Add a custom Debug formatter for ClippingRegion

This will make display list dumps more likely to fit on a single line.

components/gfx/display_list/mod.rs

@@ -897,7 +897,7 @@ impl BaseDisplayItem {
 /// A clipping region for a display item. Currently, this can describe rectangles, rounded
 /// rectangles (for `border-radius`), or arbitrary intersections of the two. Arbitrary transforms
 /// are not supported because those are handled by the higher-level `StackingContext` abstraction.
-#[derive(Clone, PartialEq, Debug, HeapSizeOf, Deserialize, Serialize)]
+#[derive(Clone, PartialEq, HeapSizeOf, Deserialize, Serialize)]
 pub struct ClippingRegion {
     /// The main rectangular region. This does not include any corners.
     pub main: Rect<Au>,
@@ -1043,6 +1043,20 @@ impl ClippingRegion {
     }
 }
 
+impl fmt::Debug for ClippingRegion {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if *self == ClippingRegion::max() {
+            write!(f, "ClippingRegion::Max")
+        } else if *self == ClippingRegion::empty() {
+            write!(f, "ClippingRegion::Empty")
+        } else if self.main == max_rect() {
+            write!(f, "ClippingRegion(Complex={:?})", self.complex)
+        } else {
+            write!(f, "ClippingRegion(Rect={:?}, Complex={:?})", self.main, self.complex)
+        }
+    }
+}
+
 impl ComplexClippingRegion {
     // TODO(pcwalton): This could be more aggressive by considering points that touch the inside of
     // the border radius ellipse.