Log element during selector matching

MozReview-Commit-ID: D8eFyRCy5BR

components/style/restyle_hints.rs

@@ -29,6 +29,7 @@ use smallvec::SmallVec;
 use std::cell::Cell;
 use std::clone::Clone;
 use std::cmp;
+use std::fmt;
 
 /// When the ElementState of an element (like IN_HOVER_STATE) changes,
 /// certain pseudo-classes (like :hover) may require us to restyle that
@@ -607,6 +608,15 @@ impl<'a, E> ElementWrapper<'a, E>
     }
 }
 
+impl<'a, E> fmt::Debug for ElementWrapper<'a, E>
+    where E: TElement,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        // Ignore other fields for now, can change later if needed.
+        self.element.fmt(f)
+    }
+}
+
 #[cfg(feature = "gecko")]
 fn dir_selector_to_state(s: &[u16]) -> ElementState {
     // Jump through some hoops to deal with our Box<[u16]> thing.