selectors: Add a MatchingContext::nest function, make nesting_level private.

components/selectors/context.rs

@@ -122,9 +122,9 @@ where
 
     /// The current nesting level of selectors that we're matching.
     ///
-    /// FIXME(emilio): Move this somewhere else and make MatchingContext
+    /// FIXME(emilio): Consider putting the mutable stuff in a Cell.
     /// immutable again.
-    pub nesting_level: usize,
+    nesting_level: usize,
 
     /// An optional hook function for checking whether a pseudo-element
     /// should match when matching_mode is ForStatelessPseudoElement.
@@ -181,6 +181,12 @@ where
         }
     }
 
+    /// How many times deep are we in a selector.
+    #[inline]
+    pub fn nesting_level(&self) -> usize {
+        self.nesting_level
+    }
+
     /// The quirks mode of the document.
     #[inline]
     pub fn quirks_mode(&self) -> QuirksMode {
@@ -192,4 +198,16 @@ where
     pub fn classes_and_ids_case_sensitivity(&self) -> CaseSensitivity {
         self.classes_and_ids_case_sensitivity
     }
+
+    /// Runs F with a deeper nesting level.
+    #[inline]
+    pub fn nest<F, R>(&mut self, f: F) -> R
+    where
+        F: FnOnce(&mut Self) -> R,
+    {
+        self.nesting_level += 1;
+        let result = f(self);
+        self.nesting_level -= 1;
+        result
+    }
 }

components/selectors/matching.rs

@@ -289,7 +289,7 @@ where
     // If this is the special pseudo-element mode, consume the ::pseudo-element
     // before proceeding, since the caller has already handled that part.
     if context.matching_mode == MatchingMode::ForStatelessPseudoElement &&
-        context.nesting_level == 0 {
+        context.nesting_level() == 0 {
         // Consume the pseudo.
         match *iter.next().unwrap() {
             Component::PseudoElement(ref pseudo) => {
@@ -347,7 +347,7 @@ fn matches_hover_and_active_quirk<Impl: SelectorImpl>(
         return MatchesHoverAndActiveQuirk::No;
     }
 
-    if context.nesting_level != 0 {
+    if context.nesting_level() != 0 {
         return MatchesHoverAndActiveQuirk::No;
     }
 
@@ -639,17 +639,15 @@ where
     match *selector {
         Component::Combinator(_) => unreachable!(),
         Component::Slotted(ref selector) => {
-            context.shared.nesting_level += 1;
-            let result =
+            context.shared.nest(|context| {
                 element.assigned_slot().is_some() &&
                 matches_complex_selector(
                     selector.iter(),
                     element,
-                    context.shared,
+                    context,
                     flags_setter,
-                );
-            context.shared.nesting_level -= 1;
-            result
+                )
+            })
         }
         Component::PseudoElement(ref pseudo) => {
             element.match_pseudo_element(pseudo, context.shared)
@@ -731,7 +729,7 @@ where
         }
         Component::NonTSPseudoClass(ref pc) => {
             if context.matches_hover_and_active_quirk == MatchesHoverAndActiveQuirk::Yes &&
-               context.shared.nesting_level == 0 &&
+               context.shared.nesting_level() == 0 &&
                E::Impl::is_active_or_hover(pc) &&
                !element.is_link() {
                 return false;
@@ -790,17 +788,22 @@ where
             matches_generic_nth_child(element, context, 0, 1, true, true, flags_setter)
         }
         Component::Negation(ref negated) => {
-            context.shared.nesting_level += 1;
-            let result = !negated.iter().all(|ss| {
-                matches_simple_selector(
-                    ss,
-                    element,
-                    context,
-                    flags_setter,
-                )
-            });
-            context.shared.nesting_level -= 1;
-            result
+            let visited_handling = context.visited_handling;
+            context.shared.nest(|context| {
+                let mut local_context = LocalMatchingContext {
+                    visited_handling,
+                    matches_hover_and_active_quirk: MatchesHoverAndActiveQuirk::No,
+                    shared: context,
+                };
+                !negated.iter().all(|ss| {
+                    matches_simple_selector(
+                        ss,
+                        element,
+                        &mut local_context,
+                        flags_setter,
+                    )
+                })
+            })
         }
     }
 }

components/style/gecko/wrapper.rs

@@ -2119,12 +2119,11 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             }
             NonTSPseudoClass::MozPlaceholder => false,
             NonTSPseudoClass::MozAny(ref sels) => {
-                context.nesting_level += 1;
-                let result = sels.iter().any(|s| {
-                    matches_complex_selector(s.iter(), self, context, flags_setter)
-                });
-                context.nesting_level -= 1;
-                result
+                context.nest(|context| {
+                    sels.iter().any(|s| {
+                        matches_complex_selector(s.iter(), self, context, flags_setter)
+                    })
+                })
             }
             NonTSPseudoClass::Lang(ref lang_arg) => {
                 self.match_element_lang(None, lang_arg)

components/style/invalidation/element/element_wrapper.rs

@@ -162,12 +162,11 @@ impl<'a, E> Element for ElementWrapper<'a, E>
             #[cfg(feature = "gecko")]
             NonTSPseudoClass::MozAny(ref selectors) => {
                 use selectors::matching::matches_complex_selector;
-                context.nesting_level += 1;
-                let result = selectors.iter().any(|s| {
-                    matches_complex_selector(s.iter(), self, context, _setter)
+                return context.nest(|context| {
+                    selectors.iter().any(|s| {
+                        matches_complex_selector(s.iter(), self, context, _setter)
+                    })
                 });
-                context.nesting_level -= 1;
-                return result
             }
 
             // :dir is implemented in terms of state flags, but which state flag