Restart selector matching from an appropriate compound selector

Introducing 3 matching failure statuses,

    NotMatchedAndRestartFromClosestLaterSibling
    NotMatchedAndRestartFromClosestDescendant
    NotMatchedGlobally

When NotMatchedGlobally appears, stop selector matching completely since
the succeeding selector never matches.
It is raised when
    Child combinator cannot find the candidate element
    Descendant combinator cannot find the candidate element

When NotMatchedAndRestartFromClosestDescendant appears, the selector
matching does backtracking and restarts from the closest Descendant
combinator.
It is raised when
    NextSibling combinator cannot find the candidate element.
    LaterSibling combinator cannot find the candidate element.
    Child combinator doesn't match on the found element.

When NotMatchedAndRestartFromClosestLaterSibling appears, the selector
matching does backtracking and restarts from the closest LaterSibling
combinator.
It is raised when
    NextSibling combinator doesn't match on the found element.

For example, when the selector "d1 d2 a" is provided and we cannot *find*
an appropriate ancestor node for "d1", this selector matching raises
NotMatchedGlobally since even if "d2" is moved to more upper node, the
candidates for "d1" becomes less than before and d1 .

The next example is siblings. When the selector "b1 + b2 ~ d1 a" is
providied and we cannot *find* an appropriate brother node for b1,
the selector matching raises NotMatchedAndRestartFromClosestDescendant.
The selectors ("b1 + b2 ~") doesn't match and matching restart from "d1".

The additional example is child and sibling. When the selector
"b1 + c1 > b2 ~ d1 a" is provided and the selector "b1" doesn't match on
the element, this "b1" raises NotMatchedAndRestartFromClosestLaterSibling.
However since the selector "c1" raises
NotMatchedAndRestartFromClosestDescendant. So the selector
"b1 + c1 > b2 ~ " doesn't match and restart matching from "d1".

src/components/style/selector_matching.rs

@@ -527,19 +527,80 @@ fn matches_compound_selector<E:TElement,
                              element: &N,
                              shareable: &mut bool)
                              -> bool {
+    match matches_compound_selector_internal(selector, element, shareable) {
+        Matched => true,
+        _ => false
+    }
+}
+
+/// A result of selector matching, includes 3 failure types,
+///
+///     NotMatchedAndRestartFromClosestLaterSibling
+///     NotMatchedAndRestartFromClosestDescendant
+///     NotMatchedGlobally
+///
+/// When NotMatchedGlobally appears, stop selector matching completely since
+/// the succeeding selectors never matches.
+/// It is raised when
+///     Child combinator cannot find the candidate element.
+///     Descendant combinator cannot find the candidate element.
+///
+/// When NotMatchedAndRestartFromClosestDescendant appears, the selector
+/// matching does backtracking and restarts from the closest Descendant
+/// combinator.
+/// It is raised when
+///     NextSibling combinator cannot find the candidate element.
+///     LaterSibling combinator cannot find the candidate element.
+///     Child combinator doesn't match on the found element.
+///
+/// When NotMatchedAndRestartFromClosestLaterSibling appears, the selector
+/// matching does backtracking and restarts from the closest LaterSibling
+/// combinator.
+/// It is raised when
+///     NextSibling combinator doesn't match on the found element.
+///
+/// For example, when the selector "d1 d2 a" is provided and we cannot *find*
+/// an appropriate ancestor node for "d1", this selector matching raises
+/// NotMatchedGlobally since even if "d2" is moved to more upper node, the
+/// candidates for "d1" becomes less than before and d1 .
+///
+/// The next example is siblings. When the selector "b1 + b2 ~ d1 a" is
+/// providied and we cannot *find* an appropriate brother node for b1,
+/// the selector matching raises NotMatchedAndRestartFromClosestDescendant.
+/// The selectors ("b1 + b2 ~") doesn't match and matching restart from "d1".
+///
+/// The additional example is child and sibling. When the selector
+/// "b1 + c1 > b2 ~ d1 a" is provided and the selector "b1" doesn't match on
+/// the element, this "b1" raises NotMatchedAndRestartFromClosestLaterSibling.
+/// However since the selector "c1" raises
+/// NotMatchedAndRestartFromClosestDescendant. So the selector
+/// "b1 + c1 > b2 ~ " doesn't match and restart matching from "d1".
+enum SelectorMatchingResult {
+    Matched,
+    NotMatchedAndRestartFromClosestLaterSibling,
+    NotMatchedAndRestartFromClosestDescendant,
+    NotMatchedGlobally,
+}
+
+fn matches_compound_selector_internal<E:TElement,
+                                      N:TNode<E>>(
+                                      selector: &CompoundSelector,
+                                      element: &N,
+                                      shareable: &mut bool)
+                                      -> SelectorMatchingResult {
     if !selector.simple_selectors.iter().all(|simple_selector| {
             matches_simple_selector(simple_selector, element, shareable)
     }) {
-        return false
+        return NotMatchedAndRestartFromClosestLaterSibling
     }
     match selector.next {
-        None => true,
+        None => Matched,
         Some((ref next_selector, combinator)) => {
-            let (siblings, just_one) = match combinator {
+            let (siblings, candidate_not_found) = match combinator {
-                Child => (false, true),
+                Child => (false, NotMatchedGlobally),
-                Descendant => (false, false),
+                Descendant => (false, NotMatchedGlobally),
-                NextSibling => (true, true),
+                NextSibling => (true, NotMatchedAndRestartFromClosestDescendant),
-                LaterSibling => (true, false),
+                LaterSibling => (true, NotMatchedAndRestartFromClosestDescendant),
             };
             let mut node = (*element).clone();
             loop {
@@ -549,14 +610,37 @@ fn matches_compound_selector<E:TElement,
                     node.parent_node()
                 };
                 match next_node {
-                    None => return false,
+                    None => return candidate_not_found,
                     Some(next_node) => node = next_node,
                 }
                 if node.is_element() {
-                    if matches_compound_selector(&**next_selector, &node, shareable) {
+                    let result = matches_compound_selector_internal(&**next_selector,
-                        return true
+                                                                    &node,
-                    } else if just_one {
+                                                                    shareable);
-                        return false
+                    match (result, combinator) {
+                        // Return the status immediately.
+                        (Matched, _) => return result,
+                        (NotMatchedGlobally, _) => return result,
+
+                        // Upgrade the failure status to
+                        // NotMatchedAndRestartFromClosestDescendant.
+                        (_, Child) => return NotMatchedAndRestartFromClosestDescendant,
+
+                        // Return the status directly.
+                        (_, NextSibling) => return result,
+
+                        // If the failure status is NotMatchedAndRestartFromClosestDescendant
+                        // and combinator is LaterSibling, give up this LaterSibling matching
+                        // and restart from the closest descendant combinator.
+                        (NotMatchedAndRestartFromClosestDescendant, LaterSibling) => return result,
+
+                        // The Descendant combinator and the status is
+                        // NotMatchedAndRestartFromClosestLaterSibling or
+                        // NotMatchedAndRestartFromClosestDescendant,
+                        // or the LaterSibling combinator and the status is
+                        // NotMatchedAndRestartFromClosestDescendant
+                        // can continue to matching on the next candidate element.
+                        _ => {},
                     }
                 }
             }