Bug 1681691 - Add ascent-, descent- and line-gap-override descriptors…

… to the style system. r=emilio

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

layout/style/FontFace.cpp

@@ -635,6 +635,30 @@ Maybe<StyleFontLanguageOverride> FontFace::GetFontLanguageOverride() const {
   return Some(langOverride);
 }
 
+Maybe<StylePercentage> FontFace::GetAscentOverride() const {
+  StylePercentage ascent{0};
+  if (!Servo_FontFaceRule_GetAscentOverride(GetData(), &ascent)) {
+    return Nothing();
+  }
+  return Some(ascent);
+}
+
+Maybe<StylePercentage> FontFace::GetDescentOverride() const {
+  StylePercentage descent{0};
+  if (!Servo_FontFaceRule_GetDescentOverride(GetData(), &descent)) {
+    return Nothing();
+  }
+  return Some(descent);
+}
+
+Maybe<StylePercentage> FontFace::GetLineGapOverride() const {
+  StylePercentage lineGap{0};
+  if (!Servo_FontFaceRule_GetLineGapOverride(GetData(), &lineGap)) {
+    return Nothing();
+  }
+  return Some(lineGap);
+}
+
 bool FontFace::HasLocalSrc() const {
   AutoTArray<StyleFontFaceSourceListComponent, 8> components;
   GetSources(components);

layout/style/FontFace.h

@@ -91,6 +91,9 @@ class FontFace final : public nsISupports, public nsWrapperCache {
   void GetFontVariationSettings(nsTArray<gfxFontVariation>&) const;
   void GetSources(nsTArray<StyleFontFaceSourceListComponent>&) const;
   Maybe<StyleFontLanguageOverride> GetFontLanguageOverride() const;
+  Maybe<StylePercentage> GetAscentOverride() const;
+  Maybe<StylePercentage> GetDescentOverride() const;
+  Maybe<StylePercentage> GetLineGapOverride() const;
 
   gfxUserFontEntry* CreateUserFontEntry();
   gfxUserFontEntry* GetUserFontEntry() const { return mUserFontEntry; }

layout/style/nsCSSFontDescList.h

@@ -14,3 +14,6 @@ CSS_FONT_DESC(font-feature-settings, FontFeatureSettings)
 CSS_FONT_DESC(font-variation-settings, FontVariationSettings)
 CSS_FONT_DESC(font-language-override, FontLanguageOverride)
 CSS_FONT_DESC(font-display, Display)
+CSS_FONT_DESC(ascent-override, AscentOverride)
+CSS_FONT_DESC(descent-override, DescentOverride)
+CSS_FONT_DESC(line-gap-override, LineGapOverride)

modules/libpref/init/StaticPrefList.yaml

@@ -6396,6 +6396,13 @@
   value: true
   mirror: always
 
+# Is support for the @font-face metrics override descriptors enabled?
+- name: layout.css.font-metrics-overrides.enabled
+  type: RelaxedAtomicBool
+  value: @IS_NIGHTLY_BUILD@
+  mirror: always
+  rust: true
+
 # Is support for variation fonts enabled?
 - name: layout.css.font-variations.enabled
   type: RelaxedAtomicBool

servo/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")]
@@ -419,6 +419,15 @@ 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
     };
@@ -577,6 +586,15 @@ 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,
     ]
 }
 

servo/components/style/values/specified/font.rs

@@ -2193,6 +2193,37 @@ 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,

servo/ports/geckolib/glue.rs

@@ -2865,6 +2865,9 @@ macro_rules! apply_font_desc_list {
                 eCSSFontDesc_FontVariationSettings => variation_settings,
                 eCSSFontDesc_FontLanguageOverride => language_override,
                 eCSSFontDesc_Display => display,
+                eCSSFontDesc_AscentOverride => ascent_override,
+                eCSSFontDesc_DescentOverride => descent_override,
+                eCSSFontDesc_LineGapOverride => line_gap_override,
             ]
             invalid: [
                 eCSSFontDesc_UNKNOWN,
@@ -2980,6 +2983,36 @@ pub unsafe extern "C" fn Servo_FontFaceRule_GetFontLanguageOverride(
     simple_font_descriptor_getter_impl!(rule, out, language_override, compute_non_system)
 }
 
+// Returns a Percentage of -1.0 if the override descriptor is present but 'normal'
+// rather than an actual percentage value.
+#[no_mangle]
+pub extern "C" fn Servo_FontFaceRule_GetAscentOverride(
+    rule: &RawServoFontFaceRule,
+    out: &mut computed::Percentage,
+) -> bool {
+    simple_font_descriptor_getter_impl!(rule, out, ascent_override, compute)
+}
+
+// Returns a Percentage of -1.0 if the override descriptor is present but 'normal'
+// rather than an actual percentage value.
+#[no_mangle]
+pub extern "C" fn Servo_FontFaceRule_GetDescentOverride(
+    rule: &RawServoFontFaceRule,
+    out: &mut computed::Percentage,
+) -> bool {
+    simple_font_descriptor_getter_impl!(rule, out, descent_override, compute)
+}
+
+// Returns a Percentage of -1.0 if the override descriptor is present but 'normal'
+// rather than an actual percentage value.
+#[no_mangle]
+pub extern "C" fn Servo_FontFaceRule_GetLineGapOverride(
+    rule: &RawServoFontFaceRule,
+    out: &mut computed::Percentage,
+) -> bool {
+    simple_font_descriptor_getter_impl!(rule, out, line_gap_override, compute)
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn Servo_FontFaceRule_GetFamilyName(
     rule: &RawServoFontFaceRule,