path: root/components/layout/wrapper.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

//! A safe wrapper for DOM nodes that prevents layout from mutating the DOM, from letting DOM nodes
//! escape, and from generally doing anything that it isn't supposed to. This is accomplished via
//! a simple whitelist of allowed operations, along with some lifetime magic to prevent nodes from
//! escaping.
//!
//! As a security wrapper is only as good as its whitelist, be careful when adding operations to
//! this list. The cardinal rules are:
//!
//! 1. Layout is not allowed to mutate the DOM.
//!
//! 2. Layout is not allowed to see anything with `LayoutJS` in the name, because it could hang
//!    onto these objects and cause use-after-free.
//!
//! When implementing wrapper functions, be careful that you do not touch the borrow flags, or you
//! will race and cause spurious thread failure. (Note that I do not believe these races are
//! exploitable, but they'll result in brokenness nonetheless.)
//!
//! Rules of the road for this file:
//!
//! * Do not call any methods on DOM nodes without checking to see whether they use borrow flags.
//!
//!   o Instead of `get_attr()`, use `.get_attr_val_for_layout()`.
//!
//!   o Instead of `html_element_in_html_document()`, use
//!     `html_element_in_html_document_for_layout()`.

#![allow(unsafe_code)]

use core::nonzero::NonZero;
use data::{LayoutDataFlags, PrivateLayoutData};
use gfx::display_list::OpaqueNode;
use gfx::text::glyph::ByteIndex;
use gfx_traits::{LayerId, LayerType};
use msg::constellation_msg::PipelineId;
use opaque_node::OpaqueNodeMethods;
use range::Range;
use script::layout_interface::{CAN_BE_FRAGMENTED, HAS_CHANGED, HAS_DIRTY_DESCENDANTS, IS_DIRTY};
use script::layout_interface::{CharacterDataTypeId, Document, Element, ElementTypeId};
use script::layout_interface::{HTMLCanvasData, HTMLElementTypeId, LayoutCharacterDataHelpers};
use script::layout_interface::{LayoutDocumentHelpers, LayoutElementHelpers, LayoutJS};
use script::layout_interface::{LayoutNodeHelpers, Node, NodeTypeId};
use script::layout_interface::{RawLayoutElementHelpers, Text, TrustedNodeAddress};
use script_layout_interface::LayoutNodeType;
use script_layout_interface::restyle_damage::RestyleDamage;
use script_layout_interface::{OpaqueStyleAndLayoutData, PartialStyleAndLayoutData};
use selectors::matching::{DeclarationBlock, ElementFlags};
use selectors::parser::{AttrSelector, NamespaceConstraint};
use smallvec::VecLike;
use std::cell::{Ref, RefCell, RefMut};
use std::marker::PhantomData;
use std::mem::{transmute, transmute_copy};
use std::sync::Arc;
use string_cache::{Atom, BorrowedAtom, BorrowedNamespace, Namespace};
use style::attr::AttrValue;
use style::computed_values::content::ContentItem;
use style::computed_values::{content, display};
use style::dom::{PresentationalHintsSynthetizer, TDocument, TElement, TNode, UnsafeNode};
use style::element_state::*;
use style::properties::{ComputedValues, ServoComputedValues};
use style::properties::{PropertyDeclaration, PropertyDeclarationBlock};
use style::restyle_hints::ElementSnapshot;
use style::selector_impl::{NonTSPseudoClass, PseudoElement, PseudoElementCascadeType, ServoSelectorImpl};
use style::servo::{PrivateStyleData, SharedStyleContext};
use url::Url;
use util::str::is_whitespace;

pub type NonOpaqueStyleAndLayoutData = *mut RefCell<PrivateLayoutData>;

/// A wrapper so that layout can access only the methods that it should have access to. Layout must
/// only ever see these and must never see instances of `LayoutJS`.

pub trait LayoutNode: TNode {
    type ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode;
    fn to_threadsafe(&self) -> Self::ConcreteThreadSafeLayoutNode;

    /// Returns the type ID of this node.
    fn type_id(&self) -> LayoutNodeType;

    fn get_style_data(&self) -> Option<&RefCell<PartialStyleAndLayoutData>>;

    fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData);
    fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData>;
}

pub trait LayoutNodeLayoutData {
    /// Similar to borrow_data*, but returns the full PrivateLayoutData rather
    /// than only the PrivateStyleData.
    unsafe fn borrow_layout_data_unchecked(&self) -> Option<*const PrivateLayoutData>;
    fn borrow_layout_data(&self) -> Option<Ref<PrivateLayoutData>>;
    fn mutate_layout_data(&self) -> Option<RefMut<PrivateLayoutData>>;
    fn initialize_data(self);
    fn flow_debug_id(self) -> usize;
}

#[derive(Copy, Clone)]
pub struct ServoLayoutNode<'a> {
    /// The wrapped node.
    node: LayoutJS<Node>,

    /// Being chained to a PhantomData prevents `LayoutNode`s from escaping.
    chain: PhantomData<&'a ()>,
}

impl<'a> PartialEq for ServoLayoutNode<'a> {
    #[inline]
    fn eq(&self, other: &ServoLayoutNode) -> bool {
        self.node == other.node
    }
}

impl<'ln> ServoLayoutNode<'ln> {
    fn from_layout_js(n: LayoutJS<Node>) -> ServoLayoutNode<'ln> {
        ServoLayoutNode {
            node: n,
            chain: PhantomData,
        }
    }

    pub unsafe fn new(address: &TrustedNodeAddress) -> ServoLayoutNode {
        ServoLayoutNode::from_layout_js(LayoutJS::from_trusted_node_address(*address))
    }

    /// Creates a new layout node with the same lifetime as this layout node.
    pub unsafe fn new_with_this_lifetime(&self, node: &LayoutJS<Node>) -> ServoLayoutNode<'ln> {
        ServoLayoutNode {
            node: *node,
            chain: self.chain,
        }
    }

    fn script_type_id(&self) -> NodeTypeId {
        unsafe {
            self.node.type_id_for_layout()
        }
    }
}

impl<'ln> TNode for ServoLayoutNode<'ln> {
    type ConcreteComputedValues = ServoComputedValues;
    type ConcreteElement = ServoLayoutElement<'ln>;
    type ConcreteDocument = ServoLayoutDocument<'ln>;
    type ConcreteRestyleDamage = RestyleDamage;

    fn to_unsafe(&self) -> UnsafeNode {
        unsafe {
            let ptr: usize = transmute_copy(self);
            (ptr, 0)
        }
    }

    unsafe fn from_unsafe(n: &UnsafeNode) -> Self {
        let (node, _) = *n;
        transmute(node)
    }

    fn is_text_node(&self) -> bool {
        self.script_type_id() == NodeTypeId::CharacterData(CharacterDataTypeId::Text)
    }

    fn is_element(&self) -> bool {
        unsafe {
            self.node.is_element_for_layout()
        }
    }

    fn dump(self) {
        self.dump_indent(0);
    }

    fn opaque(&self) -> OpaqueNode {
        OpaqueNodeMethods::from_jsmanaged(unsafe { self.get_jsmanaged() })
    }

    fn layout_parent_node(self, reflow_root: OpaqueNode) -> Option<ServoLayoutNode<'ln>> {
        if self.opaque() == reflow_root {
            None
        } else {
            self.parent_node()
        }
    }

    fn debug_id(self) -> usize {
        self.opaque().to_untrusted_node_address().0 as usize
    }

    fn children_count(&self) -> u32 {
        unsafe { self.node.children_count() }
    }

    fn as_element(&self) -> Option<ServoLayoutElement<'ln>> {
        as_element(self.node)
    }

    fn as_document(&self) -> Option<ServoLayoutDocument<'ln>> {
        self.node.downcast().map(ServoLayoutDocument::from_layout_js)
    }

    fn has_changed(&self) -> bool {
        unsafe { self.node.get_flag(HAS_CHANGED) }
    }

    unsafe fn set_changed(&self, value: bool) {
        self.node.set_flag(HAS_CHANGED, value)
    }

    fn is_dirty(&self) -> bool {
        unsafe { self.node.get_flag(IS_DIRTY) }
    }

    unsafe fn set_dirty(&self, value: bool) {
        self.node.set_flag(IS_DIRTY, value)
    }

    fn has_dirty_descendants(&self) -> bool {
        unsafe { self.node.get_flag(HAS_DIRTY_DESCENDANTS) }
    }

    unsafe fn set_dirty_descendants(&self, value: bool) {
        self.node.set_flag(HAS_DIRTY_DESCENDANTS, value)
    }

    fn can_be_fragmented(&self) -> bool {
        unsafe { self.node.get_flag(CAN_BE_FRAGMENTED) }
    }

    unsafe fn set_can_be_fragmented(&self, value: bool) {
        self.node.set_flag(CAN_BE_FRAGMENTED, value)
    }

    unsafe fn borrow_data_unchecked(&self) -> Option<*const PrivateStyleData> {
        self.get_style_data().map(|d| {
            &(*d.as_unsafe_cell().get()).style_data as *const _
        })
    }

    fn borrow_data(&self) -> Option<Ref<PrivateStyleData>> {
        self.get_style_data().map(|d| {
            Ref::map(d.borrow(), |d| &d.style_data)
        })
    }

    fn mutate_data(&self) -> Option<RefMut<PrivateStyleData>> {
        self.get_style_data().map(|d| {
            RefMut::map(d.borrow_mut(), |d| &mut d.style_data)
        })
    }

    fn restyle_damage(self) -> RestyleDamage {
        self.get_style_data().unwrap().borrow().restyle_damage
    }

    fn set_restyle_damage(self, damage: RestyleDamage) {
        self.get_style_data().unwrap().borrow_mut().restyle_damage = damage;
    }

    fn parent_node(&self) -> Option<ServoLayoutNode<'ln>> {
        unsafe {
            self.node.parent_node_ref().map(|node| self.new_with_this_lifetime(&node))
        }
    }

    fn first_child(&self) -> Option<ServoLayoutNode<'ln>> {
        unsafe {
            self.node.first_child_ref().map(|node| self.new_with_this_lifetime(&node))
        }
    }

    fn last_child(&self) -> Option<ServoLayoutNode<'ln>> {
        unsafe {
            self.node.last_child_ref().map(|node| self.new_with_this_lifetime(&node))
        }
    }

    fn prev_sibling(&self) -> Option<ServoLayoutNode<'ln>> {
        unsafe {
            self.node.prev_sibling_ref().map(|node| self.new_with_this_lifetime(&node))
        }
    }

    fn next_sibling(&self) -> Option<ServoLayoutNode<'ln>> {
        unsafe {
            self.node.next_sibling_ref().map(|node| self.new_with_this_lifetime(&node))
        }
    }
}

impl<'ln> LayoutNode for ServoLayoutNode<'ln> {
    type ConcreteThreadSafeLayoutNode = ServoThreadSafeLayoutNode<'ln>;

    fn to_threadsafe(&self) -> Self::ConcreteThreadSafeLayoutNode {
        ServoThreadSafeLayoutNode::new(self)
    }

    fn type_id(&self) -> LayoutNodeType {
        self.script_type_id().into()
    }

    fn get_style_data(&self) -> Option<&RefCell<PartialStyleAndLayoutData>> {
        unsafe {
            self.get_jsmanaged().get_style_and_layout_data().map(|d| {
                &**d.ptr
            })
        }
    }

    fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData) {
        unsafe {
            self.get_jsmanaged().init_style_and_layout_data(data);
        }
    }

    fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData> {
        unsafe {
            self.get_jsmanaged().get_style_and_layout_data()
        }
    }
}

impl<T: LayoutNode> LayoutNodeLayoutData for T {
    unsafe fn borrow_layout_data_unchecked(&self) -> Option<*const PrivateLayoutData> {
        self.get_style_and_layout_data().map(|opaque| {
            let container = *opaque.ptr as NonOpaqueStyleAndLayoutData;
            &(*(*container).as_unsafe_cell().get()) as *const PrivateLayoutData
        })
    }

    fn borrow_layout_data(&self) -> Option<Ref<PrivateLayoutData>> {
        unsafe {
            self.get_style_and_layout_data().map(|opaque| {
                let container = *opaque.ptr as NonOpaqueStyleAndLayoutData;
                (*container).borrow()
            })
        }
    }

    fn mutate_layout_data(&self) -> Option<RefMut<PrivateLayoutData>> {
        unsafe {
            self.get_style_and_layout_data().map(|opaque| {
                let container = *opaque.ptr as NonOpaqueStyleAndLayoutData;
                (*container).borrow_mut()
            })
        }
    }

    fn initialize_data(self) {
        if unsafe { self.borrow_data_unchecked() }.is_none() {
            let ptr: NonOpaqueStyleAndLayoutData =
                Box::into_raw(box RefCell::new(PrivateLayoutData::new()));
            let opaque = OpaqueStyleAndLayoutData {
                ptr: unsafe { NonZero::new(ptr as *mut RefCell<PartialStyleAndLayoutData>) }
            };
            self.init_style_and_layout_data(opaque);
        }
    }

    fn flow_debug_id(self) -> usize {
        self.borrow_layout_data().map_or(0, |d| d.flow_construction_result.debug_id())
    }
}


impl<'ln> ServoLayoutNode<'ln> {
    fn dump_indent(self, indent: u32) {
        let mut s = String::new();
        for _ in 0..indent {
            s.push_str("  ");
        }

        s.push_str(&self.debug_str());
        println!("{}", s);

        for kid in self.children() {
            kid.dump_indent(indent + 1);
        }
    }

    fn debug_str(self) -> String {
        format!("{:?}: changed={} dirty={} dirty_descendants={}",
                self.script_type_id(), self.has_changed(), self.is_dirty(), self.has_dirty_descendants())
    }

    /// Returns the interior of this node as a `LayoutJS`. This is highly unsafe for layout to
    /// call and as such is marked `unsafe`.
    unsafe fn get_jsmanaged(&self) -> &LayoutJS<Node> {
        &self.node
    }
}

// A wrapper around documents that ensures ayout can only ever access safe properties.
#[derive(Copy, Clone)]
pub struct ServoLayoutDocument<'ld> {
    document: LayoutJS<Document>,
    chain: PhantomData<&'ld ()>,
}

impl<'ld> TDocument for ServoLayoutDocument<'ld> {
    type ConcreteNode = ServoLayoutNode<'ld>;
    type ConcreteElement = ServoLayoutElement<'ld>;

    fn as_node(&self) -> ServoLayoutNode<'ld> {
        ServoLayoutNode::from_layout_js(self.document.upcast())
    }

    fn root_node(&self) -> Option<ServoLayoutNode<'ld>> {
        self.as_node().children().find(ServoLayoutNode::is_element)
    }

    fn drain_modified_elements(&self) -> Vec<(ServoLayoutElement<'ld>, ElementSnapshot)> {
        let elements =  unsafe { self.document.drain_modified_elements() };
        elements.into_iter().map(|(el, snapshot)| (ServoLayoutElement::from_layout_js(el), snapshot)).collect()
    }
}

impl<'ld> ServoLayoutDocument<'ld> {
    fn from_layout_js(doc: LayoutJS<Document>) -> ServoLayoutDocument<'ld> {
        ServoLayoutDocument {
            document: doc,
            chain: PhantomData,
        }
    }
}

/// A wrapper around elements that ensures layout can only ever access safe properties.
#[derive(Copy, Clone)]
pub struct ServoLayoutElement<'le> {
    element: LayoutJS<Element>,
    chain: PhantomData<&'le ()>,
}

impl<'le> PresentationalHintsSynthetizer for ServoLayoutElement<'le> {
    fn synthesize_presentational_hints_for_legacy_attributes<V>(&self, hints: &mut V)
        where V: VecLike<DeclarationBlock<Vec<PropertyDeclaration>>>
    {
        unsafe {
            self.element.synthesize_presentational_hints_for_legacy_attributes(hints);
        }
    }
}

impl<'le> TElement for ServoLayoutElement<'le> {
    type ConcreteNode = ServoLayoutNode<'le>;
    type ConcreteDocument = ServoLayoutDocument<'le>;

    fn as_node(&self) -> ServoLayoutNode<'le> {
        ServoLayoutNode::from_layout_js(self.element.upcast())
    }

    fn style_attribute(&self) -> &Option<PropertyDeclarationBlock> {
        unsafe {
            &*self.element.style_attribute()
        }
    }

    fn get_state(&self) -> ElementState {
        self.element.get_state_for_layout()
    }

    #[inline]
    fn get_attr(&self, namespace: &Namespace, name: &Atom) -> Option<&str> {
        unsafe {
            (*self.element.unsafe_get()).get_attr_val_for_layout(namespace, name)
        }
    }

    #[inline]
    fn get_attrs(&self, name: &Atom) -> Vec<&str> {
        unsafe {
            (*self.element.unsafe_get()).get_attr_vals_for_layout(name)
        }
    }
}


impl<'le> ServoLayoutElement<'le> {
    fn from_layout_js(el: LayoutJS<Element>) -> ServoLayoutElement<'le> {
        ServoLayoutElement {
            element: el,
            chain: PhantomData,
        }
    }
}

fn as_element<'le>(node: LayoutJS<Node>) -> Option<ServoLayoutElement<'le>> {
    node.downcast().map(ServoLayoutElement::from_layout_js)
}

impl<'le> ::selectors::Element for ServoLayoutElement<'le> {
    type Impl = ServoSelectorImpl;

    fn parent_element(&self) -> Option<ServoLayoutElement<'le>> {
        unsafe {
            self.element.upcast().parent_node_ref().and_then(as_element)
        }
    }

    fn first_child_element(&self) -> Option<ServoLayoutElement<'le>> {
        self.as_node().children().filter_map(|n| n.as_element()).next()
    }

    fn last_child_element(&self) -> Option<ServoLayoutElement<'le>> {
        self.as_node().rev_children().filter_map(|n| n.as_element()).next()
    }

    fn prev_sibling_element(&self) -> Option<ServoLayoutElement<'le>> {
        let mut node = self.as_node();
        while let Some(sibling) = node.prev_sibling() {
            if let Some(element) = sibling.as_element() {
                return Some(element)
            }
            node = sibling;
        }
        None
    }

    fn next_sibling_element(&self) -> Option<ServoLayoutElement<'le>> {
        let mut node = self.as_node();
        while let Some(sibling) = node.next_sibling() {
            if let Some(element) = sibling.as_element() {
                return Some(element)
            }
            node = sibling;
        }
        None
    }

    fn is_root(&self) -> bool {
        match self.as_node().parent_node() {
            None => false,
            Some(node) => {
                match node.script_type_id() {
                    NodeTypeId::Document(_) => true,
                    _ => false
                }
            },
        }
    }

    fn is_empty(&self) -> bool {
        self.as_node().children().all(|node| match node.script_type_id() {
            NodeTypeId::Element(..) => false,
            NodeTypeId::CharacterData(CharacterDataTypeId::Text) => unsafe {
                node.node.downcast().unwrap().data_for_layout().is_empty()
            },
            _ => true
        })
    }

    #[inline]
    fn get_local_name<'a>(&'a self) -> BorrowedAtom<'a> {
        BorrowedAtom(self.element.local_name())
    }

    #[inline]
    fn get_namespace<'a>(&'a self) -> BorrowedNamespace<'a> {
        BorrowedNamespace(self.element.namespace())
    }

    fn match_non_ts_pseudo_class(&self, pseudo_class: NonTSPseudoClass) -> bool {
        match pseudo_class {
            // https://github.com/servo/servo/issues/8718
            NonTSPseudoClass::Link |
            NonTSPseudoClass::AnyLink => unsafe {
                match self.as_node().script_type_id() {
                    // https://html.spec.whatwg.org/multipage/#selector-link
                    NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLAnchorElement)) |
                    NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLAreaElement)) |
                    NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLLinkElement)) =>
                        (*self.element.unsafe_get()).get_attr_val_for_layout(&ns!(), &atom!("href")).is_some(),
                    _ => false,
                }
            },
            NonTSPseudoClass::Visited => false,

            NonTSPseudoClass::ServoNonZeroBorder => unsafe {
                match (*self.element.unsafe_get()).get_attr_for_layout(&ns!(), &atom!("border")) {
                    None | Some(&AttrValue::UInt(_, 0)) => false,
                    _ => true,
                }
            },

            NonTSPseudoClass::ReadOnly =>
                !self.element.get_state_for_layout().contains(pseudo_class.state_flag()),

            NonTSPseudoClass::Active |
            NonTSPseudoClass::Focus |
            NonTSPseudoClass::Hover |
            NonTSPseudoClass::Enabled |
            NonTSPseudoClass::Disabled |
            NonTSPseudoClass::Checked |
            NonTSPseudoClass::Indeterminate |
            NonTSPseudoClass::ReadWrite |
            NonTSPseudoClass::PlaceholderShown =>
                self.element.get_state_for_layout().contains(pseudo_class.state_flag())
        }
    }

    #[inline]
    fn get_id(&self) -> Option<Atom> {
        unsafe {
            (*self.element.id_attribute()).clone()
        }
    }

    #[inline]
    fn has_class(&self, name: &Atom) -> bool {
        unsafe {
            self.element.has_class_for_layout(name)
        }
    }

    #[inline(always)]
    fn each_class<F>(&self, mut callback: F) where F: FnMut(&Atom) {
        unsafe {
            if let Some(ref classes) = self.element.get_classes_for_layout() {
                for class in *classes {
                    callback(class)
                }
            }
        }
    }

    fn match_attr<F>(&self, attr: &AttrSelector, test: F) -> bool where F: Fn(&str) -> bool {
        let name = if self.is_html_element_in_html_document() {
            &attr.lower_name
        } else {
            &attr.name
        };
        match attr.namespace {
            NamespaceConstraint::Specific(ref ns) => {
                self.get_attr(ns, name).map_or(false, |attr| test(attr))
            },
            NamespaceConstraint::Any => {
                self.get_attrs(name).iter().any(|attr| test(*attr))
            }
        }
    }

    fn is_html_element_in_html_document(&self) -> bool {
        unsafe {
            self.element.html_element_in_html_document_for_layout()
        }
    }

    fn insert_flags(&self, flags: ElementFlags) {
        self.element.insert_atomic_flags(flags);
    }
}

#[derive(Copy, PartialEq, Clone)]
pub enum PseudoElementType<T> {
    Normal,
    Before(T),
    After(T),
    DetailsSummary(T),
    DetailsContent(T),
}

impl<T> PseudoElementType<T> {
    pub fn is_before(&self) -> bool {
        match *self {
            PseudoElementType::Before(_) => true,
            _ => false,
        }
    }

    pub fn is_replaced_content(&self) -> bool {
        match *self {
            PseudoElementType::Before(_) | PseudoElementType::After(_) => true,
            _ => false,
        }
    }

    pub fn strip(&self) -> PseudoElementType<()> {
        match *self {
            PseudoElementType::Normal => PseudoElementType::Normal,
            PseudoElementType::Before(_) => PseudoElementType::Before(()),
            PseudoElementType::After(_) => PseudoElementType::After(()),
            PseudoElementType::DetailsSummary(_) => PseudoElementType::DetailsSummary(()),
            PseudoElementType::DetailsContent(_) => PseudoElementType::DetailsContent(()),
        }
    }

    pub fn style_pseudo_element(&self) -> PseudoElement {
        match *self {
            PseudoElementType::Normal => unreachable!("style_pseudo_element called with PseudoElementType::Normal"),
            PseudoElementType::Before(_) => PseudoElement::Before,
            PseudoElementType::After(_) => PseudoElement::After,
            PseudoElementType::DetailsSummary(_) => PseudoElement::DetailsSummary,
            PseudoElementType::DetailsContent(_) => PseudoElement::DetailsContent,
        }
    }
}

/// A thread-safe version of `LayoutNode`, used during flow construction. This type of layout
/// node does not allow any parents or siblings of nodes to be accessed, to avoid races.

pub trait ThreadSafeLayoutNode: Clone + Copy + Sized + PartialEq {
    type ConcreteThreadSafeLayoutElement:
        ThreadSafeLayoutElement<ConcreteThreadSafeLayoutNode = Self>
        + ::selectors::Element<Impl=ServoSelectorImpl>;
    type ChildrenIterator: Iterator<Item = Self> + Sized;

    /// Creates a new `ThreadSafeLayoutNode` for the same `LayoutNode`
    /// with a different pseudo-element type.
    fn with_pseudo(&self, pseudo: PseudoElementType<Option<display::T>>) -> Self;

    /// Converts self into an `OpaqueNode`.
    fn opaque(&self) -> OpaqueNode;

    /// Returns the type ID of this node.
    /// Returns `None` if this is a pseudo-element; otherwise, returns `Some`.
    fn type_id(&self) -> Option<LayoutNodeType>;

    /// Returns the type ID of this node, without discarding pseudo-elements as
    /// `type_id` does.
    fn type_id_without_excluding_pseudo_elements(&self) -> LayoutNodeType;

    #[inline]
    fn is_element_or_elements_pseudo(&self) -> bool {
        match self.type_id_without_excluding_pseudo_elements() {
            LayoutNodeType::Element(..) => true,
            _ => false,
        }
    }

    fn debug_id(self) -> usize;

    /// Returns an iterator over this node's children.
    fn children(&self) -> Self::ChildrenIterator;

    #[inline]
    fn is_element(&self) -> bool { if let Some(LayoutNodeType::Element(_)) = self.type_id() { true } else { false } }

    /// If this is an element, accesses the element data. Fails if this is not an element node.
    #[inline]
    fn as_element(&self) -> Self::ConcreteThreadSafeLayoutElement;

    #[inline]
    fn get_pseudo_element_type(&self) -> PseudoElementType<Option<display::T>>;

    #[inline]
    fn get_before_pseudo(&self) -> Option<Self> {
        if self.get_style_data()
               .unwrap()
               .borrow()
               .style_data
               .per_pseudo
               .contains_key(&PseudoElement::Before) {
            Some(self.with_pseudo(PseudoElementType::Before(None)))
        } else {
            None
        }
    }

    #[inline]
    fn get_after_pseudo(&self) -> Option<Self> {
        if self.get_style_data()
               .unwrap()
               .borrow()
               .style_data
               .per_pseudo
               .contains_key(&PseudoElement::After) {
            Some(self.with_pseudo(PseudoElementType::After(None)))
        } else {
            None
        }
    }

    #[inline]
    fn get_details_summary_pseudo(&self) -> Option<Self> {
        if self.is_element() &&
           self.as_element().get_local_name() == atom!("details") &&
           self.as_element().get_namespace() == ns!(html) {
            Some(self.with_pseudo(PseudoElementType::DetailsSummary(None)))
        } else {
            None
        }
    }

    #[inline]
    fn get_details_content_pseudo(&self) -> Option<Self> {
        if self.is_element() &&
           self.as_element().get_local_name() == atom!("details") &&
           self.as_element().get_namespace() == ns!(html) {
            let display = if self.as_element().get_attr(&ns!(), &atom!("open")).is_some() {
                None // Specified by the stylesheet
            } else {
                Some(display::T::none)
            };
            Some(self.with_pseudo(PseudoElementType::DetailsContent(display)))
        } else {
            None
        }
    }

    fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData>;

    /// Returns the style results for the given node. If CSS selector matching
    /// has not yet been performed, fails.
    ///
    /// Unlike the version on TNode, this handles pseudo-elements.
    #[inline]
    fn style(&self, context: &SharedStyleContext) -> Ref<Arc<ServoComputedValues>> {
        match self.get_pseudo_element_type() {
            PseudoElementType::Normal => {
                Ref::map(self.get_style_data().unwrap().borrow(), |data| {
                    data.style_data.style.as_ref().unwrap()
                })
            },
            other => {
                // Precompute non-eagerly-cascaded pseudo-element styles if not
                // cached before.
                let style_pseudo = other.style_pseudo_element();
                match style_pseudo.cascade_type() {
                    // Already computed during the cascade.
                    PseudoElementCascadeType::Eager => {},
                    PseudoElementCascadeType::Precomputed => {
                        if !self.get_style_data()
                                .unwrap()
                                .borrow()
                                .style_data
                                .per_pseudo.contains_key(&style_pseudo) {
                            let mut data = self.get_style_data().unwrap().borrow_mut();
                            let new_style =
                                context.stylist
                                       .precomputed_values_for_pseudo(&style_pseudo,
                                                                      data.style_data.style.as_ref());
                            data.style_data.per_pseudo
                                .insert(style_pseudo.clone(), new_style.unwrap());
                        }
                    }
                    PseudoElementCascadeType::Lazy => {
                        debug_assert!(self.is_element_or_elements_pseudo());
                        if !self.get_style_data()
                                .unwrap()
                                .borrow()
                                .style_data
                                .per_pseudo.contains_key(&style_pseudo) {
                            let mut data = self.get_style_data().unwrap().borrow_mut();
                            let new_style =
                                context.stylist
                                       .lazily_compute_pseudo_element_style(
                                           &self.as_element(),
                                           &style_pseudo,
                                           data.style_data.style.as_ref().unwrap());
                            data.style_data.per_pseudo
                                .insert(style_pseudo.clone(), new_style.unwrap());
                        }
                    }
                }

                Ref::map(self.get_style_data().unwrap().borrow(), |data| {
                    data.style_data.per_pseudo.get(&style_pseudo).unwrap()
                })
            }
        }
    }

    /// Returns the already resolved style of the node.
    ///
    /// This differs from `style(ctx)` in that if the pseudo-element has not yet
    /// been computed it would panic.
    ///
    /// This should be used just for querying layout, or when we know the
    /// element style is precomputed, not from general layout itself.
    #[inline]
    fn resolved_style(&self) -> Ref<Arc<ServoComputedValues>> {
        Ref::map(self.get_style_data().unwrap().borrow(), |data| {
            match self.get_pseudo_element_type() {
                PseudoElementType::Normal
                    => data.style_data.style.as_ref().unwrap(),
                other
                    => data.style_data.per_pseudo.get(&other.style_pseudo_element()).unwrap(),
            }
        })
    }

    #[inline]
    fn selected_style(&self, _context: &SharedStyleContext) -> Ref<Arc<ServoComputedValues>> {
        Ref::map(self.get_style_data().unwrap().borrow(), |data| {
            data.style_data.per_pseudo
                .get(&PseudoElement::Selection)
                .unwrap_or(data.style_data.style.as_ref().unwrap())
        })
    }

    /// Removes the style from this node.
    ///
    /// Unlike the version on TNode, this handles pseudo-elements.
    fn unstyle(self) {
        let mut data = self.get_style_data().unwrap().borrow_mut();

        match self.get_pseudo_element_type() {
            PseudoElementType::Normal => {
                data.style_data.style = None;
            }
            other => {
                data.style_data.per_pseudo.remove(&other.style_pseudo_element());
            }
        };
    }

    fn is_ignorable_whitespace(&self, context: &SharedStyleContext) -> bool;

    fn restyle_damage(self) -> RestyleDamage;

    fn set_restyle_damage(self, damage: RestyleDamage);

    /// Returns true if this node contributes content. This is used in the implementation of
    /// `empty_cells` per CSS 2.1 § 17.6.1.1.
    fn is_content(&self) -> bool {
        match self.type_id() {
            Some(LayoutNodeType::Element(..)) | Some(LayoutNodeType::Text) => true,
            _ => false
        }
    }

    fn can_be_fragmented(&self) -> bool;

    fn node_text_content(&self) -> String;

    /// If the insertion point is within this node, returns it. Otherwise, returns `None`.
    fn selection(&self) -> Option<Range<ByteIndex>>;

    /// If this is an image element, returns its URL. If this is not an image element, fails.
    ///
    /// FIXME(pcwalton): Don't copy URLs.
    fn image_url(&self) -> Option<Url>;

    fn canvas_data(&self) -> Option<HTMLCanvasData>;

    /// If this node is an iframe element, returns its pipeline ID. If this node is
    /// not an iframe element, fails.
    fn iframe_pipeline_id(&self) -> PipelineId;

    fn get_colspan(&self) -> u32;

    fn layer_id(&self) -> LayerId {
        let layer_type = match self.get_pseudo_element_type() {
            PseudoElementType::Normal => LayerType::FragmentBody,
            PseudoElementType::Before(_) => LayerType::BeforePseudoContent,
            PseudoElementType::After(_) => LayerType::AfterPseudoContent,
            PseudoElementType::DetailsSummary(_) => LayerType::FragmentBody,
            PseudoElementType::DetailsContent(_) => LayerType::FragmentBody,
        };
        LayerId::new_of_type(layer_type, self.opaque().id() as usize)
    }

    fn layer_id_for_overflow_scroll(&self) -> LayerId {
        LayerId::new_of_type(LayerType::OverflowScroll, self.opaque().id() as usize)
    }

    fn get_style_data(&self) -> Option<&RefCell<PartialStyleAndLayoutData>>;
}

pub trait ThreadSafeLayoutNodeHelpers {
    fn flow_debug_id(self) -> usize;

    unsafe fn borrow_layout_data_unchecked(&self) -> Option<*const PrivateLayoutData>;

    /// Borrows the layout data immutably. Fails on a conflicting borrow.
    ///
    /// TODO(pcwalton): Make this private. It will let us avoid borrow flag checks in some cases.
    #[inline(always)]
    fn borrow_layout_data(&self) -> Option<Ref<PrivateLayoutData>>;

    /// Borrows the layout data mutably. Fails on a conflicting borrow.
    ///
    /// TODO(pcwalton): Make this private. It will let us avoid borrow flag checks in some cases.
    #[inline(always)]
    fn mutate_layout_data(&self) -> Option<RefMut<PrivateLayoutData>>;

    /// Returns the layout data flags for this node.
    fn flags(self) -> LayoutDataFlags;

    /// Adds the given flags to this node.
    fn insert_flags(self, new_flags: LayoutDataFlags);

    /// Removes the given flags from this node.
    fn remove_flags(self, flags: LayoutDataFlags);

    /// If this is a text node, generated content, or a form element, copies out
    /// its content. Otherwise, panics.
    ///
    /// FIXME(pcwalton): This might have too much copying and/or allocation. Profile this.
    fn text_content(&self) -> TextContent;
}

// This trait is only public so that it can be implemented by the gecko wrapper.
// It can be used to violate thread-safety, so don't use it elsewhere in layout!
pub trait DangerousThreadSafeLayoutNode: ThreadSafeLayoutNode {
    unsafe fn dangerous_first_child(&self) -> Option<Self>;
    unsafe fn dangerous_next_sibling(&self) -> Option<Self>;
}

pub trait ThreadSafeLayoutElement: Clone + Copy + Sized +
                                   ::selectors::Element<Impl=ServoSelectorImpl> +
                                   PresentationalHintsSynthetizer {
    type ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode<ConcreteThreadSafeLayoutElement = Self>;

    #[inline]
    fn get_attr(&self, namespace: &Namespace, name: &Atom) -> Option<&str>;

    #[inline]
    fn get_local_name<'a>(&'a self) -> BorrowedAtom<'a>;

    #[inline]
    fn get_namespace<'a>(&'a self) -> BorrowedNamespace<'a>;
}

#[derive(Copy, Clone)]
pub struct ServoThreadSafeLayoutNode<'ln> {
    /// The wrapped node.
    node: ServoLayoutNode<'ln>,

    /// The pseudo-element type, with (optionally),
    /// an specified display value to override the stylesheet.
    pseudo: PseudoElementType<Option<display::T>>,
}

impl<'a> PartialEq for ServoThreadSafeLayoutNode<'a> {
    #[inline]
    fn eq(&self, other: &ServoThreadSafeLayoutNode<'a>) -> bool {
        self.node == other.node
    }
}

impl<'ln> DangerousThreadSafeLayoutNode for ServoThreadSafeLayoutNode<'ln> {
    unsafe fn dangerous_first_child(&self) -> Option<Self> {
            self.get_jsmanaged().first_child_ref()
                .map(|node| self.new_with_this_lifetime(&node))
    }
    unsafe fn dangerous_next_sibling(&self) -> Option<Self> {
            self.get_jsmanaged().next_sibling_ref()
                .map(|node| self.new_with_this_lifetime(&node))
    }
}

impl<'ln> ServoThreadSafeLayoutNode<'ln> {
    /// Creates a new layout node with the same lifetime as this layout node.
    pub unsafe fn new_with_this_lifetime(&self, node: &LayoutJS<Node>) -> ServoThreadSafeLayoutNode<'ln> {
        ServoThreadSafeLayoutNode {
            node: self.node.new_with_this_lifetime(node),
            pseudo: PseudoElementType::Normal,
        }
    }

    /// Creates a new `ServoThreadSafeLayoutNode` from the given `ServoLayoutNode`.
    pub fn new<'a>(node: &ServoLayoutNode<'a>) -> ServoThreadSafeLayoutNode<'a> {
        ServoThreadSafeLayoutNode {
            node: node.clone(),
            pseudo: PseudoElementType::Normal,
        }
    }

    /// Returns the interior of this node as a `LayoutJS`. This is highly unsafe for layout to
    /// call and as such is marked `unsafe`.
    unsafe fn get_jsmanaged(&self) -> &LayoutJS<Node> {
        self.node.get_jsmanaged()
    }
}

impl<'ln> ThreadSafeLayoutNode for ServoThreadSafeLayoutNode<'ln> {
    type ConcreteThreadSafeLayoutElement = ServoThreadSafeLayoutElement<'ln>;
    type ChildrenIterator = ThreadSafeLayoutNodeChildrenIterator<Self>;

    fn with_pseudo(&self,
                   pseudo: PseudoElementType<Option<display::T>>) -> ServoThreadSafeLayoutNode<'ln> {
        ServoThreadSafeLayoutNode {
            node: self.node.clone(),
            pseudo: pseudo,
        }
    }

    fn opaque(&self) -> OpaqueNode {
        OpaqueNodeMethods::from_jsmanaged(unsafe { self.get_jsmanaged() })
    }

    fn type_id(&self) -> Option<LayoutNodeType> {
        if self.pseudo != PseudoElementType::Normal {
            return None
        }

        Some(self.node.type_id())
    }

    #[inline]
    fn type_id_without_excluding_pseudo_elements(&self) -> LayoutNodeType {
        self.node.type_id()
    }

    fn debug_id(self) -> usize {
        self.node.debug_id()
    }

    fn children(&self) -> Self::ChildrenIterator {
        ThreadSafeLayoutNodeChildrenIterator::new(*self)
    }

    fn as_element(&self) -> ServoThreadSafeLayoutElement<'ln> {
        unsafe {
            let element = match self.get_jsmanaged().downcast() {
                Some(e) => e.unsafe_get(),
                None => panic!("not an element")
            };
            // FIXME(pcwalton): Workaround until Rust gets multiple lifetime parameters on
            // implementations.
            ServoThreadSafeLayoutElement {
                element: &*element,
            }
        }
    }

    fn get_pseudo_element_type(&self) -> PseudoElementType<Option<display::T>> {
        self.pseudo
    }

    fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData> {
        self.node.get_style_and_layout_data()
    }

    fn is_ignorable_whitespace(&self, context: &SharedStyleContext) -> bool {
        unsafe {
            let text: LayoutJS<Text> = match self.get_jsmanaged().downcast() {
                Some(text) => text,
                None => return false
            };

            if !is_whitespace(text.upcast().data_for_layout()) {
                return false
            }

            // NB: See the rules for `white-space` here:
            //
            //    http://www.w3.org/TR/CSS21/text.html#propdef-white-space
            //
            // If you implement other values for this property, you will almost certainly
            // want to update this check.
            !self.style(context).get_inheritedtext().white_space.preserve_newlines()
        }
    }

    fn restyle_damage(self) -> RestyleDamage {
        self.node.restyle_damage()
    }

    fn set_restyle_damage(self, damage: RestyleDamage) {
        self.node.set_restyle_damage(damage)
    }

    fn can_be_fragmented(&self) -> bool {
        self.node.can_be_fragmented()
    }

    fn node_text_content(&self) -> String {
        let this = unsafe { self.get_jsmanaged() };
        return this.text_content();
    }

    fn selection(&self) -> Option<Range<ByteIndex>> {
        let this = unsafe { self.get_jsmanaged() };

        this.selection().map(|range| {
            Range::new(ByteIndex(range.start as isize),
                       ByteIndex(range.len() as isize))
        })
    }

    fn image_url(&self) -> Option<Url> {
        let this = unsafe { self.get_jsmanaged() };
        this.image_url()
    }

    fn canvas_data(&self) -> Option<HTMLCanvasData> {
        let this = unsafe { self.get_jsmanaged() };
        this.canvas_data()
    }

    fn iframe_pipeline_id(&self) -> PipelineId {
        let this = unsafe { self.get_jsmanaged() };
        this.iframe_pipeline_id()
    }

    fn get_colspan(&self) -> u32 {
        unsafe {
            self.get_jsmanaged().downcast::<Element>().unwrap().get_colspan()
        }
    }

    fn get_style_data(&self) -> Option<&RefCell<PartialStyleAndLayoutData>> {
        self.node.get_style_data()
    }
}

impl<T: ThreadSafeLayoutNode> ThreadSafeLayoutNodeHelpers for T {
    fn flow_debug_id(self) -> usize {
        self.borrow_layout_data().map_or(0, |d| d.flow_construction_result.debug_id())
    }

    unsafe fn borrow_layout_data_unchecked(&self) -> Option<*const PrivateLayoutData> {
        self.get_style_and_layout_data().map(|opaque| {
            let container = *opaque.ptr as NonOpaqueStyleAndLayoutData;
            &(*(*container).as_unsafe_cell().get()) as *const PrivateLayoutData
        })
    }

    fn borrow_layout_data(&self) -> Option<Ref<PrivateLayoutData>> {
        unsafe {
            self.get_style_and_layout_data().map(|opaque| {
                let container = *opaque.ptr as NonOpaqueStyleAndLayoutData;
                (*container).borrow()
            })
        }
    }

    fn mutate_layout_data(&self) -> Option<RefMut<PrivateLayoutData>> {
        unsafe {
            self.get_style_and_layout_data().map(|opaque| {
                let container = *opaque.ptr as NonOpaqueStyleAndLayoutData;
                (*container).borrow_mut()
            })
        }
    }

    fn flags(self) -> LayoutDataFlags {
        unsafe {
            (*self.borrow_layout_data_unchecked().unwrap()).flags
        }
    }

    fn insert_flags(self, new_flags: LayoutDataFlags) {
        self.mutate_layout_data().unwrap().flags.insert(new_flags);
    }

    fn remove_flags(self, flags: LayoutDataFlags) {
        self.mutate_layout_data().unwrap().flags.remove(flags);
    }

    fn text_content(&self) -> TextContent {
        if self.get_pseudo_element_type().is_replaced_content() {
            let style = self.resolved_style();

            return match style.as_ref().get_counters().content {
                content::T::Content(ref value) if !value.is_empty() => {
                    TextContent::GeneratedContent((*value).clone())
                }
                _ => TextContent::GeneratedContent(vec![]),
            };
        }

        return TextContent::Text(self.node_text_content());
    }
}

pub struct ThreadSafeLayoutNodeChildrenIterator<ConcreteNode: ThreadSafeLayoutNode> {
    current_node: Option<ConcreteNode>,
    parent_node: ConcreteNode,
}

impl<ConcreteNode> ThreadSafeLayoutNodeChildrenIterator<ConcreteNode>
                   where ConcreteNode: DangerousThreadSafeLayoutNode {
    pub fn new(parent: ConcreteNode) -> Self {
        let first_child: Option<ConcreteNode> = match parent.get_pseudo_element_type() {
            PseudoElementType::Normal => {
                parent.get_before_pseudo().or_else(|| parent.get_details_summary_pseudo()).or_else(|| {
                    unsafe { parent.dangerous_first_child() }
                })
            },
            PseudoElementType::DetailsContent(_) | PseudoElementType::DetailsSummary(_) => {
                unsafe { parent.dangerous_first_child() }
            },
            _ => None,
        };
        ThreadSafeLayoutNodeChildrenIterator {
            current_node: first_child,
            parent_node: parent,
        }
    }
}

impl<ConcreteNode> Iterator for ThreadSafeLayoutNodeChildrenIterator<ConcreteNode>
                            where ConcreteNode: DangerousThreadSafeLayoutNode {
    type Item = ConcreteNode;
    fn next(&mut self) -> Option<ConcreteNode> {
        match self.parent_node.get_pseudo_element_type() {
            PseudoElementType::Before(_) | PseudoElementType::After(_) => None,

            PseudoElementType::DetailsSummary(_) => {
                let mut current_node = self.current_node.clone();
                loop {
                    let next_node = if let Some(ref node) = current_node {
                        if node.is_element() &&
                           node.as_element().get_local_name() == atom!("summary") &&
                           node.as_element().get_namespace() == ns!(html) {
                            self.current_node = None;
                            return Some(node.clone());
                        }
                        unsafe { node.dangerous_next_sibling() }
                    } else {
                        self.current_node = None;
                        return None
                    };
                    current_node = next_node;
                }
            }

            PseudoElementType::DetailsContent(_) => {
                let node = self.current_node.clone();
                let node = node.and_then(|node| {
                    if node.is_element() &&
                       node.as_element().get_local_name() == atom!("summary") &&
                       node.as_element().get_namespace() == ns!(html) {
                        unsafe { node.dangerous_next_sibling() }
                    } else {
                        Some(node)
                    }
                });
                self.current_node = node.and_then(|node| unsafe { node.dangerous_next_sibling() });
                node
            }

            PseudoElementType::Normal => {
                let node = self.current_node.clone();
                if let Some(ref node) = node {
                    self.current_node = match node.get_pseudo_element_type() {
                        PseudoElementType::Before(_) => {
                            let first = self.parent_node.get_details_summary_pseudo().or_else(|| unsafe {
                                self.parent_node.dangerous_first_child()
                            });
                            match first {
                                Some(first) => Some(first),
                                None => self.parent_node.get_after_pseudo(),
                            }
                        },
                        PseudoElementType::Normal => {
                            match unsafe { node.dangerous_next_sibling() } {
                                Some(next) => Some(next),
                                None => self.parent_node.get_after_pseudo(),
                            }
                        },
                        PseudoElementType::DetailsSummary(_) => self.parent_node.get_details_content_pseudo(),
                        PseudoElementType::DetailsContent(_) => self.parent_node.get_after_pseudo(),
                        PseudoElementType::After(_) => {
                            None
                        },
                    };
                }
                node
            }

        }
    }
}

/// A wrapper around elements that ensures layout can only
/// ever access safe properties and cannot race on elements.
#[derive(Copy, Clone)]
pub struct ServoThreadSafeLayoutElement<'le> {
    element: &'le Element,
}

impl<'le> ThreadSafeLayoutElement for ServoThreadSafeLayoutElement<'le> {
    type ConcreteThreadSafeLayoutNode = ServoThreadSafeLayoutNode<'le>;

    fn get_attr<'a>(&'a self, namespace: &Namespace, name: &Atom) -> Option<&'a str> {
        unsafe {
            self.element.get_attr_val_for_layout(namespace, name)
        }
    }

    #[inline]
    fn get_local_name<'a>(&'a self) -> BorrowedAtom<'a> {
        BorrowedAtom(self.element.local_name())
    }

    #[inline]
    fn get_namespace<'a>(&'a self) -> BorrowedNamespace<'a> {
        BorrowedNamespace(self.element.namespace())
    }
}

pub enum TextContent {
    Text(String),
    GeneratedContent(Vec<ContentItem>),
}

impl TextContent {
    pub fn is_empty(&self) -> bool {
        match *self {
            TextContent::Text(_) => false,
            TextContent::GeneratedContent(ref content) => content.is_empty(),
        }
    }
}

/// This implementation of `::selectors::Element` is used for implementing lazy
/// pseudo-elements.
///
/// Lazy pseudo-elements in Servo only allows selectors using safe properties,
/// i.e., local_name, attributes, so they can only be used for **private**
/// pseudo-elements (like `::-servo-details-content`).
///
/// Probably a few more of this functions can be implemented (like `has_class`,
/// `each_class`, etc), but they have no use right now.
///
/// Note that the element implementation is needed only for selector matching,
/// not for inheritance (styles are inherited appropiately).
impl <'le> ::selectors::Element for ServoThreadSafeLayoutElement<'le> {
    type Impl = ServoSelectorImpl;

    fn parent_element(&self) -> Option<Self> {
        warn!("ServoThreadSafeLayoutElement::parent_element called");
        None
    }

    fn first_child_element(&self) -> Option<Self> {
        warn!("ServoThreadSafeLayoutElement::first_child_element called");
        None
    }

    // Skips non-element nodes
    fn last_child_element(&self) -> Option<Self> {
        warn!("ServoThreadSafeLayoutElement::last_child_element called");
        None
    }

    // Skips non-element nodes
    fn prev_sibling_element(&self) -> Option<Self> {
        warn!("ServoThreadSafeLayoutElement::prev_sibling_element called");
        None
    }

    // Skips non-element nodes
    fn next_sibling_element(&self) -> Option<Self> {
        warn!("ServoThreadSafeLayoutElement::next_sibling_element called");
        None
    }

    fn is_html_element_in_html_document(&self) -> bool {
        debug!("ServoThreadSafeLayoutElement::is_html_element_in_html_document called");
        true
    }

    #[inline]
    fn get_local_name<'a>(&'a self) -> BorrowedAtom<'a> {
        ThreadSafeLayoutElement::get_local_name(self)
    }

    #[inline]
    fn get_namespace<'a>(&'a self) -> BorrowedNamespace<'a> {
        ThreadSafeLayoutElement::get_namespace(self)
    }

    fn match_non_ts_pseudo_class(&self, _: NonTSPseudoClass) -> bool {
        // NB: This could maybe be implemented
        warn!("ServoThreadSafeLayoutElement::match_non_ts_pseudo_class called");
        false
    }

    fn get_id(&self) -> Option<Atom> {
        debug!("ServoThreadSafeLayoutElement::get_id called");
        None
    }

    fn has_class(&self, _name: &Atom) -> bool {
        debug!("ServoThreadSafeLayoutElement::has_class called");
        false
    }

    fn match_attr<F>(&self, attr: &AttrSelector, test: F) -> bool
        where F: Fn(&str) -> bool {
        match attr.namespace {
            NamespaceConstraint::Specific(ref ns) => {
                self.get_attr(ns, &attr.name).map_or(false, |attr| test(attr))
            },
            NamespaceConstraint::Any => {
                unsafe {
                    self.element.get_attr_vals_for_layout(&attr.name).iter()
                        .any(|attr| test(*attr))
                }
            }
        }
    }

    fn is_empty(&self) -> bool {
        warn!("ServoThreadSafeLayoutElement::is_empty called");
        false
    }

    fn is_root(&self) -> bool {
        warn!("ServoThreadSafeLayoutElement::is_root called");
        false
    }

    fn each_class<F>(&self, _callback: F)
        where F: FnMut(&Atom) {
        warn!("ServoThreadSafeLayoutElement::each_class called");
    }
}

impl<'le> PresentationalHintsSynthetizer for ServoThreadSafeLayoutElement<'le> {
    fn synthesize_presentational_hints_for_legacy_attributes<V>(&self, _hints: &mut V)
        where V: VecLike<DeclarationBlock<Vec<PropertyDeclaration>>> {}
}