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>

components/script/xpath/context.rs

@@ -0,0 +1,95 @@
+/* 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::iter::Enumerate;
+use std::vec::IntoIter;
+
+use super::Node;
+use crate::dom::bindings::root::DomRoot;
+
+/// The context during evaluation of an XPath expression.
+pub struct EvaluationCtx {
+    /// Where we started at
+    pub starting_node: DomRoot<Node>,
+    /// The "current" node in the evaluation
+    pub context_node: DomRoot<Node>,
+    /// Details needed for evaluating a predicate list
+    pub predicate_ctx: Option<PredicateCtx>,
+    /// The nodes we're currently matching against
+    pub predicate_nodes: Option<Vec<DomRoot<Node>>>,
+}
+
+#[derive(Clone, Copy)]
+pub struct PredicateCtx {
+    pub index: usize,
+    pub size: usize,
+}
+
+impl EvaluationCtx {
+    /// Prepares the context used while evaluating the XPath expression
+    pub fn new(context_node: &Node) -> EvaluationCtx {
+        EvaluationCtx {
+            starting_node: DomRoot::from_ref(context_node),
+            context_node: DomRoot::from_ref(context_node),
+            predicate_ctx: None,
+            predicate_nodes: None,
+        }
+    }
+
+    /// Creates a new context using the provided node as the context node
+    pub fn subcontext_for_node(&self, node: &Node) -> EvaluationCtx {
+        EvaluationCtx {
+            starting_node: self.starting_node.clone(),
+            context_node: DomRoot::from_ref(node),
+            predicate_ctx: self.predicate_ctx,
+            predicate_nodes: self.predicate_nodes.clone(),
+        }
+    }
+
+    pub fn update_predicate_nodes(&self, nodes: Vec<&Node>) -> EvaluationCtx {
+        EvaluationCtx {
+            starting_node: self.starting_node.clone(),
+            context_node: self.context_node.clone(),
+            predicate_ctx: None,
+            predicate_nodes: Some(nodes.into_iter().map(DomRoot::from_ref).collect()),
+        }
+    }
+
+    pub fn subcontext_iter_for_nodes(&self) -> EvalNodesetIter {
+        let size = self.predicate_nodes.as_ref().map_or(0, |v| v.len());
+        EvalNodesetIter {
+            ctx: self,
+            nodes_iter: self
+                .predicate_nodes
+                .as_ref()
+                .map_or_else(|| Vec::new().into_iter(), |v| v.clone().into_iter())
+                .enumerate(),
+            size,
+        }
+    }
+}
+
+/// When evaluating predicates, we need to keep track of the current node being evaluated and
+/// the index of that node in the nodeset we're operating on.
+pub struct EvalNodesetIter<'a> {
+    ctx: &'a EvaluationCtx,
+    nodes_iter: Enumerate<IntoIter<DomRoot<Node>>>,
+    size: usize,
+}
+
+impl<'a> Iterator for EvalNodesetIter<'a> {
+    type Item = EvaluationCtx;
+
+    fn next(&mut self) -> Option<EvaluationCtx> {
+        self.nodes_iter.next().map(|(idx, node)| EvaluationCtx {
+            starting_node: self.ctx.starting_node.clone(),
+            context_node: node.clone(),
+            predicate_nodes: self.ctx.predicate_nodes.clone(),
+            predicate_ctx: Some(PredicateCtx {
+                index: idx + 1,
+                size: self.size,
+            }),
+        })
+    }
+}

components/script/xpath/eval.rs

@@ -0,0 +1,589 @@
+/* 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::fmt;
+
+use html5ever::{local_name, namespace_prefix, namespace_url, ns, QualName};
+
+use super::parser::{
+    AdditiveOp, Axis, EqualityOp, Expr, FilterExpr, KindTest, Literal, MultiplicativeOp, NodeTest,
+    NumericLiteral, PathExpr, PredicateExpr, PredicateListExpr, PrimaryExpr,
+    QName as ParserQualName, RelationalOp, StepExpr, UnaryOp,
+};
+use super::{EvaluationCtx, Value};
+use crate::dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
+use crate::dom::bindings::inheritance::{Castable, CharacterDataTypeId, NodeTypeId};
+use crate::dom::bindings::root::DomRoot;
+use crate::dom::bindings::xmlname::validate_and_extract;
+use crate::dom::element::Element;
+use crate::dom::node::{Node, ShadowIncluding};
+use crate::dom::processinginstruction::ProcessingInstruction;
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Error {
+    NotANodeset,
+    InvalidPath,
+    UnknownFunction { name: QualName },
+    UnknownVariable { name: QualName },
+    UnknownNamespace { prefix: String },
+    InvalidQName { qname: ParserQualName },
+    FunctionEvaluation { fname: String },
+    Internal { msg: String },
+}
+
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Error::NotANodeset => write!(f, "expression did not evaluate to a nodeset"),
+            Error::InvalidPath => write!(f, "invalid path expression"),
+            Error::UnknownFunction { name } => write!(f, "unknown function {:?}", name),
+            Error::UnknownVariable { name } => write!(f, "unknown variable {:?}", name),
+            Error::UnknownNamespace { prefix } => {
+                write!(f, "unknown namespace prefix {:?}", prefix)
+            },
+            Error::InvalidQName { qname } => {
+                write!(f, "invalid QName {:?}", qname)
+            },
+            Error::FunctionEvaluation { fname } => {
+                write!(f, "error while evaluating function: {}", fname)
+            },
+            Error::Internal { msg } => {
+                write!(f, "internal error: {}", msg)
+            },
+        }
+    }
+}
+
+impl std::error::Error for Error {}
+
+pub fn try_extract_nodeset(v: Value) -> Result<Vec<DomRoot<Node>>, Error> {
+    match v {
+        Value::Nodeset(ns) => Ok(ns),
+        _ => Err(Error::NotANodeset),
+    }
+}
+
+pub trait Evaluatable: fmt::Debug {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error>;
+    /// Returns true if this expression evaluates to a primitive value, without needing to touch the DOM
+    fn is_primitive(&self) -> bool;
+}
+
+impl<T: ?Sized> Evaluatable for Box<T>
+where
+    T: Evaluatable,
+{
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        (**self).evaluate(context)
+    }
+
+    fn is_primitive(&self) -> bool {
+        (**self).is_primitive()
+    }
+}
+
+impl Evaluatable for Expr {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        match self {
+            Expr::And(left, right) => {
+                let left_bool = left.evaluate(context)?.boolean();
+                let v = left_bool && right.evaluate(context)?.boolean();
+                Ok(Value::Boolean(v))
+            },
+            Expr::Or(left, right) => {
+                let left_bool = left.evaluate(context)?.boolean();
+                let v = left_bool || right.evaluate(context)?.boolean();
+                Ok(Value::Boolean(v))
+            },
+            Expr::Equality(left, equality_op, right) => {
+                let left_val = left.evaluate(context)?;
+                let right_val = right.evaluate(context)?;
+
+                let v = match equality_op {
+                    EqualityOp::Eq => left_val == right_val,
+                    EqualityOp::NotEq => left_val != right_val,
+                };
+
+                Ok(Value::Boolean(v))
+            },
+            Expr::Relational(left, relational_op, right) => {
+                let left_val = left.evaluate(context)?.number();
+                let right_val = right.evaluate(context)?.number();
+
+                let v = match relational_op {
+                    RelationalOp::Lt => left_val < right_val,
+                    RelationalOp::Gt => left_val > right_val,
+                    RelationalOp::LtEq => left_val <= right_val,
+                    RelationalOp::GtEq => left_val >= right_val,
+                };
+                Ok(Value::Boolean(v))
+            },
+            Expr::Additive(left, additive_op, right) => {
+                let left_val = left.evaluate(context)?.number();
+                let right_val = right.evaluate(context)?.number();
+
+                let v = match additive_op {
+                    AdditiveOp::Add => left_val + right_val,
+                    AdditiveOp::Sub => left_val - right_val,
+                };
+                Ok(Value::Number(v))
+            },
+            Expr::Multiplicative(left, multiplicative_op, right) => {
+                let left_val = left.evaluate(context)?.number();
+                let right_val = right.evaluate(context)?.number();
+
+                let v = match multiplicative_op {
+                    MultiplicativeOp::Mul => left_val * right_val,
+                    MultiplicativeOp::Div => left_val / right_val,
+                    MultiplicativeOp::Mod => left_val % right_val,
+                };
+                Ok(Value::Number(v))
+            },
+            Expr::Unary(unary_op, expr) => {
+                let v = expr.evaluate(context)?.number();
+
+                match unary_op {
+                    UnaryOp::Minus => Ok(Value::Number(-v)),
+                }
+            },
+            Expr::Union(left, right) => {
+                let as_nodes = |e: &Expr| e.evaluate(context).and_then(try_extract_nodeset);
+
+                let mut left_nodes = as_nodes(left)?;
+                let right_nodes = as_nodes(right)?;
+
+                left_nodes.extend(right_nodes);
+                Ok(Value::Nodeset(left_nodes))
+            },
+            Expr::Path(path_expr) => path_expr.evaluate(context),
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        match self {
+            Expr::Or(left, right) => left.is_primitive() && right.is_primitive(),
+            Expr::And(left, right) => left.is_primitive() && right.is_primitive(),
+            Expr::Equality(left, _, right) => left.is_primitive() && right.is_primitive(),
+            Expr::Relational(left, _, right) => left.is_primitive() && right.is_primitive(),
+            Expr::Additive(left, _, right) => left.is_primitive() && right.is_primitive(),
+            Expr::Multiplicative(left, _, right) => left.is_primitive() && right.is_primitive(),
+            Expr::Unary(_, expr) => expr.is_primitive(),
+            Expr::Union(_, _) => false,
+            Expr::Path(path_expr) => path_expr.is_primitive(),
+        }
+    }
+}
+
+impl Evaluatable for PathExpr {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        let mut current_nodes = vec![context.context_node.clone()];
+
+        // If path starts with '//', add an implicit descendant-or-self::node() step
+        if self.is_descendant {
+            current_nodes = current_nodes
+                .iter()
+                .flat_map(|n| n.traverse_preorder(ShadowIncluding::No))
+                .collect();
+        }
+
+        trace!("[PathExpr] Evaluating path expr: {:?}", self);
+
+        let have_multiple_steps = self.steps.len() > 1;
+
+        for step in &self.steps {
+            let mut next_nodes = Vec::new();
+            for node in current_nodes {
+                let step_context = context.subcontext_for_node(&node);
+                let step_result = step.evaluate(&step_context)?;
+                match (have_multiple_steps, step_result) {
+                    (_, Value::Nodeset(mut nodes)) => {
+                        // as long as we evaluate to nodesets, keep going
+                        next_nodes.append(&mut nodes);
+                    },
+                    (false, value) => {
+                        trace!("[PathExpr] Got single primitive value: {:?}", value);
+                        return Ok(value);
+                    },
+                    (true, value) => {
+                        error!(
+                        "Expected nodeset from step evaluation, got: {:?} node: {:?}, step: {:?}",
+                        value, node, step
+                    );
+                        return Ok(value);
+                    },
+                }
+            }
+            current_nodes = next_nodes;
+        }
+
+        trace!("[PathExpr] Got nodes: {:?}", current_nodes);
+
+        Ok(Value::Nodeset(current_nodes))
+    }
+
+    fn is_primitive(&self) -> bool {
+        !self.is_absolute &&
+            !self.is_descendant &&
+            self.steps.len() == 1 &&
+            self.steps[0].is_primitive()
+    }
+}
+
+impl TryFrom<&ParserQualName> for QualName {
+    type Error = Error;
+
+    fn try_from(qname: &ParserQualName) -> Result<Self, Self::Error> {
+        let qname_as_str = qname.to_string();
+        if let Ok((ns, prefix, local)) = validate_and_extract(None, &qname_as_str) {
+            Ok(QualName { prefix, ns, local })
+        } else {
+            Err(Error::InvalidQName {
+                qname: qname.clone(),
+            })
+        }
+    }
+}
+
+pub enum NameTestComparisonMode {
+    /// Namespaces must match exactly
+    XHtml,
+    /// Missing namespace information is treated as the HTML namespace
+    Html,
+}
+
+pub fn element_name_test(
+    expected_name: QualName,
+    element_qualname: QualName,
+    comparison_mode: NameTestComparisonMode,
+) -> bool {
+    let is_wildcard = expected_name.local == local_name!("*");
+
+    let test_prefix = expected_name
+        .prefix
+        .clone()
+        .unwrap_or(namespace_prefix!(""));
+    let test_ns_uri = match test_prefix {
+        namespace_prefix!("*") => ns!(*),
+        namespace_prefix!("html") => ns!(html),
+        namespace_prefix!("xml") => ns!(xml),
+        namespace_prefix!("xlink") => ns!(xlink),
+        namespace_prefix!("svg") => ns!(svg),
+        namespace_prefix!("mathml") => ns!(mathml),
+        namespace_prefix!("") => {
+            if matches!(comparison_mode, NameTestComparisonMode::XHtml) {
+                ns!()
+            } else {
+                ns!(html)
+            }
+        },
+        _ => {
+            // We don't support custom namespaces, use fallback or panic depending on strictness
+            if matches!(comparison_mode, NameTestComparisonMode::XHtml) {
+                panic!("Unrecognized namespace prefix: {}", test_prefix)
+            } else {
+                ns!(html)
+            }
+        },
+    };
+
+    if is_wildcard {
+        test_ns_uri == element_qualname.ns
+    } else {
+        test_ns_uri == element_qualname.ns && expected_name.local == element_qualname.local
+    }
+}
+
+fn apply_node_test(test: &NodeTest, node: &Node) -> Result<bool, Error> {
+    let result = match test {
+        NodeTest::Name(qname) => {
+            // Convert the unvalidated "parser QualName" into the proper QualName structure
+            let wanted_name: QualName = qname.try_into()?;
+            if matches!(node.type_id(), NodeTypeId::Element(_)) {
+                let element = node.downcast::<Element>().unwrap();
+                let comparison_mode = if node.owner_doc().is_xhtml_document() {
+                    NameTestComparisonMode::XHtml
+                } else {
+                    NameTestComparisonMode::Html
+                };
+                let element_qualname = QualName::new(
+                    element.prefix().as_ref().cloned(),
+                    element.namespace().clone(),
+                    element.local_name().clone(),
+                );
+                element_name_test(wanted_name, element_qualname, comparison_mode)
+            } else {
+                false
+            }
+        },
+        NodeTest::Wildcard => true,
+        NodeTest::Kind(kind) => match kind {
+            KindTest::PI(target) => {
+                if NodeTypeId::CharacterData(CharacterDataTypeId::ProcessingInstruction) ==
+                    node.type_id()
+                {
+                    let pi = node.downcast::<ProcessingInstruction>().unwrap();
+                    match (target, pi.target()) {
+                        (Some(target_name), node_target_name)
+                            if target_name == &node_target_name.to_string() =>
+                        {
+                            true
+                        },
+                        (Some(_), _) => false,
+                        (None, _) => true,
+                    }
+                } else {
+                    false
+                }
+            },
+            KindTest::Comment => matches!(
+                node.type_id(),
+                NodeTypeId::CharacterData(CharacterDataTypeId::Comment)
+            ),
+            KindTest::Text => matches!(
+                node.type_id(),
+                NodeTypeId::CharacterData(CharacterDataTypeId::Text(_))
+            ),
+            KindTest::Node => true,
+        },
+    };
+    Ok(result)
+}
+
+impl Evaluatable for StepExpr {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        match self {
+            StepExpr::Filter(filter_expr) => filter_expr.evaluate(context),
+            StepExpr::Axis(axis_step) => {
+                let nodes: Vec<DomRoot<Node>> = match axis_step.axis {
+                    Axis::Child => context.context_node.children().collect(),
+                    Axis::Descendant => context
+                        .context_node
+                        .traverse_preorder(ShadowIncluding::No)
+                        .skip(1)
+                        .collect(),
+                    Axis::Parent => vec![context.context_node.GetParentNode()]
+                        .into_iter()
+                        .flatten()
+                        .collect(),
+                    Axis::Ancestor => context.context_node.ancestors().collect(),
+                    Axis::Following => context
+                        .context_node
+                        .following_nodes(&context.context_node)
+                        .skip(1)
+                        .collect(),
+                    Axis::Preceding => context
+                        .context_node
+                        .preceding_nodes(&context.context_node)
+                        .skip(1)
+                        .collect(),
+                    Axis::FollowingSibling => context.context_node.following_siblings().collect(),
+                    Axis::PrecedingSibling => context.context_node.preceding_siblings().collect(),
+                    Axis::Attribute => {
+                        if matches!(Node::type_id(&context.context_node), NodeTypeId::Element(_)) {
+                            let element = context.context_node.downcast::<Element>().unwrap();
+                            element
+                                .attrs()
+                                .iter()
+                                .map(|attr| attr.upcast::<Node>())
+                                .map(DomRoot::from_ref)
+                                .collect()
+                        } else {
+                            vec![]
+                        }
+                    },
+                    Axis::Self_ => vec![context.context_node.clone()],
+                    Axis::DescendantOrSelf => context
+                        .context_node
+                        .traverse_preorder(ShadowIncluding::No)
+                        .collect(),
+                    Axis::AncestorOrSelf => context
+                        .context_node
+                        .inclusive_ancestors(ShadowIncluding::No)
+                        .collect(),
+                    Axis::Namespace => Vec::new(), // Namespace axis is not commonly implemented
+                };
+
+                trace!("[StepExpr] Axis {:?} got nodes {:?}", axis_step.axis, nodes);
+
+                // Filter nodes according to the step's node_test. Will error out if any NodeTest
+                // application errors out.
+                let filtered_nodes: Vec<DomRoot<Node>> = nodes
+                    .into_iter()
+                    .map(|node| {
+                        apply_node_test(&axis_step.node_test, &node)
+                            .map(|matches| matches.then_some(node))
+                    })
+                    .collect::<Result<Vec<_>, _>>()?
+                    .into_iter()
+                    .flatten()
+                    .collect();
+
+                trace!("[StepExpr] Filtering got nodes {:?}", filtered_nodes);
+
+                if axis_step.predicates.predicates.is_empty() {
+                    trace!(
+                        "[StepExpr] No predicates, returning nodes {:?}",
+                        filtered_nodes
+                    );
+                    Ok(Value::Nodeset(filtered_nodes))
+                } else {
+                    // Apply predicates
+                    let predicate_list_subcontext = context
+                        .update_predicate_nodes(filtered_nodes.iter().map(|n| &**n).collect());
+                    axis_step.predicates.evaluate(&predicate_list_subcontext)
+                }
+            },
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        match self {
+            StepExpr::Filter(filter_expr) => filter_expr.is_primitive(),
+            StepExpr::Axis(_) => false,
+        }
+    }
+}
+
+impl Evaluatable for PredicateListExpr {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        if let Some(ref predicate_nodes) = context.predicate_nodes {
+            // Initializing: every node the predicates act on is matched
+            let mut matched_nodes: Vec<DomRoot<Node>> = predicate_nodes.clone();
+
+            // apply each predicate to the nodes matched by the previous predicate
+            for predicate_expr in &self.predicates {
+                let context_for_predicate =
+                    context.update_predicate_nodes(matched_nodes.iter().map(|n| &**n).collect());
+
+                let narrowed_nodes = predicate_expr
+                    .evaluate(&context_for_predicate)
+                    .and_then(try_extract_nodeset)?;
+                matched_nodes = narrowed_nodes;
+                trace!(
+                    "[PredicateListExpr] Predicate {:?} matched nodes {:?}",
+                    predicate_expr,
+                    matched_nodes
+                );
+            }
+            Ok(Value::Nodeset(matched_nodes))
+        } else {
+            Err(Error::Internal {
+                msg: "[PredicateListExpr] No nodes on stack for predicate to operate on"
+                    .to_string(),
+            })
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        self.predicates.len() == 1 && self.predicates[0].is_primitive()
+    }
+}
+
+impl Evaluatable for PredicateExpr {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        let narrowed_nodes: Result<Vec<DomRoot<Node>>, Error> = context
+            .subcontext_iter_for_nodes()
+            .filter_map(|ctx| {
+                if let Some(predicate_ctx) = ctx.predicate_ctx {
+                    let eval_result = self.expr.evaluate(&ctx);
+
+                    let v = match eval_result {
+                        Ok(Value::Number(v)) => Ok(predicate_ctx.index == v as usize),
+                        Ok(v) => Ok(v.boolean()),
+                        Err(e) => Err(e),
+                    };
+
+                    match v {
+                        Ok(true) => Some(Ok(ctx.context_node)),
+                        Ok(false) => None,
+                        Err(e) => Some(Err(e)),
+                    }
+                } else {
+                    Some(Err(Error::Internal {
+                        msg: "[PredicateExpr] No predicate context set".to_string(),
+                    }))
+                }
+            })
+            .collect();
+
+        Ok(Value::Nodeset(narrowed_nodes?))
+    }
+
+    fn is_primitive(&self) -> bool {
+        self.expr.is_primitive()
+    }
+}
+
+impl Evaluatable for FilterExpr {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        let primary_result = self.primary.evaluate(context)?;
+        let have_predicates = !self.predicates.predicates.is_empty();
+
+        match (have_predicates, &primary_result) {
+            (false, _) => {
+                trace!(
+                    "[FilterExpr] No predicates, returning primary result: {:?}",
+                    primary_result
+                );
+                Ok(primary_result)
+            },
+            (true, Value::Nodeset(vec)) => {
+                let predicate_list_subcontext =
+                    context.update_predicate_nodes(vec.iter().map(|n| &**n).collect());
+                let result_filtered_by_predicates =
+                    self.predicates.evaluate(&predicate_list_subcontext);
+                trace!(
+                    "[FilterExpr] Result filtered by predicates: {:?}",
+                    result_filtered_by_predicates
+                );
+                result_filtered_by_predicates
+            },
+            // You can't use filtering expressions `[]` on other than node-sets
+            (true, _) => Err(Error::NotANodeset),
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        self.predicates.predicates.is_empty() && self.primary.is_primitive()
+    }
+}
+
+impl Evaluatable for PrimaryExpr {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        match self {
+            PrimaryExpr::Literal(literal) => literal.evaluate(context),
+            PrimaryExpr::Variable(_qname) => todo!(),
+            PrimaryExpr::Parenthesized(expr) => expr.evaluate(context),
+            PrimaryExpr::ContextItem => Ok(Value::Nodeset(vec![context.context_node.clone()])),
+            PrimaryExpr::Function(core_function) => core_function.evaluate(context),
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        match self {
+            PrimaryExpr::Literal(_) => true,
+            PrimaryExpr::Variable(_qname) => false,
+            PrimaryExpr::Parenthesized(expr) => expr.is_primitive(),
+            PrimaryExpr::ContextItem => false,
+            PrimaryExpr::Function(_) => false,
+        }
+    }
+}
+
+impl Evaluatable for Literal {
+    fn evaluate(&self, _context: &EvaluationCtx) -> Result<Value, Error> {
+        match self {
+            Literal::Numeric(numeric_literal) => match numeric_literal {
+                // We currently make no difference between ints and floats
+                NumericLiteral::Integer(v) => Ok(Value::Number(*v as f64)),
+                NumericLiteral::Decimal(v) => Ok(Value::Number(*v)),
+            },
+            Literal::String(s) => Ok(Value::String(s.into())),
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        true
+    }
+}

components/script/xpath/eval_function.rs

@@ -0,0 +1,357 @@
+/* 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 super::context::EvaluationCtx;
+use super::eval::{try_extract_nodeset, Error, Evaluatable};
+use super::parser::CoreFunction;
+use super::Value;
+use crate::dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
+use crate::dom::bindings::inheritance::{Castable, NodeTypeId};
+use crate::dom::element::Element;
+use crate::dom::node::Node;
+
+/// Returns e.g. "rect" for `<svg:rect>`
+fn local_name(node: &Node) -> Option<String> {
+    if matches!(Node::type_id(node), NodeTypeId::Element(_)) {
+        let element = node.downcast::<Element>().unwrap();
+        Some(element.local_name().to_string())
+    } else {
+        None
+    }
+}
+
+/// Returns e.g. "svg:rect" for `<svg:rect>`
+fn name(node: &Node) -> Option<String> {
+    if matches!(Node::type_id(node), NodeTypeId::Element(_)) {
+        let element = node.downcast::<Element>().unwrap();
+        if let Some(prefix) = element.prefix().as_ref() {
+            Some(format!("{}:{}", prefix, element.local_name()))
+        } else {
+            Some(element.local_name().to_string())
+        }
+    } else {
+        None
+    }
+}
+
+/// Returns e.g. the SVG namespace URI for `<svg:rect>`
+fn namespace_uri(node: &Node) -> Option<String> {
+    if matches!(Node::type_id(node), NodeTypeId::Element(_)) {
+        let element = node.downcast::<Element>().unwrap();
+        Some(element.namespace().to_string())
+    } else {
+        None
+    }
+}
+
+/// Returns the text contents of the Node, or empty string if none.
+fn string_value(node: &Node) -> String {
+    node.GetTextContent().unwrap_or_default().to_string()
+}
+
+/// If s2 is found inside s1, return everything *before* s2. Return all of s1 otherwise.
+fn substring_before(s1: &str, s2: &str) -> String {
+    match s1.find(s2) {
+        Some(pos) => s1[..pos].to_string(),
+        None => String::new(),
+    }
+}
+
+/// If s2 is found inside s1, return everything *after* s2. Return all of s1 otherwise.
+fn substring_after(s1: &str, s2: &str) -> String {
+    match s1.find(s2) {
+        Some(pos) => s1[pos + s2.len()..].to_string(),
+        None => String::new(),
+    }
+}
+
+fn substring(s: &str, start_idx: isize, len: Option<isize>) -> String {
+    let s_len = s.len();
+    let len = len.unwrap_or(s_len as isize).max(0) as usize;
+    let start_idx = start_idx.max(0) as usize;
+    let end_idx = (start_idx + len.max(0)).min(s_len);
+    s[start_idx..end_idx].to_string()
+}
+
+/// <https://www.w3.org/TR/1999/REC-xpath-19991116/#function-normalize-space>
+pub fn normalize_space(s: &str) -> String {
+    let mut result = String::with_capacity(s.len());
+    let mut last_was_whitespace = true; // Handles leading whitespace
+
+    for c in s.chars() {
+        match c {
+            '\x20' | '\x09' | '\x0D' | '\x0A' => {
+                if !last_was_whitespace {
+                    result.push(' ');
+                    last_was_whitespace = true;
+                }
+            },
+            other => {
+                result.push(other);
+                last_was_whitespace = false;
+            },
+        }
+    }
+
+    if last_was_whitespace {
+        result.pop();
+    }
+
+    result
+}
+
+impl Evaluatable for CoreFunction {
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        match self {
+            CoreFunction::Last => {
+                let predicate_ctx = context.predicate_ctx.ok_or_else(|| Error::Internal {
+                    msg: "[CoreFunction] last() is only usable as a predicate".to_string(),
+                })?;
+                Ok(Value::Number(predicate_ctx.size as f64))
+            },
+            CoreFunction::Position => {
+                let predicate_ctx = context.predicate_ctx.ok_or_else(|| Error::Internal {
+                    msg: "[CoreFunction] position() is only usable as a predicate".to_string(),
+                })?;
+                Ok(Value::Number(predicate_ctx.index as f64))
+            },
+            CoreFunction::Count(expr) => {
+                let nodes = expr.evaluate(context).and_then(try_extract_nodeset)?;
+                Ok(Value::Number(nodes.len() as f64))
+            },
+            CoreFunction::String(expr_opt) => match expr_opt {
+                Some(expr) => Ok(Value::String(expr.evaluate(context)?.string())),
+                None => Ok(Value::String(string_value(&context.context_node))),
+            },
+            CoreFunction::Concat(exprs) => {
+                let strings: Result<Vec<_>, _> = exprs
+                    .iter()
+                    .map(|e| Ok(e.evaluate(context)?.string()))
+                    .collect();
+                Ok(Value::String(strings?.join("")))
+            },
+            CoreFunction::Id(_expr) => todo!(),
+            CoreFunction::LocalName(expr_opt) => {
+                let node = match expr_opt {
+                    Some(expr) => expr
+                        .evaluate(context)
+                        .and_then(try_extract_nodeset)?
+                        .first()
+                        .cloned(),
+                    None => Some(context.context_node.clone()),
+                };
+                let name = node.and_then(|n| local_name(&n)).unwrap_or_default();
+                Ok(Value::String(name.to_string()))
+            },
+            CoreFunction::NamespaceUri(expr_opt) => {
+                let node = match expr_opt {
+                    Some(expr) => expr
+                        .evaluate(context)
+                        .and_then(try_extract_nodeset)?
+                        .first()
+                        .cloned(),
+                    None => Some(context.context_node.clone()),
+                };
+                let ns = node.and_then(|n| namespace_uri(&n)).unwrap_or_default();
+                Ok(Value::String(ns.to_string()))
+            },
+            CoreFunction::Name(expr_opt) => {
+                let node = match expr_opt {
+                    Some(expr) => expr
+                        .evaluate(context)
+                        .and_then(try_extract_nodeset)?
+                        .first()
+                        .cloned(),
+                    None => Some(context.context_node.clone()),
+                };
+                let name = node.and_then(|n| name(&n)).unwrap_or_default();
+                Ok(Value::String(name))
+            },
+            CoreFunction::StartsWith(str1, str2) => {
+                let s1 = str1.evaluate(context)?.string();
+                let s2 = str2.evaluate(context)?.string();
+                Ok(Value::Boolean(s1.starts_with(&s2)))
+            },
+            CoreFunction::Contains(str1, str2) => {
+                let s1 = str1.evaluate(context)?.string();
+                let s2 = str2.evaluate(context)?.string();
+                Ok(Value::Boolean(s1.contains(&s2)))
+            },
+            CoreFunction::SubstringBefore(str1, str2) => {
+                let s1 = str1.evaluate(context)?.string();
+                let s2 = str2.evaluate(context)?.string();
+                Ok(Value::String(substring_before(&s1, &s2)))
+            },
+            CoreFunction::SubstringAfter(str1, str2) => {
+                let s1 = str1.evaluate(context)?.string();
+                let s2 = str2.evaluate(context)?.string();
+                Ok(Value::String(substring_after(&s1, &s2)))
+            },
+            CoreFunction::Substring(str1, start, length_opt) => {
+                let s = str1.evaluate(context)?.string();
+                let start_idx = start.evaluate(context)?.number().round() as isize - 1;
+                let len = match length_opt {
+                    Some(len_expr) => Some(len_expr.evaluate(context)?.number().round() as isize),
+                    None => None,
+                };
+                Ok(Value::String(substring(&s, start_idx, len)))
+            },
+            CoreFunction::StringLength(expr_opt) => {
+                let s = match expr_opt {
+                    Some(expr) => expr.evaluate(context)?.string(),
+                    None => string_value(&context.context_node),
+                };
+                Ok(Value::Number(s.chars().count() as f64))
+            },
+            CoreFunction::NormalizeSpace(expr_opt) => {
+                let s = match expr_opt {
+                    Some(expr) => expr.evaluate(context)?.string(),
+                    None => string_value(&context.context_node),
+                };
+
+                Ok(Value::String(normalize_space(&s)))
+            },
+            CoreFunction::Translate(str1, str2, str3) => {
+                let s = str1.evaluate(context)?.string();
+                let from = str2.evaluate(context)?.string();
+                let to = str3.evaluate(context)?.string();
+                let result = s
+                    .chars()
+                    .map(|c| match from.find(c) {
+                        Some(i) if i < to.chars().count() => to.chars().nth(i).unwrap(),
+                        _ => c,
+                    })
+                    .collect();
+                Ok(Value::String(result))
+            },
+            CoreFunction::Number(expr_opt) => {
+                let val = match expr_opt {
+                    Some(expr) => expr.evaluate(context)?,
+                    None => Value::String(string_value(&context.context_node)),
+                };
+                Ok(Value::Number(val.number()))
+            },
+            CoreFunction::Sum(expr) => {
+                let nodes = expr.evaluate(context).and_then(try_extract_nodeset)?;
+                let sum = nodes
+                    .iter()
+                    .map(|n| Value::String(string_value(n)).number())
+                    .sum();
+                Ok(Value::Number(sum))
+            },
+            CoreFunction::Floor(expr) => {
+                let num = expr.evaluate(context)?.number();
+                Ok(Value::Number(num.floor()))
+            },
+            CoreFunction::Ceiling(expr) => {
+                let num = expr.evaluate(context)?.number();
+                Ok(Value::Number(num.ceil()))
+            },
+            CoreFunction::Round(expr) => {
+                let num = expr.evaluate(context)?.number();
+                Ok(Value::Number(num.round()))
+            },
+            CoreFunction::Boolean(expr) => Ok(Value::Boolean(expr.evaluate(context)?.boolean())),
+            CoreFunction::Not(expr) => Ok(Value::Boolean(!expr.evaluate(context)?.boolean())),
+            CoreFunction::True => Ok(Value::Boolean(true)),
+            CoreFunction::False => Ok(Value::Boolean(false)),
+            CoreFunction::Lang(_) => Ok(Value::Nodeset(vec![])), // Not commonly used in the DOM, short-circuit it
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        match self {
+            CoreFunction::Last => false,
+            CoreFunction::Position => false,
+            CoreFunction::Count(_) => false,
+            CoreFunction::Id(_) => false,
+            CoreFunction::LocalName(_) => false,
+            CoreFunction::NamespaceUri(_) => false,
+            CoreFunction::Name(_) => false,
+            CoreFunction::String(expr_opt) => expr_opt
+                .as_ref()
+                .map(|expr| expr.is_primitive())
+                .unwrap_or(false),
+            CoreFunction::Concat(vec) => vec.iter().all(|expr| expr.is_primitive()),
+            CoreFunction::StartsWith(expr, substr) => expr.is_primitive() && substr.is_primitive(),
+            CoreFunction::Contains(expr, substr) => expr.is_primitive() && substr.is_primitive(),
+            CoreFunction::SubstringBefore(expr, substr) => {
+                expr.is_primitive() && substr.is_primitive()
+            },
+            CoreFunction::SubstringAfter(expr, substr) => {
+                expr.is_primitive() && substr.is_primitive()
+            },
+            CoreFunction::Substring(expr, start_pos, length_opt) => {
+                expr.is_primitive() &&
+                    start_pos.is_primitive() &&
+                    length_opt
+                        .as_ref()
+                        .map(|length| length.is_primitive())
+                        .unwrap_or(false)
+            },
+            CoreFunction::StringLength(expr_opt) => expr_opt
+                .as_ref()
+                .map(|expr| expr.is_primitive())
+                .unwrap_or(false),
+            CoreFunction::NormalizeSpace(expr_opt) => expr_opt
+                .as_ref()
+                .map(|expr| expr.is_primitive())
+                .unwrap_or(false),
+            CoreFunction::Translate(expr, from_chars, to_chars) => {
+                expr.is_primitive() && from_chars.is_primitive() && to_chars.is_primitive()
+            },
+            CoreFunction::Number(expr_opt) => expr_opt
+                .as_ref()
+                .map(|expr| expr.is_primitive())
+                .unwrap_or(false),
+            CoreFunction::Sum(expr) => expr.is_primitive(),
+            CoreFunction::Floor(expr) => expr.is_primitive(),
+            CoreFunction::Ceiling(expr) => expr.is_primitive(),
+            CoreFunction::Round(expr) => expr.is_primitive(),
+            CoreFunction::Boolean(expr) => expr.is_primitive(),
+            CoreFunction::Not(expr) => expr.is_primitive(),
+            CoreFunction::True => true,
+            CoreFunction::False => true,
+            CoreFunction::Lang(_) => false,
+        }
+    }
+}
+#[cfg(test)]
+mod tests {
+    use super::{substring, substring_after, substring_before};
+
+    #[test]
+    fn test_substring_before() {
+        assert_eq!(substring_before("hello world", "world"), "hello ");
+        assert_eq!(substring_before("prefix:name", ":"), "prefix");
+        assert_eq!(substring_before("no-separator", "xyz"), "");
+        assert_eq!(substring_before("", "anything"), "");
+        assert_eq!(substring_before("multiple:colons:here", ":"), "multiple");
+        assert_eq!(substring_before("start-match-test", "start"), "");
+    }
+
+    #[test]
+    fn test_substring_after() {
+        assert_eq!(substring_after("hello world", "hello "), "world");
+        assert_eq!(substring_after("prefix:name", ":"), "name");
+        assert_eq!(substring_after("no-separator", "xyz"), "");
+        assert_eq!(substring_after("", "anything"), "");
+        assert_eq!(substring_after("multiple:colons:here", ":"), "colons:here");
+        assert_eq!(substring_after("test-end-match", "match"), "");
+    }
+
+    #[test]
+    fn test_substring() {
+        assert_eq!(substring("hello world", 0, Some(5)), "hello");
+        assert_eq!(substring("hello world", 6, Some(5)), "world");
+        assert_eq!(substring("hello", 1, Some(3)), "ell");
+        assert_eq!(substring("hello", -5, Some(2)), "he");
+        assert_eq!(substring("hello", 0, None), "hello");
+        assert_eq!(substring("hello", 2, Some(10)), "llo");
+        assert_eq!(substring("hello", 5, Some(1)), "");
+        assert_eq!(substring("", 0, Some(5)), "");
+        assert_eq!(substring("hello", 0, Some(0)), "");
+        assert_eq!(substring("hello", 0, Some(-5)), "");
+    }
+}

components/script/xpath/eval_value.rs

@@ -0,0 +1,242 @@
+/* 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::borrow::ToOwned;
+use std::collections::HashSet;
+use std::{fmt, string};
+
+use crate::dom::bindings::codegen::Bindings::NodeBinding::Node_Binding::NodeMethods;
+use crate::dom::bindings::root::DomRoot;
+use crate::dom::bindings::utils::AsVoidPtr;
+use crate::dom::node::Node;
+
+/// The primary types of values that an XPath expression returns as a result.
+pub enum Value {
+    Boolean(bool),
+    /// A IEEE-754 double-precision floating point number
+    Number(f64),
+    String(String),
+    /// A collection of not-necessarily-unique nodes
+    Nodeset(Vec<DomRoot<Node>>),
+}
+
+impl fmt::Debug for Value {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            Value::Boolean(val) => write!(f, "{}", val),
+            Value::Number(val) => write!(f, "{}", val),
+            Value::String(ref val) => write!(f, "{}", val),
+            Value::Nodeset(ref val) => write!(f, "Nodeset({:?})", val),
+        }
+    }
+}
+
+pub fn str_to_num(s: &str) -> f64 {
+    s.trim().parse().unwrap_or(f64::NAN)
+}
+
+/// Helper for PartialEq<Value> implementations
+fn str_vals(nodes: &[DomRoot<Node>]) -> HashSet<String> {
+    nodes
+        .iter()
+        .map(|n| n.GetTextContent().unwrap_or_default().to_string())
+        .collect()
+}
+
+/// Helper for PartialEq<Value> implementations
+fn num_vals(nodes: &[DomRoot<Node>]) -> Vec<f64> {
+    nodes
+        .iter()
+        .map(|n| Value::String(n.GetTextContent().unwrap_or_default().into()).number())
+        .collect()
+}
+
+impl PartialEq<Value> for Value {
+    fn eq(&self, other: &Value) -> bool {
+        match (self, other) {
+            (Value::Nodeset(left_nodes), Value::Nodeset(right_nodes)) => {
+                let left_strings = str_vals(left_nodes);
+                let right_strings = str_vals(right_nodes);
+                !left_strings.is_disjoint(&right_strings)
+            },
+            (&Value::Nodeset(ref nodes), &Value::Number(val)) |
+            (&Value::Number(val), &Value::Nodeset(ref nodes)) => {
+                let numbers = num_vals(nodes);
+                numbers.iter().any(|n| *n == val)
+            },
+            (&Value::Nodeset(ref nodes), &Value::String(ref val)) |
+            (&Value::String(ref val), &Value::Nodeset(ref nodes)) => {
+                let strings = str_vals(nodes);
+                strings.contains(val)
+            },
+            (&Value::Boolean(_), _) | (_, &Value::Boolean(_)) => self.boolean() == other.boolean(),
+            (&Value::Number(_), _) | (_, &Value::Number(_)) => self.number() == other.number(),
+            _ => self.string() == other.string(),
+        }
+    }
+}
+
+impl Value {
+    pub fn boolean(&self) -> bool {
+        match *self {
+            Value::Boolean(val) => val,
+            Value::Number(n) => n != 0.0 && !n.is_nan(),
+            Value::String(ref s) => !s.is_empty(),
+            Value::Nodeset(ref nodeset) => !nodeset.is_empty(),
+        }
+    }
+
+    pub fn into_boolean(self) -> bool {
+        self.boolean()
+    }
+
+    pub fn number(&self) -> f64 {
+        match *self {
+            Value::Boolean(val) => {
+                if val {
+                    1.0
+                } else {
+                    0.0
+                }
+            },
+            Value::Number(val) => val,
+            Value::String(ref s) => str_to_num(s),
+            Value::Nodeset(..) => str_to_num(&self.string()),
+        }
+    }
+
+    pub fn into_number(self) -> f64 {
+        self.number()
+    }
+
+    pub fn string(&self) -> string::String {
+        match *self {
+            Value::Boolean(v) => v.to_string(),
+            Value::Number(n) => {
+                if n.is_infinite() {
+                    if n.signum() < 0.0 {
+                        "-Infinity".to_owned()
+                    } else {
+                        "Infinity".to_owned()
+                    }
+                } else if n == 0.0 {
+                    // catches -0.0 also
+                    0.0.to_string()
+                } else {
+                    n.to_string()
+                }
+            },
+            Value::String(ref val) => val.clone(),
+            Value::Nodeset(ref nodes) => match nodes.document_order_first() {
+                Some(n) => n.GetTextContent().unwrap_or_default().to_string(),
+                None => "".to_owned(),
+            },
+        }
+    }
+
+    pub fn into_string(self) -> string::String {
+        match self {
+            Value::String(val) => val,
+            other => other.string(),
+        }
+    }
+}
+
+macro_rules! from_impl {
+    ($raw:ty, $variant:expr) => {
+        impl From<$raw> for Value {
+            fn from(other: $raw) -> Value {
+                $variant(other)
+            }
+        }
+    };
+}
+
+from_impl!(bool, Value::Boolean);
+from_impl!(f64, Value::Number);
+from_impl!(String, Value::String);
+impl<'a> From<&'a str> for Value {
+    fn from(other: &'a str) -> Value {
+        Value::String(other.into())
+    }
+}
+from_impl!(Vec<DomRoot<Node>>, Value::Nodeset);
+
+macro_rules! partial_eq_impl {
+    ($raw:ty, $variant:pat => $b:expr) => {
+        impl PartialEq<$raw> for Value {
+            fn eq(&self, other: &$raw) -> bool {
+                match *self {
+                    $variant => $b == other,
+                    _ => false,
+                }
+            }
+        }
+
+        impl PartialEq<Value> for $raw {
+            fn eq(&self, other: &Value) -> bool {
+                match *other {
+                    $variant => $b == self,
+                    _ => false,
+                }
+            }
+        }
+    };
+}
+
+partial_eq_impl!(bool, Value::Boolean(ref v) => v);
+partial_eq_impl!(f64, Value::Number(ref v) => v);
+partial_eq_impl!(String, Value::String(ref v) => v);
+partial_eq_impl!(&str, Value::String(ref v) => v);
+partial_eq_impl!(Vec<DomRoot<Node>>, Value::Nodeset(ref v) => v);
+
+pub trait NodesetHelpers {
+    /// Returns the node that occurs first in [document order]
+    ///
+    /// [document order]: https://www.w3.org/TR/xpath/#dt-document-order
+    fn document_order_first(&self) -> Option<DomRoot<Node>>;
+    fn document_order(&self) -> Vec<DomRoot<Node>>;
+    fn document_order_unique(&self) -> Vec<DomRoot<Node>>;
+}
+
+impl NodesetHelpers for Vec<DomRoot<Node>> {
+    fn document_order_first(&self) -> Option<DomRoot<Node>> {
+        self.iter()
+            .min_by(|a, b| {
+                if a == b {
+                    std::cmp::Ordering::Equal
+                } else if a.is_before(b) {
+                    std::cmp::Ordering::Less
+                } else {
+                    std::cmp::Ordering::Greater
+                }
+            })
+            .cloned()
+    }
+    fn document_order(&self) -> Vec<DomRoot<Node>> {
+        let mut nodes: Vec<DomRoot<Node>> = self.clone();
+        if nodes.len() == 1 {
+            return nodes;
+        }
+
+        nodes.sort_by(|a, b| {
+            if a == b {
+                std::cmp::Ordering::Equal
+            } else if a.is_before(b) {
+                std::cmp::Ordering::Less
+            } else {
+                std::cmp::Ordering::Greater
+            }
+        });
+
+        nodes
+    }
+    fn document_order_unique(&self) -> Vec<DomRoot<Node>> {
+        let mut nodes: Vec<DomRoot<Node>> = self.document_order();
+
+        nodes.dedup_by_key(|n| n.as_void_ptr());
+
+        nodes
+    }
+}

components/script/xpath/mod.rs

@@ -0,0 +1,73 @@
+/* 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 context::EvaluationCtx;
+use eval::Evaluatable;
+pub use eval_value::{NodesetHelpers, Value};
+use parser::OwnedParserError;
+pub use parser::{parse as parse_impl, Expr};
+
+use super::dom::node::Node;
+
+mod context;
+mod eval;
+mod eval_function;
+mod eval_value;
+mod parser;
+
+/// The failure modes of executing an XPath.
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    /// The XPath was syntactically invalid
+    Parsing { source: OwnedParserError },
+    /// The XPath could not be executed
+    Evaluating { source: eval::Error },
+}
+
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            Error::Parsing { source } => write!(f, "Unable to parse XPath: {}", source),
+            Error::Evaluating { source } => write!(f, "Unable to evaluate XPath: {}", source),
+        }
+    }
+}
+
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::Parsing { source } => Some(source),
+            Error::Evaluating { source } => Some(source),
+        }
+    }
+}
+
+/// Parse an XPath expression from a string
+pub fn parse(xpath: &str) -> Result<Expr, Error> {
+    match parse_impl(xpath) {
+        Ok(expr) => {
+            debug!("Parsed XPath: {:?}", expr);
+            Ok(expr)
+        },
+        Err(e) => {
+            debug!("Unable to parse XPath: {}", e);
+            Err(Error::Parsing { source: e })
+        },
+    }
+}
+
+/// Evaluate an already-parsed XPath expression
+pub fn evaluate_parsed_xpath(expr: &Expr, context_node: &Node) -> Result<Value, Error> {
+    let context = EvaluationCtx::new(context_node);
+    match expr.evaluate(&context) {
+        Ok(v) => {
+            debug!("Evaluated XPath: {:?}", v);
+            Ok(v)
+        },
+        Err(e) => {
+            debug!("Unable to evaluate XPath: {}", e);
+            Err(Error::Evaluating { source: e })
+        },
+    }
+}