style: Document the font_metrics module.

components/style/font_metrics.rs

@@ -2,6 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+//! Access to font metrics from the style system.
+
+#![deny(missing_docs)]
+
 use Atom;
 use app_units::Au;
 use euclid::Size2D;
@@ -11,13 +15,19 @@ use std::fmt;
 /// value of certain CSS units like `ex`.
 #[derive(Debug, PartialEq, Clone)]
 pub struct FontMetrics {
+    /// The x-height of the font.
     pub x_height: Au,
+    /// The zero advance.
     pub zero_advance_measure: Size2D<Au>,
 }
 
+/// The result for querying font metrics for a given font family.
 #[derive(Debug, PartialEq, Clone)]
 pub enum FontMetricsQueryResult {
+    /// The font is available, but we may or may not have found any font metrics
+    /// for it.
     Available(Option<FontMetrics>),
+    /// The font is not available.
     NotAvailable,
 }
 
@@ -26,9 +36,8 @@ pub trait FontMetricsProvider: Send + Sync + fmt::Debug {
     /// Obtain the metrics for given font family.
     ///
     /// TODO: We could make this take the full list, I guess, and save a few
-    /// virtual calls.
+    /// virtual calls in the case we are repeatedly unable to find font metrics?
-    ///
+    /// That is not too common in practice though.
-    /// This is not too common in practice though.
     fn query(&self, _font_name: &Atom) -> FontMetricsQueryResult {
         FontMetricsQueryResult::NotAvailable
     }