) => Range::Min(width.to_computed_value(&context)),
+            Range::Max(ref width) => Range::Max(width.to_computed_value(&context)),
             //Range::Eq(ref width) => Range::Eq(compute_width(width))
         }
     }

components/style/properties/gecko.mako.rs

@@ -916,6 +916,14 @@ fn static_assert() {
         }
     }
 
+    pub fn font_family_count(&self) -> usize {
+        0
+    }
+
+    pub fn font_family_at(&self, _: usize) -> longhands::font_family::computed_value::FontFamily {
+        unimplemented!()
+    }
+
     pub fn copy_font_family_from(&mut self, other: &Self) {
         unsafe { Gecko_CopyFontFamilyFrom(&mut self.gecko.mFont, &other.gecko.mFont); }
     }

components/style/properties/helpers/animated_properties.mako.rs

@@ -418,7 +418,7 @@ impl Interpolate for CalcLengthOrPercentage {
         }
 
         Ok(CalcLengthOrPercentage {
-            length: try!(interpolate_half(self.length, other.length, progress)),
+            length: try!(self.length.interpolate(&other.length, progress)),
             percentage: try!(interpolate_half(self.percentage, other.percentage, progress)),
         })
     }

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

@@ -8,7 +8,7 @@
 <% data.new_style_struct("Font",
                          inherited=True,
                          additional_methods=[Method("compute_font_hash", is_mut=True)]) %>
-<%helpers:longhand name="font-family" animatable="False">
+<%helpers:longhand name="font-family" animatable="False" need_index="True">
     use self::computed_value::FontFamily;
     use values::NoViewportPercentage;
     use values::computed::ComputedValueAsSpecified;
@@ -21,15 +21,16 @@
         use std::fmt;
         use Atom;
         use style_traits::ToCss;
+        pub use self::FontFamily as SingleComputedValue;
 
         #[derive(Debug, PartialEq, Eq, Clone, Hash)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf, Deserialize, Serialize))]
         pub enum FontFamily {
             FamilyName(Atom),
             Generic(Atom),
         }
-        impl FontFamily {
 
+        impl FontFamily {
             #[inline]
             pub fn atom(&self) -> &Atom {
                 match *self {
@@ -67,11 +68,13 @@
                 FontFamily::FamilyName(input)
             }
         }
+
         impl ToCss for FontFamily {
             fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
                 self.atom().with_str(|s| dest.write_str(s))
             }
         }
+
         impl ToCss for T {
             fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
                 let mut iter = self.0.iter();
@@ -83,6 +86,7 @@
                 Ok(())
             }
         }
+
         #[derive(Debug, Clone, PartialEq, Eq, Hash)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         pub struct T(pub Vec<FontFamily>);
@@ -307,8 +311,7 @@ ${helpers.single_keyword("font-variant",
         fn to_computed_value(&self, context: &Context) -> computed_value::T {
             match self.0 {
                 LengthOrPercentage::Length(Length::FontRelative(value)) => {
-                    value.to_computed_value(context.inherited_style().get_font().clone_font_size(),
-                                            context.style().root_font_size())
+                    value.to_computed_value(context, /* use inherited */ true)
                 }
                 LengthOrPercentage::Length(Length::ServoCharacterWidth(value)) => {
                     value.to_computed_value(context.inherited_style().get_font().clone_font_size())

components/style/properties/properties.mako.rs

@@ -25,6 +25,7 @@ use url::Url;
 #[cfg(feature = "servo")] use euclid::side_offsets::SideOffsets2D;
 use euclid::size::Size2D;
 use computed_values;
+use font_metrics::FontMetricsProvider;
 #[cfg(feature = "servo")] use logical_geometry::{LogicalMargin, PhysicalSide};
 use logical_geometry::WritingMode;
 use parser::{Parse, ParserContext, ParserContextExtraData};
@@ -1464,6 +1465,7 @@ pub fn cascade(viewport_size: Size2D<Au>,
                        inherited_style,
                        cascade_info,
                        error_reporter,
+                       None,
                        flags)
 }
 
@@ -1475,6 +1477,7 @@ pub fn apply_declarations<'a, F, I>(viewport_size: Size2D<Au>,
                                     inherited_style: &ComputedValues,
                                     mut cascade_info: Option<<&mut CascadeInfo>,
                                     mut error_reporter: StdBox<ParseErrorReporter + Send>,
+                                    font_metrics_provider: Option<<&FontMetricsProvider>,
                                     flags: CascadeFlags)
                                     -> ComputedValues
     where F: Fn() -> I, I: Iterator<Item = &'a PropertyDeclaration>
@@ -1528,6 +1531,7 @@ pub fn apply_declarations<'a, F, I>(viewport_size: Size2D<Au>,
         viewport_size: viewport_size,
         inherited_style: inherited_style,
         style: starting_style,
+        font_metrics_provider: font_metrics_provider,
     };
 
     // Set computed values, overwriting earlier declarations for the same
@@ -1562,6 +1566,7 @@ pub fn apply_declarations<'a, F, I>(viewport_size: Size2D<Au>,
                 // classification is correct.
                 let is_early_property = matches!(*declaration,
                     PropertyDeclaration::FontSize(_) |
+                    PropertyDeclaration::FontFamily(_) |
                     PropertyDeclaration::Color(_) |
                     PropertyDeclaration::Position(_) |
                     PropertyDeclaration::Float(_) |

components/style/values/computed/length.rs

@@ -17,14 +17,14 @@ pub use values::specified::{Angle, BorderStyle, Time, UrlOrNone};
 #[derive(Clone, PartialEq, Copy, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct CalcLengthOrPercentage {
-    pub length: Option<Au>,
+    pub length: Au,
     pub percentage: Option<CSSFloat>,
 }
 
 impl CalcLengthOrPercentage {
     #[inline]
     pub fn length(&self) -> Au {
-        self.length.unwrap_or(Au(0))
+        self.length
     }
 
     #[inline]
@@ -38,13 +38,13 @@ impl From<LengthOrPercentage> for CalcLengthOrPercentage {
         match len {
             LengthOrPercentage::Percentage(this) => {
                 CalcLengthOrPercentage {
-                    length: None,
+                    length: Au(0),
                     percentage: Some(this),
                 }
             }
             LengthOrPercentage::Length(this) => {
                 CalcLengthOrPercentage {
-                    length: Some(this),
+                    length: this,
                     percentage: None,
                 }
             }
@@ -60,13 +60,13 @@ impl From<LengthOrPercentageOrAuto> for Option<CalcLengthOrPercentage> {
         match len {
             LengthOrPercentageOrAuto::Percentage(this) => {
                 Some(CalcLengthOrPercentage {
-                    length: None,
+                    length: Au(0),
                     percentage: Some(this),
                 })
             }
             LengthOrPercentageOrAuto::Length(this) => {
                 Some(CalcLengthOrPercentage {
-                    length: Some(this),
+                    length: this,
                     percentage: None,
                 })
             }
@@ -83,10 +83,9 @@ impl From<LengthOrPercentageOrAuto> for Option<CalcLengthOrPercentage> {
 impl ToCss for CalcLengthOrPercentage {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         match (self.length, self.percentage) {
-            (None, Some(p)) => write!(dest, "{}%", p * 100.),
-            (Some(l), None) => write!(dest, "{}px", Au::to_px(l)),
-            (Some(l), Some(p)) => write!(dest, "calc({}px + {}%)", Au::to_px(l), p * 100.),
-            _ => unreachable!()
+            (l, Some(p)) if l == Au(0) => write!(dest, "{}%", p * 100.),
+            (l, Some(p)) => write!(dest, "calc({}px + {}%)", Au::to_px(l), p * 100.),
+            (l, None) => write!(dest, "{}px", Au::to_px(l)),
         }
     }
 }
@@ -95,16 +94,34 @@ impl ToComputedValue for specified::CalcLengthOrPercentage {
     type ComputedValue = CalcLengthOrPercentage;
 
     fn to_computed_value(&self, context: &Context) -> CalcLengthOrPercentage {
-        self.compute_from_viewport_and_font_size(context.viewport_size(),
-                                                 context.style().get_font().clone_font_size(),
-                                                 context.style().root_font_size())
+        let mut length = Au(0);
 
+        if let Some(absolute) = self.absolute {
+            length += absolute;
+        }
+
+        for val in &[self.vw, self.vh, self.vmin, self.vmax] {
+            if let Some(val) = *val {
+                length += val.to_computed_value(context.viewport_size());
+            }
+        }
+
+        for val in &[self.ch, self.em, self.ex, self.rem] {
+            if let Some(val) = *val {
+                length += val.to_computed_value(context, /* use inherited */ false);
+            }
+        }
+
+        CalcLengthOrPercentage {
+            length: length,
+            percentage: self.percentage.map(|p| p.0),
+        }
     }
 
     #[inline]
     fn from_computed_value(computed: &CalcLengthOrPercentage) -> Self {
         specified::CalcLengthOrPercentage {
-            absolute: computed.length,
+            absolute: Some(computed.length),
             percentage: computed.percentage.map(specified::Percentage),
             ..Default::default()
         }

components/style/values/computed/mod.rs

@@ -4,6 +4,7 @@
 
 use app_units::Au;
 use euclid::size::Size2D;
+use font_metrics::FontMetricsProvider;
 use properties::ComputedValues;
 use std::fmt;
 use style_traits::ToCss;
@@ -28,9 +29,11 @@ pub struct Context<'a> {
     pub viewport_size: Size2D<Au>,
     pub inherited_style: &'a ComputedValues,
 
-    /// Values access through this need to be in the properties "computed early":
-    /// color, text-decoration, font-size, display, position, float, border-*-style, outline-style
+    /// Values access 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 font_metrics_provider: Option<&'a FontMetricsProvider>,
 }
 
 impl<'a> Context<'a> {
@@ -39,6 +42,20 @@ impl<'a> Context<'a> {
     pub fn inherited_style(&self) -> &ComputedValues { &self.inherited_style }
     pub fn style(&self) -> &ComputedValues { &self.style }
     pub fn mutate_style(&mut self) -> &mut ComputedValues { &mut self.style }
+
+    /// Creates a dummy computed context for use in multiple places, like
+    /// evaluating media queries.
+    pub fn initial(viewport_size: Size2D<Au>, is_root_element: bool) -> Self {
+        let initial_style = ComputedValues::initial_values();
+        // FIXME: Enforce a font metrics provider.
+        Context {
+            is_root_element: is_root_element,
+            viewport_size: viewport_size,
+            inherited_style: initial_style,
+            style: initial_style.clone(),
+            font_metrics_provider: None,
+        }
+    }
 }
 
 pub trait ToComputedValue {
@@ -99,8 +116,7 @@ impl ToComputedValue for specified::Length {
             specified::Length::Absolute(length) => length,
             specified::Length::Calc(calc, range) => range.clamp(calc.to_computed_value(context).length()),
             specified::Length::FontRelative(length) =>
-                length.to_computed_value(context.style().get_font().clone_font_size(),
-                                         context.style().root_font_size()),
+                length.to_computed_value(context, /* use inherited */ false),
             specified::Length::ViewportPercentage(length) =>
                 length.to_computed_value(context.viewport_size()),
             specified::Length::ServoCharacterWidth(length) =>