Add XPath parser/evaluator (#34463)

* Add XPath parser/evaluator

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* Correctly annotate XPathEvaluator IDL

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* [PR review]: have bindings pass in `can_gc`

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* [PR review]: add docstrings

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* [PR review]: implement PartialEq for Value for readability

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* [PR review]: add docstrings for CoreFunctions

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* [PR review]: simplify node test code

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* [PR review]: add unit tests for string handling xpath functions

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* put xpath features behind dom.xpath.enabled pref

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* [PR review] remove rstest and insta dev-deps

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* update wpt test expectations

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* [PR review]: tweak metadata files

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

* update wpt test expectations AGAIN

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

---------

Signed-off-by: Ville Lindholm <ville@lindholm.dev>

11 files changed, 617 insertions, 7 deletions

diff --git a/components/script/dom/bindings/codegen/Bindings.conf b/components/script/dom/bindings/codegen/Bindings.conf
index a1f3de7e46a..0a15dde41d7 100644
--- a/components/script/dom/bindings/codegen/Bindings.conf
+++ b/components/script/dom/bindings/codegen/Bindings.conf
@@ -84,7 +84,7 @@ DOMInterfaces = {
 },
 
 'Document': {
-    'canGc': ['Close', 'CreateElement', 'CreateElementNS', 'ImportNode', 'SetTitle', 'Write', 'Writeln', 'CreateEvent', 'CreateRange', 'Open', 'Open_', 'CreateComment', 'CreateAttribute', 'CreateAttributeNS', 'CreateDocumentFragment', 'CreateTextNode', 'CreateCDATASection', 'CreateProcessingInstruction', 'Prepend', 'Append', 'ReplaceChildren', 'SetBgColor', 'SetFgColor', 'Fonts', 'ElementFromPoint', 'ElementsFromPoint', 'ExitFullscreen'],
+    'canGc': ['Close', 'CreateElement', 'CreateElementNS', 'ImportNode', 'SetTitle', 'Write', 'Writeln', 'CreateEvent', 'CreateRange', 'Open', 'Open_', 'CreateComment', 'CreateAttribute', 'CreateAttributeNS', 'CreateDocumentFragment', 'CreateTextNode', 'CreateCDATASection', 'CreateProcessingInstruction', 'Prepend', 'Append', 'ReplaceChildren', 'SetBgColor', 'SetFgColor', 'Fonts', 'ElementFromPoint', 'ElementsFromPoint', 'ExitFullscreen', 'CreateExpression', 'CreateNSResolver', 'Evaluate'],
 },
 
 'DocumentFragment': {
@@ -488,6 +488,14 @@ DOMInterfaces = {
     'canGc': ['Abort', 'GetResponseXML', 'Response', 'Send'],
 },
 
+'XPathEvaluator': {
+    'canGc': ['CreateExpression', 'Evaluate'],
+},
+
+'XPathExpression': {
+    'canGc': ['Evaluate'],
+},
+
 'XRBoundedReferenceSpace': {
     'canGc': ['BoundsGeometry'],
 },
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index 7021cbe62d5..99bbedf9bb1 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -71,6 +71,7 @@ use uuid::Uuid;
 use webgpu::swapchain::WebGPUContextId;
 use webrender_api::units::DeviceIntRect;
 
+use super::bindings::codegen::Bindings::XPathEvaluatorBinding::XPathEvaluatorMethods;
 use crate::animation_timeline::AnimationTimeline;
 use crate::animations::Animations;
 use crate::document_loader::{DocumentLoader, LoadType};
@@ -95,6 +96,7 @@ use crate::dom::bindings::codegen::Bindings::TouchBinding::TouchMethods;
 use crate::dom::bindings::codegen::Bindings::WindowBinding::{
     FrameRequestCallback, ScrollBehavior, WindowMethods,
 };
+use crate::dom::bindings::codegen::Bindings::XPathNSResolverBinding::XPathNSResolver;
 use crate::dom::bindings::codegen::UnionTypes::{NodeOrString, StringOrElementCreationOptions};
 use crate::dom::bindings::error::{Error, ErrorInfo, ErrorResult, Fallible};
 use crate::dom::bindings::inheritance::{Castable, ElementTypeId, HTMLElementTypeId, NodeTypeId};
@@ -180,6 +182,7 @@ use crate::dom::webgpu::gpucanvascontext::GPUCanvasContext;
 use crate::dom::wheelevent::WheelEvent;
 use crate::dom::window::{ReflowReason, Window};
 use crate::dom::windowproxy::WindowProxy;
+use crate::dom::xpathevaluator::XPathEvaluator;
 use crate::fetch::FetchCanceller;
 use crate::network_listener::{NetworkListener, PreInvoke};
 use crate::realms::{enter_realm, AlreadyInRealm, InRealm};
@@ -682,6 +685,12 @@ impl Document {
         self.is_html_document
     }
 
+    pub fn is_xhtml_document(&self) -> bool {
+        self.content_type.type_() == mime::APPLICATION &&
+            self.content_type.subtype().as_str() == "xhtml" &&
+            self.content_type.suffix() == Some(mime::XML)
+    }
+
     pub fn set_https_state(&self, https_state: HttpsState) {
         self.https_state.set(https_state);
     }
@@ -4519,11 +4528,7 @@ impl DocumentMethods<crate::DomTypeHolder> for Document {
             local_name.make_ascii_lowercase();
         }
 
-        let is_xhtml = self.content_type.type_() == mime::APPLICATION &&
-            self.content_type.subtype().as_str() == "xhtml" &&
-            self.content_type.suffix() == Some(mime::XML);
-
-        let ns = if self.is_html_document || is_xhtml {
+        let ns = if self.is_html_document || self.is_xhtml_document() {
             ns!(html)
         } else {
             ns!()
@@ -5613,6 +5618,53 @@ impl DocumentMethods<crate::DomTypeHolder> for Document {
     fn VisibilityState(&self) -> DocumentVisibilityState {
         self.visibility_state.get()
     }
+
+    fn CreateExpression(
+        &self,
+        expression: DOMString,
+        resolver: Option<Rc<XPathNSResolver>>,
+        can_gc: CanGc,
+    ) -> Fallible<DomRoot<super::types::XPathExpression>> {
+        let global = self.global();
+        let window = global.as_window();
+        let evaluator = XPathEvaluator::new(window, None, can_gc);
+        XPathEvaluatorMethods::<crate::DomTypeHolder>::CreateExpression(
+            &*evaluator,
+            expression,
+            resolver,
+            can_gc,
+        )
+    }
+
+    fn CreateNSResolver(&self, node_resolver: &Node, can_gc: CanGc) -> DomRoot<Node> {
+        let global = self.global();
+        let window = global.as_window();
+        let evaluator = XPathEvaluator::new(window, None, can_gc);
+        XPathEvaluatorMethods::<crate::DomTypeHolder>::CreateNSResolver(&*evaluator, node_resolver)
+    }
+
+    fn Evaluate(
+        &self,
+        expression: DOMString,
+        context_node: &Node,
+        resolver: Option<Rc<XPathNSResolver>>,
+        type_: u16,
+        result: Option<&super::types::XPathResult>,
+        can_gc: CanGc,
+    ) -> Fallible<DomRoot<super::types::XPathResult>> {
+        let global = self.global();
+        let window = global.as_window();
+        let evaluator = XPathEvaluator::new(window, None, can_gc);
+        XPathEvaluatorMethods::<crate::DomTypeHolder>::Evaluate(
+            &*evaluator,
+            expression,
+            context_node,
+            resolver,
+            type_,
+            result,
+            can_gc,
+        )
+    }
 }
 
 fn update_with_current_instant(marker: &Cell<Option<CrossProcessInstant>>) {
diff --git a/components/script/dom/mod.rs b/components/script/dom/mod.rs
index 3253cf6b39d..52a8559628a 100644
--- a/components/script/dom/mod.rs
+++ b/components/script/dom/mod.rs
@@ -601,4 +601,7 @@ pub mod xmlhttprequest;
 pub mod xmlhttprequesteventtarget;
 pub mod xmlhttprequestupload;
 pub mod xmlserializer;
+pub mod xpathevaluator;
+pub mod xpathexpression;
+pub mod xpathresult;
 pub use self::webgl_extensions::ext::*;
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 3c7df12b633..b8e3531508e 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -10,7 +10,7 @@ use std::default::Default;
 use std::ops::Range;
 use std::slice::from_ref;
 use std::sync::Arc as StdArc;
-use std::{cmp, iter};
+use std::{cmp, fmt, iter};
 
 use app_units::Au;
 use base::id::{BrowsingContextId, PipelineId};
@@ -168,6 +168,23 @@ pub struct Node {
     layout_data: DomRefCell<Option<Box<GenericLayoutData>>>,
 }
 
+impl fmt::Debug for Node {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if matches!(self.type_id(), NodeTypeId::Element(_)) {
+            let el = self.downcast::<Element>().unwrap();
+            el.fmt(f)
+        } else {
+            write!(f, "[Node({:?})]", self.type_id())
+        }
+    }
+}
+
+impl fmt::Debug for DomRoot<Node> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
 /// Flags for node items
 #[derive(Clone, Copy, JSTraceable, MallocSizeOf)]
 pub struct NodeFlags(u16);
diff --git a/components/script/dom/webidls/XPathEvaluator.webidl b/components/script/dom/webidls/XPathEvaluator.webidl
new file mode 100644
index 00000000000..8586b19232b
--- /dev/null
+++ b/components/script/dom/webidls/XPathEvaluator.webidl
@@ -0,0 +1,30 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+// https://dom.spec.whatwg.org/#mixin-xpathevaluatorbase
+interface mixin XPathEvaluatorBase {
+  [NewObject, Throws, Pref="dom.xpath.enabled"] XPathExpression createExpression(
+    DOMString expression,
+    optional XPathNSResolver? resolver = null
+  );
+  Node createNSResolver(Node nodeResolver); // legacy
+  // XPathResult.ANY_TYPE = 0
+  [Throws, Pref="dom.xpath.enabled"] XPathResult evaluate(
+    DOMString expression,
+    Node contextNode,
+    optional XPathNSResolver? resolver = null,
+    optional unsigned short type = 0,
+    optional XPathResult? result = null
+  );
+};
+
+Document includes XPathEvaluatorBase;
+
+// https://dom.spec.whatwg.org/#interface-xpathevaluator
+[Exposed=Window, Pref="dom.xpath.enabled"]
+interface XPathEvaluator {
+  constructor();
+};
+
+XPathEvaluator includes XPathEvaluatorBase;
diff --git a/components/script/dom/webidls/XPathExpression.webidl b/components/script/dom/webidls/XPathExpression.webidl
new file mode 100644
index 00000000000..bf5ba2b83aa
--- /dev/null
+++ b/components/script/dom/webidls/XPathExpression.webidl
@@ -0,0 +1,14 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+// https://dom.spec.whatwg.org/#interface-xpathexpression
+[Exposed=Window, Pref="dom.xpath.enabled"]
+interface XPathExpression {
+  // XPathResult.ANY_TYPE = 0
+  [Throws] XPathResult evaluate(
+    Node contextNode,
+    optional unsigned short type = 0,
+    optional XPathResult? result = null
+  );
+};
diff --git a/components/script/dom/webidls/XPathNSResolver.webidl b/components/script/dom/webidls/XPathNSResolver.webidl
new file mode 100644
index 00000000000..099f6afd105
--- /dev/null
+++ b/components/script/dom/webidls/XPathNSResolver.webidl
@@ -0,0 +1,9 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+// https://dom.spec.whatwg.org/#mixin-xpathevaluatorbase
+[Exposed=Window]
+callback interface XPathNSResolver {
+  DOMString? lookupNamespaceURI(DOMString? prefix);
+};
diff --git a/components/script/dom/webidls/XPathResult.webidl b/components/script/dom/webidls/XPathResult.webidl
new file mode 100644
index 00000000000..25e87e3024a
--- /dev/null
+++ b/components/script/dom/webidls/XPathResult.webidl
@@ -0,0 +1,29 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+// https://dom.spec.whatwg.org/#interface-xpathresult
+[Exposed=Window, Pref="dom.xpath.enabled"]
+interface XPathResult {
+  const unsigned short ANY_TYPE = 0;
+  const unsigned short NUMBER_TYPE = 1;
+  const unsigned short STRING_TYPE = 2;
+  const unsigned short BOOLEAN_TYPE = 3;
+  const unsigned short UNORDERED_NODE_ITERATOR_TYPE = 4;
+  const unsigned short ORDERED_NODE_ITERATOR_TYPE = 5;
+  const unsigned short UNORDERED_NODE_SNAPSHOT_TYPE = 6;
+  const unsigned short ORDERED_NODE_SNAPSHOT_TYPE = 7;
+  const unsigned short ANY_UNORDERED_NODE_TYPE = 8;
+  const unsigned short FIRST_ORDERED_NODE_TYPE = 9;
+
+  readonly attribute unsigned short resultType;
+  [Throws] readonly attribute unrestricted double numberValue;
+  [Throws] readonly attribute DOMString stringValue;
+  [Throws] readonly attribute boolean booleanValue;
+  [Throws] readonly attribute Node? singleNodeValue;
+  [Throws] readonly attribute boolean invalidIteratorState;
+  [Throws] readonly attribute unsigned long snapshotLength;
+
+  [Throws] Node? iterateNext();
+  [Throws] Node? snapshotItem(unsigned long index);
+};
diff --git a/components/script/dom/xpathevaluator.rs b/components/script/dom/xpathevaluator.rs
new file mode 100644
index 00000000000..967f7ef6076
--- /dev/null
+++ b/components/script/dom/xpathevaluator.rs
@@ -0,0 +1,110 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use std::rc::Rc;
+
+use dom_struct::dom_struct;
+use js::rust::HandleObject;
+
+use super::bindings::error::Error;
+use crate::dom::bindings::codegen::Bindings::XPathEvaluatorBinding::XPathEvaluatorMethods;
+use crate::dom::bindings::codegen::Bindings::XPathExpressionBinding::XPathExpression_Binding::XPathExpressionMethods;
+use crate::dom::bindings::codegen::Bindings::XPathNSResolverBinding::XPathNSResolver;
+use crate::dom::bindings::error::Fallible;
+use crate::dom::bindings::reflector::{reflect_dom_object_with_proto, DomObject, Reflector};
+use crate::dom::bindings::root::{Dom, DomRoot};
+use crate::dom::bindings::str::DOMString;
+use crate::dom::node::Node;
+use crate::dom::window::Window;
+use crate::dom::xpathexpression::XPathExpression;
+use crate::dom::xpathresult::XPathResult;
+use crate::script_runtime::CanGc;
+
+#[dom_struct]
+pub struct XPathEvaluator {
+    reflector_: Reflector,
+    window: Dom<Window>,
+}
+
+impl XPathEvaluator {
+    fn new_inherited(window: &Window) -> XPathEvaluator {
+        XPathEvaluator {
+            reflector_: Reflector::new(),
+            window: Dom::from_ref(window),
+        }
+    }
+
+    pub fn new(
+        window: &Window,
+        proto: Option<HandleObject>,
+        can_gc: CanGc,
+    ) -> DomRoot<XPathEvaluator> {
+        reflect_dom_object_with_proto(
+            Box::new(XPathEvaluator::new_inherited(window)),
+            window,
+            proto,
+            can_gc,
+        )
+    }
+}
+
+impl XPathEvaluatorMethods<crate::DomTypeHolder> for XPathEvaluator {
+    /// <https://dom.spec.whatwg.org/#dom-xpathevaluator-xpathevaluator>
+    fn Constructor(
+        window: &Window,
+        proto: Option<HandleObject>,
+        can_gc: CanGc,
+    ) -> DomRoot<XPathEvaluator> {
+        XPathEvaluator::new(window, proto, can_gc)
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathevaluatorbase-createexpression>
+    fn CreateExpression(
+        &self,
+        expression: DOMString,
+        _resolver: Option<Rc<XPathNSResolver>>,
+        can_gc: CanGc,
+    ) -> Fallible<DomRoot<XPathExpression>> {
+        let global = self.global();
+        let window = global.as_window();
+        // NB: this function is *not* Fallible according to the spec, so we swallow any parsing errors and
+        // just pass a None as the expression... it's not great.
+        let parsed_expression = crate::xpath::parse(&expression).map_err(|_e| Error::Syntax)?;
+        Ok(XPathExpression::new(
+            window,
+            None,
+            can_gc,
+            parsed_expression,
+        ))
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathevaluatorbase-creatensresolver>
+    fn CreateNSResolver(&self, node_resolver: &Node) -> DomRoot<Node> {
+        // Legacy: the spec tells us to just return `node_resolver` as-is
+        DomRoot::from_ref(node_resolver)
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathevaluatorbase-evaluate>
+    fn Evaluate(
+        &self,
+        expression_str: DOMString,
+        context_node: &Node,
+        _resolver: Option<Rc<XPathNSResolver>>,
+        result_type: u16,
+        result: Option<&XPathResult>,
+        can_gc: CanGc,
+    ) -> Fallible<DomRoot<XPathResult>> {
+        let global = self.global();
+        let window = global.as_window();
+        let parsed_expression = crate::xpath::parse(&expression_str).map_err(|_| Error::Syntax)?;
+        let expression = XPathExpression::new(window, None, can_gc, parsed_expression);
+        XPathExpressionMethods::<crate::DomTypeHolder>::Evaluate(
+            &*expression,
+            context_node,
+            result_type,
+            result,
+            can_gc,
+        )
+    }
+}
diff --git a/components/script/dom/xpathexpression.rs b/components/script/dom/xpathexpression.rs
new file mode 100644
index 00000000000..16e898c2e72
--- /dev/null
+++ b/components/script/dom/xpathexpression.rs
@@ -0,0 +1,77 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use dom_struct::dom_struct;
+use js::rust::HandleObject;
+
+use crate::dom::bindings::codegen::Bindings::XPathExpressionBinding::XPathExpressionMethods;
+use crate::dom::bindings::error::{Error, Fallible};
+use crate::dom::bindings::reflector::{reflect_dom_object_with_proto, DomObject, Reflector};
+use crate::dom::bindings::root::{Dom, DomRoot};
+use crate::dom::node::Node;
+use crate::dom::window::Window;
+use crate::dom::xpathresult::{XPathResult, XPathResultType};
+use crate::script_runtime::CanGc;
+use crate::xpath::{evaluate_parsed_xpath, Expr};
+
+#[dom_struct]
+pub struct XPathExpression {
+    reflector_: Reflector,
+    window: Dom<Window>,
+    #[no_trace]
+    parsed_expression: Expr,
+}
+
+impl XPathExpression {
+    fn new_inherited(window: &Window, parsed_expression: Expr) -> XPathExpression {
+        XPathExpression {
+            reflector_: Reflector::new(),
+            window: Dom::from_ref(window),
+            parsed_expression,
+        }
+    }
+
+    pub fn new(
+        window: &Window,
+        proto: Option<HandleObject>,
+        can_gc: CanGc,
+        parsed_expression: Expr,
+    ) -> DomRoot<XPathExpression> {
+        reflect_dom_object_with_proto(
+            Box::new(XPathExpression::new_inherited(window, parsed_expression)),
+            window,
+            proto,
+            can_gc,
+        )
+    }
+}
+
+impl XPathExpressionMethods<crate::DomTypeHolder> for XPathExpression {
+    /// <https://dom.spec.whatwg.org/#dom-xpathexpression-evaluate>
+    fn Evaluate(
+        &self,
+        context_node: &Node,
+        result_type_num: u16,
+        _result: Option<&XPathResult>,
+        can_gc: CanGc,
+    ) -> Fallible<DomRoot<XPathResult>> {
+        let result_type = XPathResultType::try_from(result_type_num)
+            .map_err(|()| Error::Type("Invalid XPath result type".to_string()))?;
+
+        let global = self.global();
+        let window = global.as_window();
+
+        let result_value = evaluate_parsed_xpath(&self.parsed_expression, context_node)
+            .map_err(|_e| Error::Operation)?;
+
+        // TODO(vlindhol): support putting results into mutable `_result` as per the spec
+        Ok(XPathResult::new(
+            window,
+            None,
+            can_gc,
+            result_type,
+            result_value.into(),
+        ))
+    }
+}
diff --git a/components/script/dom/xpathresult.rs b/components/script/dom/xpathresult.rs
new file mode 100644
index 00000000000..93905b05ff7
--- /dev/null
+++ b/components/script/dom/xpathresult.rs
@@ -0,0 +1,261 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use std::cell::Cell;
+
+use dom_struct::dom_struct;
+use js::rust::HandleObject;
+
+use crate::dom::bindings::codegen::Bindings::XPathResultBinding::{
+    XPathResultConstants, XPathResultMethods,
+};
+use crate::dom::bindings::error::{Error, Fallible};
+use crate::dom::bindings::reflector::{reflect_dom_object_with_proto, Reflector};
+use crate::dom::bindings::root::{Dom, DomRoot};
+use crate::dom::bindings::str::DOMString;
+use crate::dom::node::Node;
+use crate::dom::window::Window;
+use crate::script_runtime::CanGc;
+use crate::xpath::{NodesetHelpers, Value};
+
+#[repr(u16)]
+#[derive(Clone, Copy, Debug, Eq, JSTraceable, MallocSizeOf, Ord, PartialEq, PartialOrd)]
+pub enum XPathResultType {
+    Any = XPathResultConstants::ANY_TYPE,
+    Number = XPathResultConstants::NUMBER_TYPE,
+    String = XPathResultConstants::STRING_TYPE,
+    Boolean = XPathResultConstants::BOOLEAN_TYPE,
+    UnorderedNodeIterator = XPathResultConstants::UNORDERED_NODE_ITERATOR_TYPE,
+    OrderedNodeIterator = XPathResultConstants::ORDERED_NODE_ITERATOR_TYPE,
+    UnorderedNodeSnapshot = XPathResultConstants::UNORDERED_NODE_SNAPSHOT_TYPE,
+    OrderedNodeSnapshot = XPathResultConstants::ORDERED_NODE_SNAPSHOT_TYPE,
+    AnyUnorderedNode = XPathResultConstants::ANY_UNORDERED_NODE_TYPE,
+    FirstOrderedNode = XPathResultConstants::FIRST_ORDERED_NODE_TYPE,
+}
+
+impl TryFrom<u16> for XPathResultType {
+    type Error = ();
+
+    fn try_from(value: u16) -> Result<Self, Self::Error> {
+        match value {
+            XPathResultConstants::ANY_TYPE => Ok(Self::Any),
+            XPathResultConstants::NUMBER_TYPE => Ok(Self::Number),
+            XPathResultConstants::STRING_TYPE => Ok(Self::String),
+            XPathResultConstants::BOOLEAN_TYPE => Ok(Self::Boolean),
+            XPathResultConstants::UNORDERED_NODE_ITERATOR_TYPE => Ok(Self::UnorderedNodeIterator),
+            XPathResultConstants::ORDERED_NODE_ITERATOR_TYPE => Ok(Self::OrderedNodeIterator),
+            XPathResultConstants::UNORDERED_NODE_SNAPSHOT_TYPE => Ok(Self::UnorderedNodeSnapshot),
+            XPathResultConstants::ORDERED_NODE_SNAPSHOT_TYPE => Ok(Self::OrderedNodeSnapshot),
+            XPathResultConstants::ANY_UNORDERED_NODE_TYPE => Ok(Self::AnyUnorderedNode),
+            XPathResultConstants::FIRST_ORDERED_NODE_TYPE => Ok(Self::FirstOrderedNode),
+            _ => Err(()),
+        }
+    }
+}
+
+#[derive(JSTraceable, MallocSizeOf)]
+pub enum XPathResultValue {
+    Boolean(bool),
+    /// A IEEE-754 double-precision floating point number
+    Number(f64),
+    String(DOMString),
+    /// A collection of unique nodes
+    Nodeset(Vec<DomRoot<Node>>),
+}
+
+impl From<Value> for XPathResultValue {
+    fn from(value: Value) -> Self {
+        match value {
+            Value::Boolean(b) => XPathResultValue::Boolean(b),
+            Value::Number(n) => XPathResultValue::Number(n),
+            Value::String(s) => XPathResultValue::String(s.into()),
+            Value::Nodeset(nodes) => {
+                // Put the evaluation result into (unique) document order. This also re-roots them
+                // so that we are sure we can hold them for the lifetime of this XPathResult.
+                let rooted_nodes = nodes.document_order_unique();
+                XPathResultValue::Nodeset(rooted_nodes)
+            },
+        }
+    }
+}
+
+#[dom_struct]
+pub struct XPathResult {
+    reflector_: Reflector,
+    window: Dom<Window>,
+    result_type: XPathResultType,
+    value: XPathResultValue,
+    iterator_invalid: Cell<bool>,
+    iterator_pos: Cell<usize>,
+}
+
+impl XPathResult {
+    fn new_inherited(
+        window: &Window,
+        result_type: XPathResultType,
+        value: XPathResultValue,
+    ) -> XPathResult {
+        // TODO(vlindhol): if the wanted result type is AnyUnorderedNode | FirstOrderedNode,
+        // we could drop all nodes except one to save memory.
+        let inferred_result_type = if result_type == XPathResultType::Any {
+            match value {
+                XPathResultValue::Boolean(_) => XPathResultType::Boolean,
+                XPathResultValue::Number(_) => XPathResultType::Number,
+                XPathResultValue::String(_) => XPathResultType::String,
+                XPathResultValue::Nodeset(_) => XPathResultType::UnorderedNodeIterator,
+            }
+        } else {
+            result_type
+        };
+
+        XPathResult {
+            reflector_: Reflector::new(),
+            window: Dom::from_ref(window),
+            result_type: inferred_result_type,
+            iterator_invalid: Cell::new(false),
+            iterator_pos: Cell::new(0),
+            value,
+        }
+    }
+
+    /// NB: Blindly trusts `result_type` and constructs an object regardless of the contents
+    /// of `value`. The exception is `XPathResultType::Any`, for which we look at the value
+    /// to determine the type.
+    pub fn new(
+        window: &Window,
+        proto: Option<HandleObject>,
+        can_gc: CanGc,
+        result_type: XPathResultType,
+        value: XPathResultValue,
+    ) -> DomRoot<XPathResult> {
+        reflect_dom_object_with_proto(
+            Box::new(XPathResult::new_inherited(window, result_type, value)),
+            window,
+            proto,
+            can_gc,
+        )
+    }
+}
+
+impl XPathResultMethods<crate::DomTypeHolder> for XPathResult {
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-resulttype>
+    fn ResultType(&self) -> u16 {
+        self.result_type as u16
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-numbervalue>
+    fn GetNumberValue(&self) -> Fallible<f64> {
+        match (&self.value, self.result_type) {
+            (XPathResultValue::Number(n), XPathResultType::Number) => Ok(*n),
+            _ => Err(Error::Type(
+                "Can't get number value for non-number XPathResult".to_string(),
+            )),
+        }
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-stringvalue>
+    fn GetStringValue(&self) -> Fallible<DOMString> {
+        match (&self.value, self.result_type) {
+            (XPathResultValue::String(s), XPathResultType::String) => Ok(s.clone()),
+            _ => Err(Error::Type(
+                "Can't get string value for non-string XPathResult".to_string(),
+            )),
+        }
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-booleanvalue>
+    fn GetBooleanValue(&self) -> Fallible<bool> {
+        match (&self.value, self.result_type) {
+            (XPathResultValue::Boolean(b), XPathResultType::Boolean) => Ok(*b),
+            _ => Err(Error::Type(
+                "Can't get boolean value for non-boolean XPathResult".to_string(),
+            )),
+        }
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-iteratenext>
+    fn IterateNext(&self) -> Fallible<Option<DomRoot<Node>>> {
+        // TODO(vlindhol): actually set `iterator_invalid` somewhere
+        if self.iterator_invalid.get() {
+            return Err(Error::Range(
+                "Invalidated iterator for XPathResult, the DOM has mutated.".to_string(),
+            ));
+        }
+
+        match (&self.value, self.result_type) {
+            (
+                XPathResultValue::Nodeset(nodes),
+                XPathResultType::OrderedNodeIterator | XPathResultType::UnorderedNodeIterator,
+            ) => {
+                let pos = self.iterator_pos.get();
+                if pos >= nodes.len() {
+                    Ok(None)
+                } else {
+                    let node = nodes[pos].clone();
+                    self.iterator_pos.set(pos + 1);
+                    Ok(Some(node))
+                }
+            },
+            _ => Err(Error::Type(
+                "Can't iterate on XPathResult that is not a node-set".to_string(),
+            )),
+        }
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-invaliditeratorstate>
+    fn GetInvalidIteratorState(&self) -> Fallible<bool> {
+        let is_iterator_invalid = self.iterator_invalid.get();
+        if is_iterator_invalid ||
+            matches!(
+                self.result_type,
+                XPathResultType::OrderedNodeIterator | XPathResultType::UnorderedNodeIterator
+            )
+        {
+            Ok(is_iterator_invalid)
+        } else {
+            Err(Error::Type(
+                "Can't iterate on XPathResult that is not a node-set".to_string(),
+            ))
+        }
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-snapshotlength>
+    fn GetSnapshotLength(&self) -> Fallible<u32> {
+        match (&self.value, self.result_type) {
+            (
+                XPathResultValue::Nodeset(nodes),
+                XPathResultType::OrderedNodeSnapshot | XPathResultType::UnorderedNodeSnapshot,
+            ) => Ok(nodes.len() as u32),
+            _ => Err(Error::Type(
+                "Can't get snapshot length of XPathResult that is not a snapshot".to_string(),
+            )),
+        }
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-snapshotitem>
+    fn SnapshotItem(&self, index: u32) -> Fallible<Option<DomRoot<Node>>> {
+        match (&self.value, self.result_type) {
+            (
+                XPathResultValue::Nodeset(nodes),
+                XPathResultType::OrderedNodeSnapshot | XPathResultType::UnorderedNodeSnapshot,
+            ) => Ok(nodes.get(index as usize).cloned()),
+            _ => Err(Error::Type(
+                "Can't get snapshot item of XPathResult that is not a snapshot".to_string(),
+            )),
+        }
+    }
+
+    /// <https://dom.spec.whatwg.org/#dom-xpathresult-singlenodevalue>
+    fn GetSingleNodeValue(&self) -> Fallible<Option<DomRoot<Node>>> {
+        match (&self.value, self.result_type) {
+            (
+                XPathResultValue::Nodeset(nodes),
+                XPathResultType::AnyUnorderedNode | XPathResultType::FirstOrderedNode,
+            ) => Ok(nodes.first().cloned()),
+            _ => Err(Error::Type(
+                "Getting single value requires result type 'any unordered node' or 'first ordered node'".to_string(),
+            )),
+        }
+    }
+}