Store selectors and combinators inline in a single sequence.

This improves cache locality and reduces allocations during parsing.

Note that this reverses the iteration order within a sequence of simple selectors,
but that shouldn't matter.

components/selectors/arcslice.rs

@@ -0,0 +1,326 @@
+/* Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+ * http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+ * <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+ * option. This file may not be copied, modified, or distributed
+ * except according to those terms.
+ *
+ * See the COPYRIGHT file at the top-level directory of this distribution */
+//! A thread-safe reference-counted slice type.
+//!
+//! Forked from https://github.com/huonw/shared_slice , which doesn't work on
+//! rust stable.
+
+use std::{cmp, fmt, ops};
+use std::hash::{Hash, Hasher};
+use std::sync::{Arc, Weak};
+
+
+/// A reference-counted slice type.
+pub struct ArcSlice<T> {
+    data: *const [T],
+    counts: Arc<Box<[T]>>,
+}
+
+unsafe impl<T: Send + Sync> Send for ArcSlice<T> {}
+unsafe impl<T: Send + Sync> Sync for ArcSlice<T> {}
+
+/// A non-owning reference-counted slice type.
+///
+/// This is to `ArcSlice` as `std::sync::Weak` is to `std::sync::Arc`, and
+/// allows one to have cyclic references without stopping memory from
+/// being deallocated.
+pub struct WeakSlice<T> {
+    data: *const [T],
+    counts: Weak<Box<[T]>>,
+}
+unsafe impl<T: Send + Sync> Send for WeakSlice<T> {}
+unsafe impl<T: Send + Sync> Sync for WeakSlice<T> {}
+
+impl<T> ArcSlice<T> {
+    /// Construct a new `ArcSlice` containing the elements of `slice`.
+    ///
+    /// This reuses the allocation of `slice`.
+    pub fn new(slice: Box<[T]>) -> ArcSlice<T> {
+        ArcSlice {
+            data: &*slice,
+            counts: Arc::new(slice),
+        }
+    }
+
+    /// Downgrade self into a weak slice.
+    pub fn downgrade(&self) -> WeakSlice<T> {
+        WeakSlice {
+            data: self.data,
+            counts: Arc::downgrade(&self.counts)
+        }
+    }
+
+    /// Construct a new `ArcSlice` that only points to elements at
+    /// indices `lo` (inclusive) through `hi` (exclusive).
+    ///
+    /// This consumes `self` to avoid unnecessary reference-count
+    /// modifications. Use `.clone()` if it is necessary to refer to
+    /// `self` after calling this.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `lo > hi` or if either are strictly greater than
+    /// `self.len()`.
+    pub fn slice(mut self, lo: usize, hi: usize) -> ArcSlice<T> {
+        self.data = &self[lo..hi];
+        self
+    }
+    /// Construct a new `ArcSlice` that only points to elements at
+    /// indices up to `hi` (exclusive).
+    ///
+    /// This consumes `self` to avoid unnecessary reference-count
+    /// modifications. Use `.clone()` if it is necessary to refer to
+    /// `self` after calling this.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `hi > self.len()`.
+    pub fn slice_to(self, hi: usize) -> ArcSlice<T> {
+        self.slice(0, hi)
+    }
+    /// Construct a new `ArcSlice` that only points to elements at
+    /// indices starting at  `lo` (inclusive).
+    ///
+    /// This consumes `self` to avoid unnecessary reference-count
+    /// modifications. Use `.clone()` if it is necessary to refer to
+    /// `self` after calling this.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `lo > self.len()`.
+    pub fn slice_from(self, lo: usize) -> ArcSlice<T> {
+        let hi = self.len();
+        self.slice(lo, hi)
+    }
+}
+
+impl<T> Clone for ArcSlice<T> {
+    fn clone(&self) -> ArcSlice<T> {
+        ArcSlice {
+            data: self.data,
+            counts: self.counts.clone()
+        }
+    }
+}
+
+impl<T> ops::Deref for ArcSlice<T> {
+    type Target = [T];
+    fn deref<'a>(&'a self) -> &'a [T] {
+        unsafe { &*self.data }
+    }
+}
+
+impl<T> AsRef<[T]> for ArcSlice<T> {
+    fn as_ref(&self) -> &[T] { &**self }
+}
+
+impl<T: PartialEq> PartialEq for ArcSlice<T> {
+    fn eq(&self, other: &ArcSlice<T>) -> bool { **self == **other }
+    fn ne(&self, other: &ArcSlice<T>) -> bool { **self != **other }
+}
+impl<T: Eq> Eq for ArcSlice<T> {}
+
+impl<T: PartialOrd> PartialOrd for ArcSlice<T> {
+    fn partial_cmp(&self, other: &ArcSlice<T>) -> Option<cmp::Ordering> {
+        (**self).partial_cmp(&**other)
+    }
+    fn lt(&self, other: &ArcSlice<T>) -> bool { **self < **other }
+    fn le(&self, other: &ArcSlice<T>) -> bool { **self <= **other }
+    fn gt(&self, other: &ArcSlice<T>) -> bool { **self > **other }
+    fn ge(&self, other: &ArcSlice<T>) -> bool { **self >= **other }
+}
+impl<T: Ord> Ord for ArcSlice<T> {
+    fn cmp(&self, other: &ArcSlice<T>) -> cmp::Ordering { (**self).cmp(&**other) }
+}
+
+impl<T: Hash> Hash for ArcSlice<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        Hash::hash(&**self, state)
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for ArcSlice<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Debug::fmt(&**self, f)
+    }
+}
+
+impl<T> WeakSlice<T> {
+    /// Attempt to upgrade `self` to a strongly-counted `ArcSlice`.
+    ///
+    /// Returns `None` if this is not possible (the data has already
+    /// been freed).
+    pub fn upgrade(&self) -> Option<ArcSlice<T>> {
+        self.counts.upgrade().map(|counts| {
+            ArcSlice {
+                data: self.data,
+                counts: counts
+            }
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::cell::Cell;
+    use std::cmp::Ordering;
+    use std::sync::{Arc, Mutex};
+    use super::{ArcSlice, WeakSlice};
+    #[test]
+    fn clone() {
+        let x = ArcSlice::new(Box::new([Cell::new(false)]));
+        let y = x.clone();
+
+        assert_eq!(x[0].get(), false);
+        assert_eq!(y[0].get(), false);
+
+        x[0].set(true);
+        assert_eq!(x[0].get(), true);
+        assert_eq!(y[0].get(), true);
+    }
+
+    #[test]
+    fn test_upgrade_downgrade() {
+        let x = ArcSlice::new(Box::new([1]));
+        let y: WeakSlice<_> = x.downgrade();
+
+        assert_eq!(y.upgrade(), Some(x.clone()));
+
+        drop(x);
+
+        assert!(y.upgrade().is_none())
+    }
+
+    #[test]
+    fn test_total_cmp() {
+        let x = ArcSlice::new(Box::new([1, 2, 3]));
+        let y = ArcSlice::new(Box::new([1, 2, 3]));
+        let z = ArcSlice::new(Box::new([1, 2, 4]));
+
+        assert_eq!(x, x);
+        assert_eq!(x, y);
+        assert!(x != z);
+        assert!(y != z);
+
+        assert!(x < z);
+        assert!(x <= z);
+        assert!(!(x > z));
+        assert!(!(x >= z));
+
+        assert!(!(z < x));
+        assert!(!(z <= x));
+        assert!(z > x);
+        assert!(z >= x);
+
+        assert_eq!(x.partial_cmp(&x), Some(Ordering::Equal));
+        assert_eq!(x.partial_cmp(&y), Some(Ordering::Equal));
+        assert_eq!(x.partial_cmp(&z), Some(Ordering::Less));
+        assert_eq!(z.partial_cmp(&y), Some(Ordering::Greater));
+
+        assert_eq!(x.cmp(&x), Ordering::Equal);
+        assert_eq!(x.cmp(&y), Ordering::Equal);
+        assert_eq!(x.cmp(&z), Ordering::Less);
+        assert_eq!(z.cmp(&y), Ordering::Greater);
+    }
+
+    #[test]
+    fn test_partial_cmp() {
+        use std::f64;
+        let x = ArcSlice::new(Box::new([1.0, f64::NAN]));
+        let y = ArcSlice::new(Box::new([1.0, f64::NAN]));
+        let z = ArcSlice::new(Box::new([2.0, f64::NAN]));
+        let w = ArcSlice::new(Box::new([f64::NAN, 1.0]));
+        assert!(!(x == y));
+        assert!(x != y);
+
+        assert!(!(x < y));
+        assert!(!(x <= y));
+        assert!(!(x > y));
+        assert!(!(x >= y));
+
+        assert!(x < z);
+        assert!(x <= z);
+        assert!(!(x > z));
+        assert!(!(x >= z));
+
+        assert!(!(z < w));
+        assert!(!(z <= w));
+        assert!(!(z > w));
+        assert!(!(z >= w));
+
+        assert_eq!(x.partial_cmp(&x), None);
+        assert_eq!(x.partial_cmp(&y), None);
+        assert_eq!(x.partial_cmp(&z), Some(Ordering::Less));
+        assert_eq!(z.partial_cmp(&x), Some(Ordering::Greater));
+
+        assert_eq!(x.partial_cmp(&w), None);
+        assert_eq!(y.partial_cmp(&w), None);
+        assert_eq!(z.partial_cmp(&w), None);
+        assert_eq!(w.partial_cmp(&w), None);
+    }
+
+    #[test]
+    fn test_show() {
+        let x = ArcSlice::new(Box::new([1, 2]));
+        assert_eq!(format!("{:?}", x), "[1, 2]");
+
+        let y: ArcSlice<i32> = ArcSlice::new(Box::new([]));
+        assert_eq!(format!("{:?}", y), "[]");
+    }
+
+    #[test]
+    fn test_slice() {
+        let x = ArcSlice::new(Box::new([1, 2, 3]));
+        let real = [1, 2, 3];
+        for i in 0..3 + 1 {
+            for j in i..3 + 1 {
+                let slice: ArcSlice<_> = x.clone().slice(i, j);
+                assert_eq!(&*slice, &real[i..j]);
+            }
+            assert_eq!(&*x.clone().slice_to(i), &real[..i]);
+            assert_eq!(&*x.clone().slice_from(i), &real[i..]);
+        }
+    }
+
+
+    #[test]
+    fn test_send_sync() {
+        fn assert_send<T: Send>() {}
+        fn assert_sync<T: Send>() {}
+
+        assert_send::<ArcSlice<u8>>();
+        assert_sync::<ArcSlice<u8>>();
+        assert_send::<WeakSlice<u8>>();
+        assert_sync::<WeakSlice<u8>>();
+    }
+
+    #[test]
+    fn test_drop() {
+        let drop_flag = Arc::new(Mutex::new(0));
+        struct Foo(Arc<Mutex<i32>>);
+
+        impl Drop for Foo {
+            fn drop(&mut self) {
+                let mut n = self.0.lock().unwrap();
+                *n += 1;
+            }
+        }
+
+        let whole = ArcSlice::new(Box::new([Foo(drop_flag.clone()), Foo(drop_flag.clone())]));
+
+        drop(whole);
+        assert_eq!(*drop_flag.lock().unwrap(), 2);
+
+        *drop_flag.lock().unwrap() = 0;
+
+        let whole = ArcSlice::new(Box::new([Foo(drop_flag.clone()), Foo(drop_flag.clone())]));
+        let part = whole.slice(1, 2);
+        drop(part);
+        assert_eq!(*drop_flag.lock().unwrap(), 2);
+    }
+}

components/selectors/lib.rs

@@ -9,6 +9,7 @@ extern crate fnv;
 extern crate precomputed_hash;
 extern crate smallvec;
 
+pub mod arcslice;
 pub mod bloom;
 pub mod matching;
 pub mod parser;

components/selectors/matching.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 use bloom::BloomFilter;
 use parser::{CaseSensitivity, Combinator, ComplexSelector, LocalName};
-use parser::{SimpleSelector, Selector, SelectorInner};
+use parser::{SimpleSelector, Selector, SelectorInner, SelectorIter};
 use std::borrow::Borrow;
 use tree::Element;
 
@@ -207,7 +207,7 @@ pub fn matches_complex_selector<E, F>(selector: &ComplexSelector<E::Impl>,
      where E: Element,
            F: FnMut(&E, ElementSelectorFlags),
 {
-    match matches_complex_selector_internal(selector,
+    match matches_complex_selector_internal(selector.iter(),
                                             element,
                                             relations,
                                             flags_setter) {
@@ -216,7 +216,7 @@ pub fn matches_complex_selector<E, F>(selector: &ComplexSelector<E::Impl>,
     }
 }
 
-fn matches_complex_selector_internal<E, F>(selector: &ComplexSelector<E::Impl>,
+fn matches_complex_selector_internal<E, F>(mut selector_iter: SelectorIter<E::Impl>,
                                            element: &E,
                                            relations: &mut StyleRelations,
                                            flags_setter: &mut F)
@@ -224,14 +224,12 @@ fn matches_complex_selector_internal<E, F>(selector: &ComplexSelector<E::Impl>,
      where E: Element,
            F: FnMut(&E, ElementSelectorFlags),
 {
-    let matches_all_simple_selectors = selector.compound_selector.iter().all(|simple| {
+    let matches_all_simple_selectors = selector_iter.all(|simple| {
         matches_simple_selector(simple, element, relations, flags_setter)
     });
 
-    let siblings = selector.next.as_ref().map_or(false, |&(_, combinator)| {
-        matches!(combinator, Combinator::NextSibling | Combinator::LaterSibling)
-    });
-
+    let combinator = selector_iter.next_sequence();
+    let siblings = combinator.map_or(false, |c| c.is_sibling());
     if siblings {
         flags_setter(element, HAS_SLOW_SELECTOR_LATER_SIBLINGS);
     }
@@ -240,9 +238,9 @@ fn matches_complex_selector_internal<E, F>(selector: &ComplexSelector<E::Impl>,
         return SelectorMatchingResult::NotMatchedAndRestartFromClosestLaterSibling;
     }
 
-    match selector.next {
+    match combinator {
         None => SelectorMatchingResult::Matched,
-        Some((ref next_selector, combinator)) => {
+        Some(c) => {
             let (mut next_element, candidate_not_found) = if siblings {
                 (element.prev_sibling_element(),
                  SelectorMatchingResult::NotMatchedAndRestartFromClosestDescendant)
@@ -256,11 +254,11 @@ fn matches_complex_selector_internal<E, F>(selector: &ComplexSelector<E::Impl>,
                     None => return candidate_not_found,
                     Some(next_element) => next_element,
                 };
-                let result = matches_complex_selector_internal(&**next_selector,
+                let result = matches_complex_selector_internal(selector_iter.clone(),
                                                                &element,
                                                                relations,
                                                                flags_setter);
-                match (result, combinator) {
+                match (result, c) {
                     // Return the status immediately.
                     (SelectorMatchingResult::Matched, _) => return result,
                     (SelectorMatchingResult::NotMatchedGlobally, _) => return result,
@@ -319,6 +317,7 @@ fn matches_simple_selector<E, F>(
     }
 
     match *selector {
+        SimpleSelector::Combinator(_) => unreachable!(),
         SimpleSelector::LocalName(LocalName { ref name, ref lower_name }) => {
             let name = if element.is_html_element_in_html_document() { lower_name } else { name };
             element.get_local_name() == name.borrow()
@@ -421,7 +420,7 @@ fn matches_simple_selector<E, F>(
         }
         SimpleSelector::Negation(ref negated) => {
             !negated.iter().all(|s| {
-                match matches_complex_selector_internal(s,
+                match matches_complex_selector_internal(s.iter(),
                                                         element,
                                                         relations,
                                                         flags_setter) {

components/selectors/parser.rs

@@ -2,6 +2,7 @@
  * 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/. */
 
+use arcslice::ArcSlice;
 use cssparser::{Token, Parser as CssParser, parse_nth, ToCss, serialize_identifier, CssStringWriter};
 use precomputed_hash::PrecomputedHash;
 use std::ascii::AsciiExt;
@@ -9,8 +10,9 @@ use std::borrow::{Borrow, Cow};
 use std::cmp;
 use std::fmt::{self, Display, Debug, Write};
 use std::hash::Hash;
+use std::iter::Rev;
 use std::ops::Add;
-use std::sync::Arc;
+use std::slice;
 use tree::SELECTOR_WHITESPACE;
 use visitor::SelectorVisitor;
 
@@ -132,7 +134,7 @@ const NUM_ANCESTOR_HASHES: usize = 4;
 #[derive(PartialEq, Eq, Hash, Clone)]
 pub struct SelectorInner<Impl: SelectorImpl> {
     /// The selector data.
-    pub complex: Arc<ComplexSelector<Impl>>,
+    pub complex: ComplexSelector<Impl>,
     /// Ancestor hashes for the bloom filter. We precompute these and store
     /// them inline to optimize cache performance during selector matching.
     /// This matters a lot.
@@ -140,15 +142,14 @@ pub struct SelectorInner<Impl: SelectorImpl> {
 }
 
 impl<Impl: SelectorImpl> SelectorInner<Impl> {
-    pub fn new(c: Arc<ComplexSelector<Impl>>) -> Self {
+    pub fn new(c: ComplexSelector<Impl>) -> Self {
         let mut hashes = [0; NUM_ANCESTOR_HASHES];
         {
             // Compute ancestor hashes for the bloom filter.
-            let mut hash_iter =
-                iter_ancestors(&c).flat_map(|x| x.compound_selector.iter())
-                                  .map(|x| x.ancestor_hash())
-                                  .filter(|x| x.is_some())
-                                  .map(|x| x.unwrap());
+            let mut hash_iter = c.iter_ancestors()
+                                 .map(|x| x.ancestor_hash())
+                                 .filter(|x| x.is_some())
+                                 .map(|x| x.unwrap());
             for i in 0..NUM_ANCESTOR_HASHES {
                 hashes[i] = match hash_iter.next() {
                     Some(x) => x,
@@ -162,6 +163,12 @@ impl<Impl: SelectorImpl> SelectorInner<Impl> {
             ancestor_hashes: hashes,
         }
     }
+
+    /// Creates a SelectorInner from a Vec of SimpleSelectors. Used in tests.
+    pub fn from_vec(vec: Vec<SimpleSelector<Impl>>) -> Self {
+        let complex = ComplexSelector::from_vec(vec);
+        Self::new(complex)
+    }
 }
 
 #[derive(PartialEq, Eq, Hash, Clone)]
@@ -194,25 +201,22 @@ impl<Impl: SelectorImpl> SelectorMethods for ComplexSelector<Impl> {
     fn visit<V>(&self, visitor: &mut V) -> bool
         where V: SelectorVisitor<Impl = Impl>,
     {
-        let mut current = self;
+        let mut current = self.iter();
         let mut combinator = None;
         loop {
-            if !visitor.visit_complex_selector(current, combinator) {
+            if !visitor.visit_complex_selector(current.clone(), combinator) {
                 return false;
             }
 
-            for selector in &current.compound_selector {
+            for selector in &mut current {
                 if !selector.visit(visitor) {
                     return false;
                 }
             }
 
-            match current.next {
-                Some((ref next, next_combinator)) => {
-                    current = next;
-                    combinator = Some(next_combinator);
-                }
-                None => break,
+            combinator = current.next_sequence();
+            if combinator.is_none() {
+                break;
             }
         }
 
@@ -262,38 +266,139 @@ impl<Impl: SelectorImpl> SelectorMethods for SimpleSelector<Impl> {
     }
 }
 
+/// A ComplexSelectors stores a sequence of simple selectors and combinators. The
+/// iterator classes allow callers to iterate at either the raw sequence level or
+/// at the level of sequences of simple selectors separated by combinators. Most
+/// callers want the higher-level iterator.
+///
+/// We store selectors internally left-to-right (in parsing order), but the
+/// canonical iteration order is right-to-left (selector matching order). The
+/// iterators abstract over these details.
 #[derive(Clone, Eq, Hash, PartialEq)]
-pub struct ComplexSelector<Impl: SelectorImpl> {
-    pub compound_selector: Vec<SimpleSelector<Impl>>,
-    pub next: Option<(Arc<ComplexSelector<Impl>>, Combinator)>,  // c.next is left of c
-}
+pub struct ComplexSelector<Impl: SelectorImpl>(ArcSlice<SimpleSelector<Impl>>);
 
-struct AncestorIterator<'a, Impl: 'a + SelectorImpl> {
-    curr: Option<&'a Arc<ComplexSelector<Impl>>>,
-}
-
-impl<'a, Impl: SelectorImpl> Iterator for AncestorIterator<'a, Impl> {
-    type Item = &'a Arc<ComplexSelector<Impl>>;
-    fn next(&mut self) -> Option<Self::Item> {
-        while let Some(sel) = self.curr.take() {
-            let (next_sel, is_ancestor) = match sel.next {
-                None => (None, true),
-                Some((ref sel, comb)) =>
-                    (Some(sel), matches!(comb, Combinator::Child | Combinator::Descendant)),
-            };
-            self.curr = next_sel;
-            if is_ancestor {
-                break;
-            }
+impl<Impl: SelectorImpl> ComplexSelector<Impl> {
+    /// Returns an iterator over the next sequence of simple selectors. When
+    /// a combinator is reached, the iterator will return None, and
+    /// next_sequence() may be called to continue to the next sequence.
+    pub fn iter(&self) -> SelectorIter<Impl> {
+        SelectorIter {
+            iter: self.iter_raw(),
+            next_combinator: None,
         }
+    }
 
-        self.curr
+    /// Returns an iterator over the entire sequence of simple selectors and combinators,
+    /// from right to left.
+    pub fn iter_raw(&self) -> Rev<slice::Iter<SimpleSelector<Impl>>> {
+        self.iter_raw_rev().rev()
+    }
+
+    /// Returns an iterator over the entire sequence of simple selectors and combinators,
+    /// from left to right.
+    pub fn iter_raw_rev(&self) -> slice::Iter<SimpleSelector<Impl>> {
+        self.0.iter()
+    }
+
+    /// Returns an iterator over ancestor simple selectors. All combinators and
+    /// non-ancestor simple selectors will be skipped.
+    pub fn iter_ancestors(&self) -> AncestorIter<Impl> {
+        AncestorIter::new(self.iter())
+    }
+
+    /// Returns a ComplexSelector identical to |self| but with the rightmost |index|
+    /// entries removed.
+    pub fn slice_from(&self, index: usize) -> Self {
+        // Note that we convert the slice_from to slice_to because selectors are
+        // stored left-to-right but logical order is right-to-left.
+        ComplexSelector(self.0.clone().slice_to(self.0.len() - index))
+    }
+
+    /// Creates a ComplexSelector from a vec of SimpleSelectors. Used in tests.
+    pub fn from_vec(vec: Vec<SimpleSelector<Impl>>) -> Self {
+        ComplexSelector(ArcSlice::new(vec.into_boxed_slice()))
     }
 }
 
-fn iter_ancestors<Impl: SelectorImpl>(sel: &Arc<ComplexSelector<Impl>>) -> AncestorIterator<Impl> {
-    AncestorIterator {
-        curr: Some(sel)
+pub struct SelectorIter<'a, Impl: 'a + SelectorImpl> {
+    iter: Rev<slice::Iter<'a, SimpleSelector<Impl>>>,
+    next_combinator: Option<Combinator>,
+}
+
+// NB: Deriving this doesn't work for some reason, because it expects Impl to
+// implement Clone.
+impl<'a, Impl: 'a + SelectorImpl> Clone for SelectorIter<'a, Impl> {
+    fn clone(&self) -> Self {
+        SelectorIter {
+            iter: self.iter.clone(),
+            next_combinator: self.next_combinator.clone(),
+        }
+    }
+}
+
+impl<'a, Impl: 'a + SelectorImpl> SelectorIter<'a, Impl> {
+    /// Prepares this iterator to point to the next sequence to the left,
+    /// returning the combinator if the sequence was found.
+    pub fn next_sequence(&mut self) -> Option<Combinator> {
+        self.next_combinator.take()
+    }
+}
+
+impl<'a, Impl: SelectorImpl> Iterator for SelectorIter<'a, Impl> {
+    type Item = &'a SimpleSelector<Impl>;
+    fn next(&mut self) -> Option<Self::Item> {
+        debug_assert!(self.next_combinator.is_none(), "Should call take_combinator!");
+        match self.iter.next() {
+            None => None,
+            Some(&SimpleSelector::Combinator(c)) => {
+                self.next_combinator = Some(c);
+                None
+            },
+            Some(x) => Some(x),
+        }
+    }
+}
+
+/// An iterator over all simple selectors belonging to ancestors.
+pub struct AncestorIter<'a, Impl: 'a + SelectorImpl>(SelectorIter<'a, Impl>);
+impl<'a, Impl: 'a + SelectorImpl> AncestorIter<'a, Impl> {
+    /// Creates an AncestorIter. The passed-in iterator is assumed to point to
+    /// the beginning of the child sequence, which will be skipped.
+    fn new(inner: SelectorIter<'a, Impl>) -> Self {
+        let mut result = AncestorIter(inner);
+        result.skip_until_ancestor();
+        result
+    }
+
+    /// Skips a sequence of simple selectors and all subsequent sequences until an
+    /// ancestor combinator is reached.
+    fn skip_until_ancestor(&mut self) {
+        loop {
+            while let Some(_) = self.0.next() {}
+            if self.0.next_sequence().map_or(true, |x| x.is_ancestor()) {
+                break;
+            }
+        }
+    }
+}
+
+impl<'a, Impl: SelectorImpl> Iterator for AncestorIter<'a, Impl> {
+    type Item = &'a SimpleSelector<Impl>;
+    fn next(&mut self) -> Option<Self::Item> {
+        // Grab the next simple selector in the sequence if available.
+        let next = self.0.next();
+        if next.is_some() {
+            return next;
+        }
+
+        // See if there are more sequences. If so, skip any non-ancestor sequences.
+        if let Some(combinator) = self.0.next_sequence() {
+            if !combinator.is_ancestor() {
+                self.skip_until_ancestor();
+            }
+        }
+
+        self.0.next()
     }
 }
 
@@ -305,8 +410,29 @@ pub enum Combinator {
     LaterSibling,  // ~
 }
 
+impl Combinator {
+    /// Returns true if this combinator is a child or descendant combinator.
+    pub fn is_ancestor(&self) -> bool {
+        matches!(*self, Combinator::Child | Combinator::Descendant)
+    }
+
+    /// Returns true if this combinator is a next- or later-sibling combinator.
+    pub fn is_sibling(&self) -> bool {
+        matches!(*self, Combinator::NextSibling | Combinator::LaterSibling)
+    }
+}
+
+/// A CSS simple selector or combinator. We store both in the same enum for
+/// optimal packing and cache performance, see [1].
+///
+/// We could rename this SimpleSelectorOrCombinator, but that's annoying to
+/// type everywhere, and the combinators are generally filtered out and
+/// handled separately by the iterator classes anyway.
+///
+/// [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1357973
 #[derive(Eq, PartialEq, Clone, Hash)]
 pub enum SimpleSelector<Impl: SelectorImpl> {
+    Combinator(Combinator),
     ID(Impl::Identifier),
     Class(Impl::ClassName),
     LocalName(LocalName<Impl>),
@@ -327,7 +453,7 @@ pub enum SimpleSelector<Impl: SelectorImpl> {
     AttrSuffixNeverMatch(AttrSelector<Impl>, Impl::AttrValue),  // empty value
 
     // Pseudo-classes
-    Negation(Vec<Arc<ComplexSelector<Impl>>>),
+    Negation(Vec<ComplexSelector<Impl>>),
     FirstChild, LastChild, OnlyChild,
     Root,
     Empty,
@@ -368,6 +494,11 @@ impl<Impl: SelectorImpl> SimpleSelector<Impl> {
             _ => None,
         }
     }
+
+    /// Returns true if this is a combinator.
+    pub fn is_combinator(&self) -> bool {
+        matches!(*self, SimpleSelector::Combinator(_))
+    }
 }
 
 #[derive(Eq, PartialEq, Clone, Hash, Copy, Debug)]
@@ -466,31 +597,11 @@ impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
 
 impl<Impl: SelectorImpl> ToCss for ComplexSelector<Impl> {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-       use smallvec::SmallVec;
-       let mut current = self;
-       let mut nodes = SmallVec::<[&Self;8]>::new();
-       nodes.push(current);
-
-       loop {
-           match current.next {
-               None => break,
-               Some((ref next, _)) => {
-                   current = &**next;
-                   nodes.push(next);
-               }
-           }
+       for item in self.iter_raw_rev() {
+           item.to_css(dest)?;
        }
 
-       for selector in nodes.iter().rev() {
-           if let Some((_, ref combinator)) = selector.next {
-               combinator.to_css(dest)?;
-           }
-
-           for simple in &selector.compound_selector {
-               simple.to_css(dest)?;
-           }
-        }
-        Ok(())
+       Ok(())
     }
 }
 
@@ -509,6 +620,9 @@ impl<Impl: SelectorImpl> ToCss for SimpleSelector<Impl> {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         use self::SimpleSelector::*;
         match *self {
+            Combinator(ref c) => {
+                c.to_css(dest)
+            }
             ID(ref s) => {
                 dest.write_char('#')?;
                 display_to_css_identifier(s, dest)
@@ -699,14 +813,15 @@ fn specificity<Impl>(complex_selector: &ComplexSelector<Impl>,
     specificity.into()
 }
 
-fn complex_selector_specificity<Impl>(mut selector: &ComplexSelector<Impl>)
+fn complex_selector_specificity<Impl>(selector: &ComplexSelector<Impl>)
                                       -> Specificity
                                       where Impl: SelectorImpl {
-    fn compound_selector_specificity<Impl>(compound_selector: &[SimpleSelector<Impl>],
+    fn compound_selector_specificity<Impl>(selector_iter: &mut SelectorIter<Impl>,
                                            specificity: &mut Specificity)
                                            where Impl: SelectorImpl {
-        for simple_selector in compound_selector.iter() {
+        for simple_selector in selector_iter {
             match *simple_selector {
+                SimpleSelector::Combinator(..) => unreachable!(),
                 SimpleSelector::LocalName(..) =>
                     specificity.element_selectors += 1,
                 SimpleSelector::ID(..) =>
@@ -749,16 +864,11 @@ fn complex_selector_specificity<Impl>(mut selector: &ComplexSelector<Impl>)
     }
 
     let mut specificity = Default::default();
-    compound_selector_specificity(&selector.compound_selector,
-                              &mut specificity);
+    let mut iter = selector.iter();
     loop {
-        match selector.next {
-            None => break,
-            Some((ref next_selector, _)) => {
-                selector = &**next_selector;
-                compound_selector_specificity(&selector.compound_selector,
-                                          &mut specificity)
-            }
+        compound_selector_specificity(&mut iter, &mut specificity);
+        if iter.next_sequence().is_none() {
+            break;
         }
     }
     specificity
@@ -775,7 +885,7 @@ fn parse_selector<P, Impl>(parser: &P, input: &mut CssParser) -> Result<Selector
         parse_complex_selector_and_pseudo_element(parser, input)?;
     Ok(Selector {
         specificity: specificity(&complex, pseudo_element.as_ref()),
-        inner: SelectorInner::new(Arc::new(complex)),
+        inner: SelectorInner::new(complex),
         pseudo_element: pseudo_element,
     })
 }
@@ -786,10 +896,16 @@ fn parse_complex_selector_and_pseudo_element<P, Impl>(
         -> Result<(ComplexSelector<Impl>, Option<Impl::PseudoElement>), ()>
     where P: Parser<Impl=Impl>, Impl: SelectorImpl
 {
-    let (first, mut pseudo_element) = parse_compound_selector(parser, input)?;
-    let mut complex = ComplexSelector { compound_selector: first, next: None };
+    let mut sequence = Vec::new();
+    let mut pseudo_element;
+    'outer_loop: loop {
+        // Parse a sequence of simple selectors.
+        pseudo_element = parse_compound_selector(parser, input, &mut sequence)?;
+        if pseudo_element.is_some() {
+            break;
+        }
 
-    'outer_loop: while pseudo_element.is_none() {
+        // Parse a combinator.
         let combinator;
         let mut any_whitespace = false;
         loop {
@@ -820,14 +936,10 @@ fn parse_complex_selector_and_pseudo_element<P, Impl>(
                 }
             }
         }
-        let (compound_selector, pseudo) = parse_compound_selector(parser, input)?;
-        complex = ComplexSelector {
-            compound_selector: compound_selector,
-            next: Some((Arc::new(complex), combinator))
-        };
-        pseudo_element = pseudo;
+        sequence.push(SimpleSelector::Combinator(combinator));
     }
 
+    let complex = ComplexSelector(ArcSlice::new(sequence.into_boxed_slice()));
     Ok((complex, pseudo_element))
 }
 
@@ -848,30 +960,29 @@ impl<Impl: SelectorImpl> ComplexSelector<Impl> {
 /// * `Err(())`: Invalid selector, abort
 /// * `Ok(None)`: Not a type selector, could be something else. `input` was not consumed.
 /// * `Ok(Some(vec))`: Length 0 (`*|*`), 1 (`*|E` or `ns|*`) or 2 (`|E` or `ns|E`)
-fn parse_type_selector<P, Impl>(parser: &P, input: &mut CssParser)
-                       -> Result<Option<Vec<SimpleSelector<Impl>>>, ()>
+fn parse_type_selector<P, Impl>(parser: &P, input: &mut CssParser, sequence: &mut Vec<SimpleSelector<Impl>>)
+                       -> Result<bool, ()>
     where P: Parser<Impl=Impl>, Impl: SelectorImpl
 {
     match parse_qualified_name(parser, input, /* in_attr_selector = */ false)? {
-        None => Ok(None),
+        None => Ok(false),
         Some((namespace, local_name)) => {
-            let mut compound_selector = vec!();
             match namespace {
                 NamespaceConstraint::Specific(ns) => {
-                    compound_selector.push(SimpleSelector::Namespace(ns))
+                    sequence.push(SimpleSelector::Namespace(ns))
                 },
                 NamespaceConstraint::Any => (),
             }
             match local_name {
                 Some(name) => {
-                    compound_selector.push(SimpleSelector::LocalName(LocalName {
+                    sequence.push(SimpleSelector::LocalName(LocalName {
                         lower_name: from_ascii_lowercase(&name),
                         name: from_cow_str(name),
                     }))
                 }
                 None => (),
             }
-            Ok(Some(compound_selector))
+            Ok(true)
         }
     }
 }
@@ -1051,7 +1162,7 @@ fn parse_negation<P, Impl>(parser: &P,
                            -> Result<SimpleSelector<Impl>, ()>
     where P: Parser<Impl=Impl>, Impl: SelectorImpl
 {
-    input.parse_comma_separated(|input| ComplexSelector::parse(parser, input).map(Arc::new))
+    input.parse_comma_separated(|input| ComplexSelector::parse(parser, input))
          .map(SimpleSelector::Negation)
 }
 
@@ -1062,8 +1173,9 @@ fn parse_negation<P, Impl>(parser: &P,
 /// `Err(())` means invalid selector
 fn parse_compound_selector<P, Impl>(
     parser: &P,
-    input: &mut CssParser)
-    -> Result<(Vec<SimpleSelector<Impl>>, Option<Impl::PseudoElement>), ()>
+    input: &mut CssParser,
+    mut sequence: &mut Vec<SimpleSelector<Impl>>)
+    -> Result<Option<Impl::PseudoElement>, ()>
     where P: Parser<Impl=Impl>, Impl: SelectorImpl
 {
     // Consume any leading whitespace.
@@ -1075,28 +1187,26 @@ fn parse_compound_selector<P, Impl>(
         }
     }
     let mut empty = true;
-    let mut compound_selector = match parse_type_selector(parser, input)? {
-        None => {
-            match parser.default_namespace() {
-                // If there was no explicit type selector, but there is a
-                // default namespace, there is an implicit "<defaultns>|*" type
-                // selector.
-                Some(url) => vec![SimpleSelector::Namespace(Namespace {
-                    prefix: None,
-                    url: url
-                })],
-                None => vec![],
-            }
+    if !parse_type_selector(parser, input, &mut sequence)? {
+        if let Some(url) = parser.default_namespace() {
+            // If there was no explicit type selector, but there is a
+            // default namespace, there is an implicit "<defaultns>|*" type
+            // selector.
+            sequence.push(SimpleSelector::Namespace(Namespace {
+                prefix: None,
+                url: url
+            }));
         }
-        Some(s) => { empty = false; s }
-    };
+    } else {
+        empty = false;
+    }
 
     let mut pseudo_element = None;
     loop {
         match parse_one_simple_selector(parser, input, /* inside_negation = */ false)? {
             None => break,
             Some(SimpleSelectorParseResult::SimpleSelector(s)) => {
-                compound_selector.push(s);
+                sequence.push(s);
                 empty = false
             }
             Some(SimpleSelectorParseResult::PseudoElement(p)) => {
@@ -1110,7 +1220,7 @@ fn parse_compound_selector<P, Impl>(
         // An empty selector is invalid.
         Err(())
     } else {
-        Ok((compound_selector, pseudo_element))
+        Ok(pseudo_element)
     }
 }
 
@@ -1244,7 +1354,6 @@ pub mod tests {
     use std::borrow::Cow;
     use std::collections::HashMap;
     use std::fmt;
-    use std::sync::Arc;
     use super::*;
 
     #[derive(PartialEq, Clone, Debug, Hash, Eq)]
@@ -1408,57 +1517,45 @@ pub mod tests {
         assert_eq!(parse(":lang(4)"), Err(())) ;
         assert_eq!(parse(":lang(en US)"), Err(())) ;
         assert_eq!(parse("EeÉ"), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(SimpleSelector::LocalName(LocalName {
+            inner: SelectorInner::from_vec(vec!(SimpleSelector::LocalName(LocalName {
                     name: DummyAtom::from("EeÉ"),
                     lower_name: DummyAtom::from("eeÉ") })),
-                next: None,
-            })),
+            ),
             pseudo_element: None,
             specificity: specificity(0, 0, 1),
         }))));
         assert_eq!(parse(".foo:lang(en-US)"), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec![
+            inner: SelectorInner::from_vec(vec![
                     SimpleSelector::Class(DummyAtom::from("foo")),
                     SimpleSelector::NonTSPseudoClass(PseudoClass::Lang("en-US".to_owned()))
-                ],
-                next: None,
-            })),
+            ]),
             pseudo_element: None,
             specificity: specificity(0, 2, 0),
         }))));
         assert_eq!(parse("#bar"), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(SimpleSelector::ID(DummyAtom::from("bar"))),
-                next: None,
-            })),
+            inner: SelectorInner::from_vec(vec!(SimpleSelector::ID(DummyAtom::from("bar")))),
             pseudo_element: None,
             specificity: specificity(1, 0, 0),
         }))));
         assert_eq!(parse("e.foo#bar"), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(SimpleSelector::LocalName(LocalName {
+            inner: SelectorInner::from_vec(vec!(SimpleSelector::LocalName(LocalName {
                                             name: DummyAtom::from("e"),
                                             lower_name: DummyAtom::from("e") }),
                                        SimpleSelector::Class(DummyAtom::from("foo")),
-                                       SimpleSelector::ID(DummyAtom::from("bar"))),
-                next: None,
-            })),
+                                       SimpleSelector::ID(DummyAtom::from("bar")))),
             pseudo_element: None,
             specificity: specificity(1, 1, 1),
         }))));
         assert_eq!(parse("e.foo #bar"), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(SimpleSelector::ID(DummyAtom::from("bar"))),
-                next: Some((Arc::new(ComplexSelector {
-                    compound_selector: vec!(SimpleSelector::LocalName(LocalName {
-                                                name: DummyAtom::from("e"),
-                                                lower_name: DummyAtom::from("e") }),
-                                           SimpleSelector::Class(DummyAtom::from("foo"))),
-                    next: None,
-                }), Combinator::Descendant)),
-            })),
+            inner: SelectorInner::from_vec(vec!(
+                           SimpleSelector::LocalName(LocalName {
+                               name: DummyAtom::from("e"),
+                               lower_name: DummyAtom::from("e")
+                           }),
+                           SimpleSelector::Class(DummyAtom::from("foo")),
+                           SimpleSelector::Combinator(Combinator::Descendant),
+                           SimpleSelector::ID(DummyAtom::from("bar")),
+                       )),
             pseudo_element: None,
             specificity: specificity(1, 1, 1),
         }))));
@@ -1466,25 +1563,22 @@ pub mod tests {
         // https://github.com/mozilla/servo/pull/1652
         let mut parser = DummyParser::default();
         assert_eq!(parse_ns("[Foo]", &parser), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(SimpleSelector::AttrExists(AttrSelector {
+            inner: SelectorInner::from_vec(vec!(
+                SimpleSelector::AttrExists(AttrSelector {
                     name: DummyAtom::from("Foo"),
                     lower_name: DummyAtom::from("foo"),
                     namespace: NamespaceConstraint::Specific(Namespace {
                         prefix: None,
                         url: "".into(),
-                    }),
-                })),
-                next: None,
-            })),
+                    }) }))),
             pseudo_element: None,
             specificity: specificity(0, 1, 0),
         }))));
         assert_eq!(parse_ns("svg|circle", &parser), Err(()));
         parser.ns_prefixes.insert(DummyAtom("svg".into()), DummyAtom(SVG.into()));
         assert_eq!(parse_ns("svg|circle", &parser), Ok(SelectorList(vec![Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec![
+            inner: SelectorInner::from_vec(
+                vec![
                     SimpleSelector::Namespace(Namespace {
                         prefix: Some(DummyAtom("svg".into())),
                         url: SVG.into(),
@@ -1493,9 +1587,7 @@ pub mod tests {
                         name: DummyAtom::from("circle"),
                         lower_name: DummyAtom::from("circle"),
                     })
-                ],
-                next: None,
-            })),
+                ]),
             pseudo_element: None,
             specificity: specificity(0, 0, 1),
         }])));
@@ -1505,8 +1597,8 @@ pub mod tests {
         // https://github.com/servo/rust-selectors/pull/82
         parser.default_ns = Some(MATHML.into());
         assert_eq!(parse_ns("[Foo]", &parser), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec![
+            inner: SelectorInner::from_vec(
+                vec![
                     SimpleSelector::Namespace(Namespace {
                         prefix: None,
                         url: MATHML.into(),
@@ -1519,16 +1611,14 @@ pub mod tests {
                             url: "".into(),
                         }),
                     }),
-                ],
-                next: None,
-            })),
+                ]),
             pseudo_element: None,
             specificity: specificity(0, 1, 0),
         }))));
         // Default namespace does apply to type selectors
         assert_eq!(parse_ns("e", &parser), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(
+            inner: SelectorInner::from_vec(
+                vec!(
                     SimpleSelector::Namespace(Namespace {
                         prefix: None,
                         url: MATHML.into(),
@@ -1536,15 +1626,13 @@ pub mod tests {
                     SimpleSelector::LocalName(LocalName {
                         name: DummyAtom::from("e"),
                         lower_name: DummyAtom::from("e") }),
-                ),
-                next: None,
-            })),
+                )),
             pseudo_element: None,
             specificity: specificity(0, 0, 1),
         }))));
         assert_eq!(parse("[attr |= \"foo\"]"), Ok(SelectorList(vec![Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec![
+            inner: SelectorInner::from_vec(
+                vec![
                     SimpleSelector::AttrDashMatch(AttrSelector {
                         name: DummyAtom::from("attr"),
                         lower_name: DummyAtom::from("attr"),
@@ -1553,70 +1641,48 @@ pub mod tests {
                             url: "".into(),
                         }),
                     }, DummyAtom::from("foo"))
-                ],
-                next: None,
-            })),
+                ]),
             pseudo_element: None,
             specificity: specificity(0, 1, 0),
         }])));
         // https://github.com/mozilla/servo/issues/1723
         assert_eq!(parse("::before"), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(),
-                next: None,
-            })),
+            inner: SelectorInner::from_vec(vec![]),
             pseudo_element: Some(PseudoElement::Before),
             specificity: specificity(0, 0, 1),
         }))));
         // https://github.com/servo/servo/issues/15335
         assert_eq!(parse(":: before"), Err(()));
         assert_eq!(parse("div ::after"), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(),
-                next: Some((Arc::new(ComplexSelector {
-                    compound_selector: vec!(SimpleSelector::LocalName(LocalName {
+            inner: SelectorInner::from_vec(
+                vec![
+                     SimpleSelector::LocalName(LocalName {
                         name: DummyAtom::from("div"),
-                        lower_name: DummyAtom::from("div") })),
-                    next: None,
-                }), Combinator::Descendant)),
-            })),
+                        lower_name: DummyAtom::from("div") }),
+                    SimpleSelector::Combinator(Combinator::Descendant),
+                ]),
             pseudo_element: Some(PseudoElement::After),
             specificity: specificity(0, 0, 2),
         }))));
         assert_eq!(parse("#d1 > .ok"), Ok(SelectorList(vec![Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec![
+            inner: SelectorInner::from_vec(
+                vec![
+                    SimpleSelector::ID(DummyAtom::from("d1")),
+                    SimpleSelector::Combinator(Combinator::Child),
                     SimpleSelector::Class(DummyAtom::from("ok")),
-                ],
-                next: Some((Arc::new(ComplexSelector {
-                    compound_selector: vec![
-                        SimpleSelector::ID(DummyAtom::from("d1")),
-                    ],
-                    next: None,
-                }), Combinator::Child)),
-            })),
+                ]),
             pseudo_element: None,
             specificity: (1 << 20) + (1 << 10) + (0 << 0),
         }])));
         assert_eq!(parse(":not(.babybel, #provel.old)"), Ok(SelectorList(vec!(Selector {
-            inner: SelectorInner::new(Arc::new(ComplexSelector {
-                compound_selector: vec!(SimpleSelector::Negation(
+            inner: SelectorInner::from_vec(vec!(SimpleSelector::Negation(
                     vec!(
-                        Arc::new(ComplexSelector {
-                            compound_selector: vec!(SimpleSelector::Class(DummyAtom::from("babybel"))),
-                            next: None
-                        }),
-                        Arc::new(ComplexSelector {
-                            compound_selector: vec!(
+                        ComplexSelector::from_vec(vec!(SimpleSelector::Class(DummyAtom::from("babybel")))),
+                        ComplexSelector::from_vec(vec!(
                                 SimpleSelector::ID(DummyAtom::from("provel")),
                                 SimpleSelector::Class(DummyAtom::from("old")),
-                            ),
-                            next: None
-                        }),
-                    )
-                )),
-                next: None,
-            })),
+                    )))
+            ))),
             pseudo_element: None,
             specificity: specificity(1, 1, 0),
         }))));

components/selectors/visitor.rs

@@ -6,7 +6,8 @@
 
 #![deny(missing_docs)]
 
-use parser::{AttrSelector, Combinator, ComplexSelector, SelectorImpl, SimpleSelector};
+use parser::{AttrSelector, Combinator, SelectorImpl};
+use parser::{SelectorIter, SimpleSelector};
 
 /// A trait to visit selector properties.
 ///
@@ -33,7 +34,7 @@ pub trait SelectorVisitor {
     /// Gets the combinator to the right of the selector, or `None` if the
     /// selector is the leftmost one.
     fn visit_complex_selector(&mut self,
-                              _: &ComplexSelector<Self::Impl>,
+                              _: SelectorIter<Self::Impl>,
                               _combinator_to_right: Option<Combinator>)
                               -> bool {
         true

components/style/gecko/selector_parser.rs

@@ -14,7 +14,6 @@ use selectors::visitor::SelectorVisitor;
 use std::borrow::Cow;
 use std::fmt;
 use std::ptr;
-use std::sync::Arc;
 use string_cache::{Atom, Namespace, WeakAtom, WeakNamespace};
 
 /// A representation of a CSS pseudo-element.
@@ -218,7 +217,7 @@ macro_rules! pseudo_class_name {
             ///
             /// TODO(emilio): We disallow combinators and pseudos here, so we
             /// should use SimpleSelector instead
-            MozAny(Vec<Arc<ComplexSelector<SelectorImpl>>>),
+            MozAny(Vec<ComplexSelector<SelectorImpl>>),
         }
     }
 }
@@ -396,10 +395,10 @@ impl<'a> ::selectors::Parser for SelectorParser<'a> {
                     }, )*
                     "-moz-any" => {
                         let selectors = parser.parse_comma_separated(|input| {
-                            ComplexSelector::parse(self, input).map(Arc::new)
+                            ComplexSelector::parse(self, input)
                         })?;
                         // Selectors inside `:-moz-any` may not include combinators.
-                        if selectors.iter().any(|s| s.next.is_some()) {
+                        if selectors.iter().flat_map(|x| x.iter_raw()).any(|s| s.is_combinator()) {
                             return Err(())
                         }
                         NonTSPseudoClass::MozAny(selectors)

components/style/restyle_hints.rs

@@ -17,10 +17,10 @@ use selector_parser::{AttrValue, NonTSPseudoClass, Snapshot, SelectorImpl};
 use selectors::{Element, MatchAttr};
 use selectors::matching::{ElementSelectorFlags, StyleRelations};
 use selectors::matching::matches_selector;
-use selectors::parser::{AttrSelector, Combinator, ComplexSelector, SelectorInner, SelectorMethods, SimpleSelector};
+use selectors::parser::{AttrSelector, Combinator, ComplexSelector, SelectorInner, SelectorIter};
+use selectors::parser::{SelectorMethods, SimpleSelector};
 use selectors::visitor::SelectorVisitor;
 use std::clone::Clone;
-use std::sync::Arc;
 
 bitflags! {
     /// When the ElementState of an element (like IN_HOVER_STATE) changes,
@@ -509,7 +509,7 @@ impl SelectorVisitor for SensitivitiesVisitor {
     type Impl = SelectorImpl;
 
     fn visit_complex_selector(&mut self,
-                              _: &ComplexSelector<SelectorImpl>,
+                              _: SelectorIter<SelectorImpl>,
                               combinator: Option<Combinator>) -> bool {
         self.hint |= combinator_to_restyle_hint(combinator);
         self.needs_revalidation |= self.hint.contains(RESTYLE_LATER_SIBLINGS);
@@ -567,50 +567,47 @@ impl DependencySet {
     /// cache revalidation, that is, whether two siblings of the same "shape"
     /// may have different style due to this selector.
     pub fn note_selector(&mut self,
-                         selector: &Arc<ComplexSelector<SelectorImpl>>)
+                         base: &ComplexSelector<SelectorImpl>)
                          -> bool
     {
+        let mut next = Some(base.clone());
         let mut combinator = None;
-        let mut current = selector;
-
         let mut needs_revalidation = false;
 
-        loop {
-            let mut sensitivities_visitor = SensitivitiesVisitor {
+        while let Some(current) = next.take() {
+            // Set up our visitor.
+            let mut visitor = SensitivitiesVisitor {
                 sensitivities: Sensitivities::new(),
-                hint: RestyleHint::empty(),
+                hint: combinator_to_restyle_hint(combinator),
                 needs_revalidation: false,
             };
 
-            for ss in &current.compound_selector {
-                ss.visit(&mut sensitivities_visitor);
+            {
+                // Visit all the simple selectors.
+                let mut iter = current.iter();
+                let mut index = 0usize;
+                for ss in &mut iter {
+                    ss.visit(&mut visitor);
+                    index += 1;
+                }
+
+                // Prepare the next sequence of simple selectors.
+                if let Some(next_combinator) = iter.next_sequence() {
+                    next = Some(current.slice_from(index + 1));
+                    combinator = Some(next_combinator);
+                }
             }
 
-            needs_revalidation |= sensitivities_visitor.needs_revalidation;
-
-            let SensitivitiesVisitor {
-                sensitivities,
-                mut hint,
-                ..
-            } = sensitivities_visitor;
-
-            hint |= combinator_to_restyle_hint(combinator);
-
-            if !sensitivities.is_empty() {
+            // Note what we found.
+            needs_revalidation |= visitor.needs_revalidation;
+            if !visitor.sensitivities.is_empty() {
                 self.add_dependency(Dependency {
-                    sensitivities: sensitivities,
-                    hint: hint,
-                    selector: SelectorInner::new(current.clone()),
+                    sensitivities: visitor.sensitivities,
+                    hint: visitor.hint,
+                    selector: SelectorInner::new(current),
                 })
             }
 
-            match current.next {
-                Some((ref next, next_combinator)) => {
-                    current = next;
-                    combinator = Some(next_combinator);
-                }
-                None => break,
-            }
         }
 
         needs_revalidation
@@ -729,7 +726,7 @@ fn smoke_restyle_hints() {
     let mut dependencies = DependencySet::new();
 
     let mut p = Parser::new(":not(:active) ~ label");
-    let selector = Arc::new(ComplexSelector::parse(&parser, &mut p).unwrap());
+    let selector = ComplexSelector::parse(&parser, &mut p).unwrap();
     dependencies.note_selector(&selector);
     assert_eq!(dependencies.len(), 1);
     assert_eq!(dependencies.state_deps.len(), 1);

components/style/stylist.rs

@@ -1070,8 +1070,7 @@ impl SelectorMap {
         // correct, and also to not trigger rule tree assertions.
         let mut important = vec![];
         for rule in self.other_rules.iter() {
-            if rule.selector.complex.compound_selector.is_empty() &&
-               rule.selector.complex.next.is_none() {
+            if rule.selector.complex.iter_raw().next().is_none() {
                 let style_rule = rule.style_rule.read_with(guard);
                 let block = style_rule.block.read_with(guard);
                 if block.any_normal() {
@@ -1180,7 +1179,7 @@ impl SelectorMap {
 
     /// Retrieve the first ID name in Rule, or None otherwise.
     pub fn get_id_name(rule: &Rule) -> Option<Atom> {
-        for ss in &rule.selector.complex.compound_selector {
+        for ss in rule.selector.complex.iter() {
             // TODO(pradeep): Implement case-sensitivity based on the
             // document type and quirks mode.
             if let SimpleSelector::ID(ref id) = *ss {
@@ -1193,7 +1192,7 @@ impl SelectorMap {
 
     /// Retrieve the FIRST class name in Rule, or None otherwise.
     pub fn get_class_name(rule: &Rule) -> Option<Atom> {
-        for ss in &rule.selector.complex.compound_selector {
+        for ss in rule.selector.complex.iter() {
             // TODO(pradeep): Implement case-sensitivity based on the
             // document type and quirks mode.
             if let SimpleSelector::Class(ref class) = *ss {
@@ -1206,7 +1205,7 @@ impl SelectorMap {
 
     /// Retrieve the name if it is a type selector, or None otherwise.
     pub fn get_local_name(rule: &Rule) -> Option<LocalNameSelector<SelectorImpl>> {
-        for ss in &rule.selector.complex.compound_selector {
+        for ss in rule.selector.complex.iter() {
             if let SimpleSelector::LocalName(ref n) = *ss {
                 return Some(LocalNameSelector {
                     name: n.name.clone(),