layout: Add a `FontMetricsProvider` for resolving font-relative units (#31966)

The only font relative unit that Servo knows how to resolve currently is
`rem` (relative to the root font size). This is because Stylo cannot do
any font queries. This adds a mechanism to allow this, exposing the
ability to properly render `ex` units in Servo.

This change only allows resolving some font size relative units thoug,
as Servo doesn't collect all the FontMetrics it needs to resolve them
all. This capability will be added in followup changes.

Some new tests fail:
 - ex-unit-001.html: This test fails because Servo does not yet have
   support for setting the weight using @font-face rules on web fonts.
 - ex-unit-004.html: This test fails because Servo does not yet have
   support for setting the Unicode range of a web font using @font-face
   rules.
 - first-available-font-001.html: This test fails because the above
   two feature are missing.

1 files changed, 23 insertions, 0 deletions

diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index 1ed1db20a91..7fec0013cc2 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -92,6 +92,7 @@ use style::media_queries::{Device, MediaList, MediaType};
 use style::properties::style_structs::Font;
 use style::properties::PropertyId;
 use style::selector_parser::{PseudoElement, SnapshotMap};
+use style::servo::media_queries::FontMetricsProvider;
 use style::servo::restyle_damage::ServoRestyleDamage;
 use style::shared_lock::{SharedRwLock, SharedRwLockReadGuard, StylesheetGuards};
 use style::stylesheets::{
@@ -263,6 +264,10 @@ impl Layout for LayoutThread {
         self.handle_request(Request::FromFontCache);
     }
 
+    fn device<'a>(&'a self) -> &'a Device {
+        self.stylist.device()
+    }
+
     fn waiting_for_web_fonts_to_load(&self) -> bool {
         self.outstanding_web_fonts.load(Ordering::SeqCst) != 0
     }
@@ -506,6 +511,7 @@ impl LayoutThread {
             QuirksMode::NoQuirks,
             window_size.initial_viewport,
             window_size.device_pixel_ratio,
+            Box::new(LayoutFontMetricsProvider),
         );
 
         // Ask the router to proxy IPC messages from the font cache thread to layout.
@@ -1477,6 +1483,7 @@ impl LayoutThread {
             self.stylist.quirks_mode(),
             window_size_data.initial_viewport,
             window_size_data.device_pixel_ratio,
+            Box::new(LayoutFontMetricsProvider),
         );
 
         // Preserve any previously computed root font size.
@@ -1684,3 +1691,19 @@ impl RegisteredPainters for RegisteredPaintersImpl {
             .map(|painter| painter as &dyn RegisteredPainter)
     }
 }
+
+#[derive(Debug)]
+struct LayoutFontMetricsProvider;
+
+impl FontMetricsProvider for LayoutFontMetricsProvider {
+    fn query_font_metrics(
+        &self,
+        _vertical: bool,
+        _font: &Font,
+        _base_size: style::values::computed::CSSPixelLength,
+        _in_media_query: bool,
+        _retrieve_math_scales: bool,
+    ) -> style::font_metrics::FontMetrics {
+        Default::default()
+    }
+}