Avoid creating a vector in can_share_style_with.

components/layout/css/matching.rs

@@ -20,6 +20,7 @@ use selectors::bloom::BloomFilter;
 use selectors::matching::{CommonStyleAffectingAttributeMode, CommonStyleAffectingAttributes};
 use selectors::matching::{common_style_affecting_attributes, rare_style_affecting_attributes};
 use selectors::parser::PseudoElement;
+use selectors::smallvec::VecLike;
 use std::borrow::ToOwned;
 use std::hash::{Hash, Hasher};
 use std::mem;
@@ -289,9 +290,29 @@ impl StyleSharingCandidate {
             return false
         }
 
-        let mut matching_rules = vec![];
+        struct RulesSink {
+            empty: bool,
+        }
+        impl<T> VecLike<T> for RulesSink {
+            #[inline]
+            fn vec_len(&self) -> usize {
+                unreachable!()
+            }
+
+            #[inline]
+            fn vec_push(&mut self, _value: T) {
+                self.empty = false;
+            }
+
+            #[inline]
+            fn vec_slice_mut<'a>(&'a mut self, _start: usize, _end: usize)
+                                 -> &'a mut [T] {
+                unreachable!()
+            }
+        }
+        let mut matching_rules = RulesSink { empty: true };
         element.synthesize_presentational_hints_for_legacy_attributes(&mut matching_rules);
-        if !matching_rules.is_empty() {
+        if !matching_rules.empty {
             return false;
         }