style: Derive debug for CascadeInputs.

It no longer has anything than rules.

components/style/context.rs

@@ -203,7 +203,7 @@ impl<'a> SharedStyleContext<'a> {
 /// within the `CurrentElementInfo`. At the end of the cascade, they are folded
 /// down into the main `ComputedValues` to reduce memory usage per element while
 /// still remaining accessible.
-#[derive(Clone, Default)]
+#[derive(Clone, Debug, Default)]
 pub struct CascadeInputs {
     /// The rule node representing the ordered list of rules matched for this
     /// node.
@@ -226,15 +226,6 @@ impl CascadeInputs {
     }
 }
 
-// We manually implement Debug for CascadeInputs so that we can avoid the
-// verbose stringification of ComputedValues for normal logging.
-impl fmt::Debug for CascadeInputs {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "CascadeInputs {{ rules: {:?}, visited_rules: {:?}, .. }}",
-               self.rules, self.visited_rules)
-    }
-}
-
 /// A list of cascade inputs for eagerly-cascaded pseudo-elements.
 /// The list is stored inline.
 #[derive(Debug)]