stylo: Store font metrics provider in thread local style context

components/style/font_metrics.rs

@@ -8,6 +8,7 @@
 
 use Atom;
 use app_units::Au;
+use context::SharedStyleContext;
 use euclid::Size2D;
 use std::fmt;
 
@@ -31,8 +32,36 @@ pub enum FontMetricsQueryResult {
     NotAvailable,
 }
 
+// TODO: Servo's font metrics provider will probably not live in this crate, so this will
+// have to be replaced with something else (perhaps a trait method on TElement)
+// when we get there
+#[derive(Debug)]
+#[cfg(feature = "servo")]
+/// Dummy metrics provider for Servo. Knows nothing about fonts and does not provide
+/// any metrics.
+pub struct ServoMetricsProvider;
+
+#[cfg(feature = "servo")]
+impl FontMetricsProvider for ServoMetricsProvider {
+    fn create_from(_: &SharedStyleContext) -> Self {
+        ServoMetricsProvider
+    }
+}
+
+#[cfg(feature = "gecko")]
+/// Construct a font metrics provider for the current product
+pub fn get_metrics_provider_for_product() -> ::gecko::wrapper::GeckoFontMetricsProvider {
+    ::gecko::wrapper::GeckoFontMetricsProvider::new()
+}
+
+#[cfg(feature = "servo")]
+/// Construct a font metrics provider for the current product
+pub fn get_metrics_provider_for_product() -> ServoMetricsProvider {
+    ServoMetricsProvider
+}
+
 /// A trait used to represent something capable of providing us font metrics.
-pub trait FontMetricsProvider: Send + Sync + fmt::Debug {
+pub trait FontMetricsProvider: Send + fmt::Debug {
     /// Obtain the metrics for given font family.
     ///
     /// TODO: We could make this take the full list, I guess, and save a few
@@ -41,4 +70,13 @@ pub trait FontMetricsProvider: Send + Sync + fmt::Debug {
     fn query(&self, _font_name: &Atom) -> FontMetricsQueryResult {
         FontMetricsQueryResult::NotAvailable
     }
+
+    /// Get default size of a given language and generic family
+    fn get_size(&self, _font_name: &Atom, _font_family: u8) -> Au {
+        unimplemented!()
+    }
+
+    /// Construct from a shared style context
+    fn create_from(context: &SharedStyleContext) -> Self where Self: Sized;
 }
+