style: Allow FontMetricsProvider to produce ex height and zero width independently.

We are always able to produce an x height, but depending on whether the
glyph exists, we sometimes can't produce a zero glyph width.

Differential Revision: https://phabricator.services.mozilla.com/D23424

components/style/font_metrics.rs

@@ -12,22 +12,12 @@ use app_units::Au;
 
 /// Represents the font metrics that style needs from a font to compute the
 /// value of certain CSS units like `ex`.
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, Default, PartialEq)]
 pub struct FontMetrics {
     /// The x-height of the font.
-    pub x_height: Au,
+    pub x_height: Option<Au>,
     /// The zero advance. This is usually writing mode dependent
-    pub zero_advance_measure: Au,
+    pub zero_advance_measure: Option<Au>,
-}
-
-/// The result for querying font metrics for a given font family.
-#[derive(Clone, Debug, PartialEq)]
-pub enum FontMetricsQueryResult {
-    /// The font is available, but we may or may not have found any font metrics
-    /// for it.
-    Available(FontMetrics),
-    /// The font is not available.
-    NotAvailable,
 }
 
 /// A trait used to represent something capable of providing us font metrics.
@@ -38,7 +28,7 @@ pub trait FontMetricsProvider {
         _context: &crate::values::computed::Context,
         _base_size: crate::values::specified::length::FontBaseSize,
     ) -> FontMetricsQueryResult {
-        FontMetricsQueryResult::NotAvailable
+        Default::default()
     }
 
     /// Get default size of a given language and generic family.

components/style/gecko/wrapper.rs

@@ -20,7 +20,7 @@ use crate::context::{PostAnimationTasks, QuirksMode, SharedStyleContext, UpdateA
 use crate::data::ElementData;
 use crate::dom::{LayoutIterator, NodeInfo, OpaqueNode, TDocument, TElement, TNode, TShadowRoot};
 use crate::element_state::{DocumentState, ElementState};
-use crate::font_metrics::{FontMetrics, FontMetricsProvider, FontMetricsQueryResult};
+use crate::font_metrics::{FontMetrics, FontMetricsProvider};
 use crate::gecko::data::GeckoStyleSheet;
 use crate::gecko::selector_parser::{NonTSPseudoClass, PseudoElement, SelectorImpl};
 use crate::gecko::snapshot_helpers;
@@ -1035,10 +1035,10 @@ impl FontMetricsProvider for GeckoFontMetricsProvider {
         &self,
         context: &crate::values::computed::Context,
         base_size: FontBaseSize,
-    ) -> FontMetricsQueryResult {
+    ) -> FontMetrics {
         let pc = match context.device().pres_context() {
             Some(pc) => pc,
-            None => return FontMetricsQueryResult::NotAvailable,
+            None => return Default::default(),
         };
 
         let size = base_size.resolve(context);
@@ -1066,11 +1066,14 @@ impl FontMetricsProvider for GeckoFontMetricsProvider {
                 !context.in_media_query,
             )
         };
-        let metrics = FontMetrics {
+        FontMetrics {
-            x_height: Au(gecko_metrics.mXSize),
+            x_height: Some(Au(gecko_metrics.mXSize)),
-            zero_advance_measure: Au(gecko_metrics.mChSize),
+            zero_advance_measure: if gecko_metrics.mChSize >= 0 {
-        };
+                Some(Au(gecko_metrics.mChSize))
-        FontMetricsQueryResult::Available(metrics)
+            } else {
+                None
+            },
+        }
     }
 }
 

components/style/values/specified/length.rs

@@ -7,7 +7,7 @@
 //! [length]: https://drafts.csswg.org/css-values/#lengths
 
 use super::{AllowQuirks, Number, Percentage, ToComputedValue};
-use crate::font_metrics::FontMetricsQueryResult;
+use crate::font_metrics::FontMetrics;
 use crate::parser::{Parse, ParserContext};
 use crate::properties::computed_value_flags::ComputedValueFlags;
 use crate::values::computed::{self, CSSPixelLength, Context};
@@ -133,7 +133,7 @@ impl FontRelativeLength {
         fn query_font_metrics(
             context: &Context,
             base_size: FontBaseSize,
-        ) -> FontMetricsQueryResult {
+        ) -> FontMetrics {
             context.font_metrics_provider.query(context, base_size)
         }
 
@@ -160,16 +160,16 @@ impl FontRelativeLength {
                     context.rule_cache_conditions.borrow_mut().set_uncacheable();
                 }
                 context.builder.add_flags(ComputedValueFlags::DEPENDS_ON_FONT_METRICS);
-                let reference_size = match query_font_metrics(context, base_size) {
+                let metrics = query_font_metrics(context, base_size);
-                    FontMetricsQueryResult::Available(metrics) => metrics.x_height,
+                let reference_size = metrics.x_height.unwrap_or_else(|| {
                     // https://drafts.csswg.org/css-values/#ex
                     //
                     //     In the cases where it is impossible or impractical to
                     //     determine the x-height, a value of 0.5em must be
                     //     assumed.
                     //
-                    FontMetricsQueryResult::NotAvailable => reference_font_size.scale_by(0.5),
+                    reference_font_size.scale_by(0.5)
-                };
+                });
                 (reference_size, length)
             },
             FontRelativeLength::Ch(length) => {
@@ -177,8 +177,8 @@ impl FontRelativeLength {
                     context.rule_cache_conditions.borrow_mut().set_uncacheable();
                 }
                 context.builder.add_flags(ComputedValueFlags::DEPENDS_ON_FONT_METRICS);
-                let reference_size = match query_font_metrics(context, base_size) {
+                let metrics = query_font_metrics(context, base_size);
-                    FontMetricsQueryResult::Available(metrics) => metrics.zero_advance_measure,
+                let reference_size = metrics.zero_advance_measure.unwrap_or_else(|| {
                     // https://drafts.csswg.org/css-values/#ch
                     //
                     //     In the cases where it is impossible or impractical to
@@ -189,14 +189,12 @@ impl FontRelativeLength {
                     //     writing-mode is vertical-rl or vertical-lr and
                     //     text-orientation is upright).
                     //
-                    FontMetricsQueryResult::NotAvailable => {
+                    if context.style().writing_mode.is_vertical() {
-                        if context.style().writing_mode.is_vertical() {
+                        reference_font_size
-                            reference_font_size
+                    } else {
-                        } else {
+                        reference_font_size.scale_by(0.5)
-                            reference_font_size.scale_by(0.5)
+                    }
-                        }
+                });
-                    },
-                };
                 (reference_size, length)
             },
             FontRelativeLength::Rem(length) => {