stylo: Stop cloning the list of stylesheets each flush.

components/style/gecko/data.rs

@@ -18,7 +18,7 @@ use std::collections::HashMap;
 use std::sync::mpsc::{Receiver, Sender, channel};
 use stylearc::Arc;
 use stylesheet_set::StylesheetSet;
-use stylesheets::{FontFaceRule, Origin, Stylesheet};
+use stylesheets::{FontFaceRule, Origin};
 use stylist::{ExtraStyleData, Stylist};
 
 /// The container for data that a Servo-backed Gecko document needs to style
@@ -102,10 +102,8 @@ impl PerDocumentStyleDataImpl {
         };
 
         let author_style_disabled = self.stylesheets.author_style_disabled();
-        let mut stylesheets = Vec::<Arc<Stylesheet>>::new();
-        self.stylesheets.flush(&mut stylesheets);
         self.stylist.clear();
-        self.stylist.rebuild(stylesheets.iter(),
+        self.stylist.rebuild(self.stylesheets.flush(),
                              &StylesheetGuards::same(guard),
                              /* ua_sheets = */ None,
                              /* stylesheets_changed = */ true,

components/style/stylesheet_set.rs

@@ -4,6 +4,7 @@
 
 //! A centralized set of stylesheets for a document.
 
+use std::slice;
 use stylearc::Arc;
 use stylesheets::Stylesheet;
 
@@ -14,6 +15,17 @@ pub struct StylesheetSetEntry {
     sheet: Arc<Stylesheet>,
 }
 
+/// A iterator over the stylesheets of a list of entries in the StylesheetSet.
+#[derive(Clone)]
+pub struct StylesheetIterator<'a>(slice::Iter<'a, StylesheetSetEntry>);
+
+impl<'a> Iterator for StylesheetIterator<'a> {
+    type Item = &'a Arc<Stylesheet>;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next().map(|entry| &entry.sheet)
+    }
+}
+
 /// The set of stylesheets effective for a given document.
 pub struct StylesheetSet {
     /// The actual list of all the stylesheets that apply to the given document,
@@ -108,12 +120,11 @@ impl StylesheetSet {
         self.dirty
     }
 
-    /// Flush the current set, unmarking it as dirty.
+    /// Flush the current set, unmarking it as dirty, and returns an iterator
-    pub fn flush(&mut self, sheets: &mut Vec<Arc<Stylesheet>>) {
+    /// over the new stylesheet list.
+    pub fn flush(&mut self) -> StylesheetIterator {
         self.dirty = false;
-        for entry in &self.entries {
+        StylesheetIterator(self.entries.iter())
-            sheets.push(entry.sheet.clone())
-        }
     }
 
     /// Mark the stylesheets as dirty, because something external may have