style: Add ascent-, descent- and line-gap-override descriptors to the style system

Differential Revision: https://phabricator.services.mozilla.com/D109287

components/style/font_face.rs

@@ -19,7 +19,7 @@ use crate::values::specified::font::SpecifiedFontFeatureSettings;
 use crate::values::specified::font::SpecifiedFontStyle;
 #[cfg(feature = "gecko")]
 use crate::values::specified::font::SpecifiedFontVariationSettings;
-use crate::values::specified::font::{AbsoluteFontWeight, FontStretch};
+use crate::values::specified::font::{AbsoluteFontWeight, FontStretch, MetricsOverride};
 use crate::values::specified::url::SpecifiedUrl;
 use crate::values::specified::Angle;
 #[cfg(feature = "gecko")]
@@ -418,6 +418,15 @@ macro_rules! is_descriptor_enabled {
     ("font-variation-settings") => {
         static_prefs::pref!("layout.css.font-variations.enabled")
     };
+    ("ascent-override") => {
+        static_prefs::pref!("layout.css.font-metrics-overrides.enabled")
+    };
+    ("descent-override") => {
+        static_prefs::pref!("layout.css.font-metrics-overrides.enabled")
+    };
+    ("line-gap-override") => {
+        static_prefs::pref!("layout.css.font-metrics-overrides.enabled")
+    };
     ($name:tt) => {
         true
     };
@@ -576,6 +585,15 @@ font_face_descriptors! {
 
         /// The language override of this font face.
         "font-language-override" language_override / mFontLanguageOverride: font_language_override::SpecifiedValue,
+
+        /// The ascent override for this font face.
+        "ascent-override" ascent_override / mAscentOverride: MetricsOverride,
+
+        /// The descent override for this font face.
+        "descent-override" descent_override / mDescentOverride: MetricsOverride,
+
+        /// The line-gap override for this font face.
+        "line-gap-override" line_gap_override / mLineGapOverride: MetricsOverride,
     ]
 }
 

components/style/values/specified/font.rs

@@ -2205,6 +2205,37 @@ impl Parse for VariationValue<Number> {
     }
 }
 
+/// A metrics override value for a @font-face descriptor
+///
+/// https://drafts.csswg.org/css-fonts/#font-metrics-override-desc
+#[derive(Clone, Copy, Debug, MallocSizeOf, Parse, PartialEq, SpecifiedValueInfo, ToCss, ToShmem)]
+pub enum MetricsOverride {
+    /// A non-negative `<percentage>` of the computed font size
+    Override(NonNegativePercentage),
+    /// Normal metrics from the font.
+    Normal,
+}
+
+impl MetricsOverride {
+    #[inline]
+    /// Get default value with `normal`
+    pub fn normal() -> MetricsOverride {
+        MetricsOverride::Normal
+    }
+
+    /// The ToComputedValue implementation, used for @font-face descriptors.
+    ///
+    /// Valid override percentages must be non-negative; we return -1.0 to indicate
+    /// the absence of an override (i.e. 'normal').
+    #[inline]
+    pub fn compute(&self) -> ComputedPercentage {
+        match *self {
+            MetricsOverride::Normal => ComputedPercentage(-1.0),
+            MetricsOverride::Override(percent) => ComputedPercentage(percent.0.get()),
+        }
+    }
+}
+
 #[derive(
     Clone,
     Copy,