Add some fmt::Debug implementations

components/script/dom/element.rs

@@ -79,6 +79,7 @@ use std::borrow::Cow;
 use std::cell::{Cell, Ref};
 use std::convert::TryFrom;
 use std::default::Default;
+use std::fmt;
 use std::mem;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicUsize, Ordering};
@@ -115,6 +116,16 @@ pub struct Element {
     atomic_flags: AtomicElementFlags,
 }
 
+impl fmt::Debug for Element {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        try!(write!(f, "<{}", self.local_name));
+        if let Some(ref id) = *self.id_attribute.borrow() {
+            try!(write!(f, " id={}", id));
+        }
+        write!(f, ">")
+    }
+}
+
 #[derive(PartialEq, HeapSizeOf)]
 pub enum ElementCreator {
     ParserCreated,

components/script/layout_wrapper.rs

@@ -49,6 +49,7 @@ use script_layout_interface::{HTMLCanvasData, LayoutNodeType, TrustedNodeAddress
 use script_layout_interface::{OpaqueStyleAndLayoutData, PartialStyleAndLayoutData};
 use selectors::matching::{DeclarationBlock, ElementFlags};
 use selectors::parser::{AttrSelector, NamespaceConstraint};
+use std::fmt;
 use std::marker::PhantomData;
 use std::mem::{transmute, transmute_copy};
 use std::sync::Arc;
@@ -405,6 +406,16 @@ pub struct ServoLayoutElement<'le> {
     chain: PhantomData<&'le ()>,
 }
 
+impl<'le> fmt::Debug for ServoLayoutElement<'le> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        try!(write!(f, "<{}", self.element.local_name()));
+        if let &Some(ref id) = unsafe { &*self.element.id_attribute() } {
+            try!(write!(f, " id={}", id));
+        }
+        write!(f, ">")
+    }
+}
+
 impl<'le> PresentationalHintsSynthetizer for ServoLayoutElement<'le> {
     fn synthesize_presentational_hints_for_legacy_attributes<V>(&self, hints: &mut V)
         where V: Push<DeclarationBlock<Vec<PropertyDeclaration>>>
@@ -926,7 +937,7 @@ impl<ConcreteNode> Iterator for ThreadSafeLayoutNodeChildrenIterator<ConcreteNod
 
 /// A wrapper around elements that ensures layout can only
 /// ever access safe properties and cannot race on elements.
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub struct ServoThreadSafeLayoutElement<'le> {
     element: &'le Element,
 }