style: Iterate the LRU cache contents from back to front.

We put the more recently used item last, so iterating then from left to right is
pointless.

components/style/cache.rs

@@ -6,7 +6,7 @@
 
 #![deny(missing_docs)]
 
-use std::slice::{Iter, IterMut};
+use std::{iter, slice};
 
 /// A LRU cache used to store a set of at most `n` elements at the same time.
 ///
@@ -16,6 +16,12 @@ pub struct LRUCache<K> {
     cache_size: usize,
 }
 
+/// A iterator over the items of the LRU cache.
+pub type LRUCacheIterator<'a, K> = iter::Rev<slice::Iter<'a, K>>;
+
+/// A iterator over the mutable items of the LRU cache.
+pub type LRUCacheMutIterator<'a, K> = iter::Rev<slice::IterMut<'a, K>>;
+
 impl<K: PartialEq> LRUCache<K> {
     /// Create a new LRU cache with `size` elements at most.
     pub fn new(size: usize) -> Self {
@@ -35,14 +41,15 @@ impl<K: PartialEq> LRUCache<K> {
         }
     }
 
-    /// Iterate over the contents of this cache.
-    pub fn iter(&self) -> Iter<K> {
-        self.entries.iter()
+    /// Iterate over the contents of this cache, from more to less recently
+    /// used.
+    pub fn iter(&self) -> LRUCacheIterator<K> {
+        self.entries.iter().rev()
     }
 
     /// Iterate mutably over the contents of this cache.
-    pub fn iter_mut(&mut self) -> IterMut<K> {
-        self.entries.iter_mut()
+    pub fn iter_mut(&mut self) -> LRUCacheMutIterator<K> {
+        self.entries.iter_mut().rev()
     }
 
     /// Insert a given key in the cache.

components/style/matching.rs

@@ -10,7 +10,7 @@
 use {Atom, LocalName};
 use animation::{self, Animation, PropertyAnimation};
 use atomic_refcell::AtomicRefMut;
-use cache::LRUCache;
+use cache::{LRUCache, LRUCacheMutIterator};
 use cascade_info::CascadeInfo;
 use context::{SequentialTask, SharedStyleContext, StyleContext};
 use data::{ComputedStyle, ElementData, ElementStyles, RestyleData};
@@ -27,7 +27,6 @@ use selectors::matching::AFFECTED_BY_PSEUDO_ELEMENTS;
 use servo_config::opts;
 use sink::ForgetfulSink;
 use std::collections::hash_map::Entry;
-use std::slice::IterMut;
 use std::sync::Arc;
 use stylist::ApplicableDeclarationBlock;
 
@@ -366,7 +365,7 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
         }
     }
 
-    fn iter_mut(&mut self) -> IterMut<StyleSharingCandidate<E>> {
+    fn iter_mut(&mut self) -> LRUCacheMutIterator<StyleSharingCandidate<E>> {
         self.cache.iter_mut()
     }