big_code_analysis_ast/

node.rs

// Metric counts (token, function, branch, argument, etc.) are stored as
// `usize` and crossed with `f64` averages, ratios, and Halstead scores
// across the cyclomatic / MI / Halstead computations. The `usize as f64`
// and `f64 as usize` casts are intentional and snapshot-anchored — every
// site is bounded by the count it came from. Allowing the lints at the
// module level keeps the metric arithmetic legible.
#![allow(
    clippy::cast_precision_loss,
    clippy::cast_possible_truncation,
    clippy::cast_sign_loss
)]

//! Tree-sitter wrappers: `Tree`, [`Node`], [`Ancestors`], and the cursors.

mod ancestors;
mod parser_cache;

#[doc(hidden)]
pub use ancestors::WIDE_PARENT;
pub use ancestors::{AncestorIter, Ancestors, FoldCache, SiblingCache};
pub(crate) use ancestors::{FoldLane, NodeMemo};

use tree_sitter::Node as OtherNode;
use tree_sitter::Tree as OtherTree;
use tree_sitter::TreeCursor;

use crate::checker::Checker;
use crate::traits::{LanguageInfo, Search};

use parser_cache::parse_on_scratch_parser;

// Sibling lookups that resolved the parent from the node, on this
// thread.
//
// [`Node::previous_sibling`] answers exactly what the chain-based
// [`Ancestors::previous_sibling`] does, so no assertion on a metric
// value can tell a walk that uses one from a walk that uses the
// other — only the cost differs, and only on a deep tree. #1096 took
// the last of these out of the metric bodies and #1100 out of the
// `exclude_tests` prune; the counter is what makes putting one back a
// test failure rather than a silent quadratic.
crate::observation::counter!(
    node_resolved_sibling_lookups,
    cfg(any(test, feature = "test-support"))
);

// Child scans that built their own `TreeCursor`, on this thread.
//
// [`Node::children_with`] yields exactly what [`Node::children`] yields
// — it exists to reuse one cursor across a traversal instead of
// heap-allocating and freeing one per visited node, and no assertion on
// a metric value can tell the two apart. #1112 moved the per-node
// traversals that could hoist a cursor onto it; the counter is what
// makes moving one back a test failure rather than a silent allocation
// per node.
//
// All five consumers are guarded: `preorder` and `act_on_node` here,
// `metrics::npa::python` and the suppression DFS through a `metrics()`
// / `suppression_markers` call, and `output::dump`'s renderer from that
// module's own tests. The accessor is `pub` (not `pub(super)`)
// precisely so the last one can be asserted from where it lives — see
// `crate::observation`.
crate::observation::counter!(child_scan_cursors, cfg(any(test, feature = "test-support")));

/// A parsed source tree wrapping a [`tree_sitter::Tree`].
///
/// The "open parse seam" (see issue #251) is reached by external
/// callers through `big_code_analysis::Ast::from_tree_sitter`, which accepts a
/// caller-built `tree_sitter::Tree` directly; this wrapper stays
/// internal so the metric walker is the only thing that observes it.
#[derive(Clone, Debug)]
pub(crate) struct Tree(OtherTree);

impl Tree {
    /// # Panics
    ///
    /// When `T`'s per-language Cargo feature is disabled in this build.
    ///
    /// [`AnyParser::parse`](crate::langs::AnyParser::parse) and
    /// [`AnyParser::from_tree`](crate::langs::AnyParser::from_tree)
    /// cfg-gate each `LANG::*` arm and return
    /// `Err(MetricsError::LanguageDisabled)` before reaching here, so
    /// every path through `big_code_analysis::Ast` is safe. Since
    /// #1376, though, `Parser<T>` and [`ParserTrait::new`] are `pub` in
    /// this crate, so a caller that names a `*Parser` alias directly
    /// bypasses that gate — check
    /// [`LANG::is_enabled`](crate::LANG::is_enabled) first, or go
    /// through `AnyParser` and match on the error.
    pub(crate) fn new<T: LanguageInfo>(code: &[u8]) -> Self {
        let language = T::lang().get_ts_language().expect(
            "invariant: the caller checked LANG::is_enabled, or reached this through AnyParser",
        );
        Self(parse_on_scratch_parser(&language, code))
    }

    pub(crate) fn from_ts_tree(tree: OtherTree) -> Self {
        Self(tree)
    }

    pub(crate) fn get_root(&self) -> Node<'_> {
        Node(self.0.root_node())
    }

    pub(crate) fn as_ts_tree(&self) -> &OtherTree {
        &self.0
    }
}

/// An `AST` node.
///
/// The inner [`tree_sitter::Node`] is reached through
/// [`Node::as_tree_sitter`] for advanced use cases that need direct
/// access to the underlying tree-sitter API; the field itself is
/// private so a `tree-sitter` version bump cannot silently reshape
/// this struct's public layout.
#[derive(Clone, Copy, Debug)]
pub struct Node<'a>(OtherNode<'a>);

impl<'a> Node<'a> {
    /// Returns the underlying [`tree_sitter::Node`] for callers that
    /// want to drive their own traversal alongside the metric walker.
    ///
    /// `tree_sitter::Node` is [`Copy`], so the node is returned by
    /// value. The returned node borrows the same source-tree lifetime
    /// as `self`.
    ///
    /// The `tree-sitter` re-export this exposes is *value-not-stable*:
    /// the underlying pin may bump in any minor release, so node shape
    /// and node-kind ids are not part of this crate's stability
    /// contract (see the [`tree_sitter`] re-export
    /// note in the crate root).
    #[must_use]
    #[inline]
    pub fn as_tree_sitter(&self) -> OtherNode<'a> {
        self.0
    }

    /// Checks if a node represents a syntax error or contains any syntax errors
    /// anywhere within it.
    ///
    /// Transitive, which is the trap: asking it of a construct to decide
    /// whether that construct's *own* shape is readable also answers `true`
    /// for an error arbitrarily deep inside one of its children. Where the
    /// question is about a node's own slot sequence, ask [`is_error`] /
    /// [`is_missing`] of each slot instead (#1381).
    ///
    /// [`is_error`]: Self::is_error
    /// [`is_missing`]: Self::is_missing
    #[must_use]
    pub fn has_error(&self) -> bool {
        self.0.has_error()
    }

    /// Whether this node *is* the `ERROR` node, as opposed to
    /// [`has_error`](Self::has_error)'s "is, or contains one".
    #[must_use]
    #[inline]
    pub fn is_error(&self) -> bool {
        self.0.is_error()
    }

    /// Whether the parser inserted this node during error recovery: a
    /// zero-width token the source does not contain, standing in for one
    /// the grammar required.
    ///
    /// A sibling of [`is_error`](Self::is_error) rather than a synonym —
    /// a MISSING token is not an `ERROR` node, but it occupies a child
    /// slot just the same, so a rule reading a child's *index* has to
    /// account for both.
    #[must_use]
    #[inline]
    pub fn is_missing(&self) -> bool {
        self.0.is_missing()
    }

    /// An id unique to this node within its tree, stable for the tree's
    /// lifetime. Suitable as a map key for per-node walk state.
    #[inline]
    #[must_use]
    pub fn id(&self) -> usize {
        self.0.id()
    }

    /// The grammar's name for this node's kind (`"function_item"`,
    /// `"identifier"`, …). Several distinct [`kind_id`](Self::kind_id)s
    /// can share one name — see the aliasing note on `kind_id`.
    #[inline]
    #[must_use]
    pub fn kind(&self) -> &'static str {
        self.0.kind()
    }

    /// The numeric kind of this node in its grammar's symbol table.
    ///
    /// This is what the per-language token enums in [`languages`]
    /// (`Rust::FunctionItem as u16`, …) and every classifier match on.
    /// The value is an index into the pinned grammar and moves whenever
    /// that grammar is bumped; it is not stable across versions.
    ///
    /// [`languages`]: crate::languages
    #[inline]
    #[must_use]
    pub fn kind_id(&self) -> u16 {
        self.0.kind_id()
    }

    /// The source text this node spans, or `None` when the span falls
    /// outside `data` or those bytes are not valid UTF-8.
    ///
    /// Indexed with `data.get(..)` rather than delegating to
    /// [`tree_sitter::Node::utf8_text`], which is
    /// `str::from_utf8(&source[start..end])` — the slice panics on a span
    /// reaching past `source`, before the `Result` its signature offers
    /// ever exists, so `.ok()` cannot catch it. Every caller today passes
    /// the same buffer the tree was parsed from and so cannot produce
    /// that span, but the guarantee is one refactor from false: this
    /// crate already runs passes that parse one buffer and read another
    /// (`preproc`, `comment_rm`), and `panic!` is banned in non-test code
    /// whether or not the branch is currently reachable. `data.get(..)`
    /// is also what `.claude/rules/grammar-dispatch.md` §10 prescribes
    /// for reading a node's bytes.
    #[inline]
    #[must_use]
    pub fn utf8_text(&self, data: &'a [u8]) -> Option<&'a str> {
        let bytes = data.get(self.0.start_byte()..self.0.end_byte())?;
        std::str::from_utf8(bytes).ok()
    }

    /// The 0-based byte offset where this node starts.
    #[inline]
    #[must_use]
    pub fn start_byte(&self) -> usize {
        self.0.start_byte()
    }

    /// The 0-based byte offset one past this node's last byte.
    #[inline]
    #[must_use]
    pub fn end_byte(&self) -> usize {
        self.0.end_byte()
    }

    /// The 0-based `(row, column)` where this node starts.
    #[inline]
    #[must_use]
    pub fn start_position(&self) -> (usize, usize) {
        let temp = self.0.start_position();
        (temp.row, temp.column)
    }

    /// The 0-based `(row, column)` one past this node's end.
    #[inline]
    #[must_use]
    pub fn end_position(&self) -> (usize, usize) {
        let temp = self.0.end_position();
        (temp.row, temp.column)
    }

    /// The 0-based row this node starts on.
    #[inline]
    #[must_use]
    pub fn start_row(&self) -> usize {
        self.0.start_position().row
    }

    /// The 0-based row of this node's end position. A node ending at
    /// column 0 does not occupy that row; see [`end_line`](Self::end_line).
    #[inline]
    #[must_use]
    pub fn end_row(&self) -> usize {
        self.0.end_position().row
    }

    /// The 1-based **last line this node occupies** — the inclusive end
    /// of its line span.
    ///
    /// Converting tree-sitter's 0-based end row needs a `+ 1` only when
    /// the node actually occupies the row it ends on. A node whose end
    /// column is 0 finished at the *start* of the row below its last
    /// content row, having absorbed the preceding newline, so that row
    /// is not part of its span.
    ///
    /// Getting this wrong is invisible in almost every grammar, because
    /// almost every function node ends just past its closing delimiter
    /// and therefore at a column above 0. Perl's trailing `sub` does
    /// not: it ends at column 0 of the row below its `}`, exactly where
    /// the file root ends, and a blanket `+ 1` then reported it ending a
    /// line past both its enclosing space and EOF (#1163) — the shape
    /// behind the release `usize` underflow in #1051.
    #[inline]
    #[must_use]
    pub fn end_line(&self) -> usize {
        let end = self.0.end_position();
        if end.column == 0 {
            end.row
        } else {
            end.row + 1
        }
    }

    /// Returns this node's parent.
    ///
    /// **`O(depth)`, not `O(1)`.** tree-sitter stores no parent pointer:
    /// `ts_node_parent` restarts at the tree root and descends. A single
    /// call in a per-node metric therefore costs `O(nodes × depth)` over a
    /// walk, and an ancestor *loop* built on it costs `O(depth²)` per call.
    ///
    /// This has bitten the analyzer for real: #1052 was a per-leaf
    /// `successors(node, Node::parent)` walk in `Tokens` that made the
    /// metric `O(leaves × depth²)`, so a 2 KB file of nested parentheses
    /// took ~19 s. Prefer inheriting state downward through the
    /// traversal (see `Walk` in `spaces::compute`) over rediscovering it
    /// upward, and where a predicate genuinely needs an ancestor, take
    /// an [`Ancestors`] rather than calling this (#1084).
    ///
    /// As of #1096 no code the metric, `ops`, `bca function`, or
    /// comment-removal walks reach calls this; the remaining callers are
    /// `Ancestors` itself (the no-chain fallback), the one-off start node
    /// of a `dump`, and tests. `rg '\.parent\(\)' src/` re-checks that.
    #[inline]
    pub fn parent(&self) -> Option<Node<'a>> {
        self.0.parent().map(Node)
    }

    /// Returns `true` if this node's parent has any direct child with
    /// the given grammar `kind_id` (the parent's children include this
    /// node itself, so a self-match counts). Delegates to [`wraps_any`]
    /// on the parent. This sits on the JS/TS arrow-function
    /// closure-classification hot path (`check_if_arrow_func!`); see
    /// #521.
    ///
    /// `ancestors` supplies the parent, for [`Node::parent`]'s
    /// `O(depth)` reason (#1088).
    ///
    /// [`wraps_any`]: Self::wraps_any
    #[inline]
    #[must_use]
    pub fn has_sibling(&self, ancestors: Ancestors<'a, '_>, id: u16) -> bool {
        ancestors
            .parent(self)
            .is_some_and(|parent| parent.is_child(id))
    }

    /// The sibling immediately before this node.
    ///
    /// **`O(depth)`, not `O(1)`**, for [`Node::parent`]'s reason:
    /// `ts_node__prev_sibling` opens with `ts_node_parent`. Callers on a
    /// walk should use [`Ancestors::previous_sibling`] instead (#1096).
    #[inline]
    pub fn previous_sibling(&self) -> Option<Node<'a>> {
        node_resolved_sibling_lookups::record();
        self.0.prev_sibling().map(Node)
    }

    /// The sibling immediately after this node.
    ///
    /// **`O(depth)`, not `O(1)`**, for [`previous_sibling`]'s reason:
    /// `ts_node_next_sibling` opens with `ts_node_parent`. There is no
    /// [`Ancestors`] counterpart because the only caller wants exactly
    /// this cost — the `exclude_tests` attribute lookahead reads forward
    /// over the parent's children whenever the parent is narrow enough,
    /// and falls back here precisely for the wide-and-shallow parent a
    /// cursor pass would make `O(width)` per node (#1431). Anything on a
    /// walk that does not budget the two against each other should be
    /// reading the child list instead.
    ///
    /// [`previous_sibling`]: Self::previous_sibling
    #[inline]
    pub(crate) fn next_sibling(&self) -> Option<Node<'a>> {
        node_resolved_sibling_lookups::record();
        self.0.next_sibling().map(Node)
    }

    /// Returns `true` if any direct child has the given grammar
    /// `kind_id`. See #217 for the motivating perf finding from the
    /// JS/TS template-literal hot path.
    #[inline]
    #[must_use]
    pub fn is_child(&self, id: u16) -> bool {
        self.wraps_any(&[id])
    }

    /// Returns `true` if any direct child matches one of the given
    /// grammar `kind_id`s. The single-id [`is_child`] delegates here, so
    /// both share one child scan (the `#[inline]` makes the
    /// single-element `contains` collapse to an equality check).
    /// Generalizing the check to a set lets the shared
    /// string-interpolation operand skip declare its rule once (issue
    /// #420).
    ///
    /// # Why a cursor rather than `child(0)` + `next_sibling()`
    ///
    /// #217 replaced the cursor walk with a `next_sibling()` chain to
    /// dodge the `TreeCursor` heap allocation, on the premise that a
    /// sibling step is `O(1)`. It is not: `ts_node_next_sibling`
    /// resolves the parent first, and `tree_sitter` stores no parent
    /// pointer — it descends from the root — so each step cost
    /// `O(depth)` and the scan `O(children × depth)`. That made every
    /// caller of this method `O(depth)` per node, which is the same
    /// defect #1084 removed from the predicates that ask for an
    /// ancestor outright. The cursor iterator is `O(children)` after one
    /// allocation, and measured faster on real input as well as on the
    /// pathological one: the `nom/nested-arrow` probe went from
    /// quadratic (17.6 s at depth 4000) to linear (6.3 ms), and a walk
    /// over the 384-file `pdf.js` corpus dropped from ~443 ms to
    /// ~370 ms (#1088).
    ///
    /// [`is_child`]: Self::is_child
    #[inline]
    #[must_use]
    pub fn wraps_any(&self, ids: &[u16]) -> bool {
        self.children().any(|c| ids.contains(&c.kind_id()))
    }

    /// The number of direct children, named and anonymous alike.
    #[inline]
    #[must_use]
    pub fn child_count(&self) -> usize {
        self.0.child_count()
    }

    /// Whether this node is a named grammar production, as opposed to an
    /// anonymous token such as a punctuation or keyword literal. Walks
    /// use it to skip tokens like the leading `|` in an or-pattern.
    #[inline]
    #[must_use]
    pub fn is_named(&self) -> bool {
        self.0.is_named()
    }

    /// Returns the direct child reached through the grammar `field_name`,
    /// if any. The child carries the underlying tree lifetime `'a` (the
    /// `tree_sitter::Node` it wraps is [`Copy`] and valid for the whole
    /// tree), so callers may hold it past the borrow of `&self` — matching
    /// the sibling accessors ([`child`], [`parent`], [`children`], …) rather
    /// than over-narrowing to the method-call borrow (see issue #786).
    ///
    /// [`child`]: Self::child
    /// [`parent`]: Self::parent
    /// [`children`]: Self::children
    #[inline]
    pub fn child_by_field_name(&self, name: &str) -> Option<Node<'a>> {
        self.0.child_by_field_name(name).map(Node)
    }

    /// The direct child at index `pos`, counting named and anonymous
    /// children alike. Prefer [`child_by_field_name`](Self::child_by_field_name)
    /// for any slot the grammar can precede with an optional preamble.
    #[inline]
    #[must_use]
    pub fn child(&self, pos: usize) -> Option<Node<'a>> {
        self.0.child(pos as u32).map(Node)
    }

    /// Returns the tree-sitter grammar field name through which this
    /// node reaches the child at `child_index`, if any. Used by the
    /// AST builder to thread the parent's `field_name` into each child
    /// without a parallel cursor walk.
    #[inline]
    #[must_use]
    pub fn field_name_for_child(&self, child_index: u32) -> Option<&'static str> {
        self.0.field_name_for_child(child_index)
    }

    /// Iterator over this node's direct children.
    ///
    /// Builds a [`Cursor`], which heap-allocates. A loop that visits
    /// many nodes should hoist one cursor and use [`children_with`]
    /// instead; see its note for when the difference is worth the
    /// plumbing.
    ///
    /// [`children_with`]: Self::children_with
    #[inline]
    #[must_use]
    pub fn children(&self) -> Children<'a> {
        child_scan_cursors::record();
        // `descend`, not `seed`: `ts_node_walk` already ran the
        // `ts_tree_cursor_init` that `Cursor::reset` would run again.
        let mut cursor = self.cursor();
        let scan = ChildScan::descend(self, &mut cursor);
        Children { cursor, scan }
    }

    /// [`children`], over a cursor the caller owns.
    ///
    /// `tree_sitter::TreeCursor` heap-allocates its stack when built and
    /// frees it when dropped, so a traversal that calls [`children`]
    /// once per visited node pays a `malloc`/`free` pair per node.
    /// `ts_tree_cursor_reset` keeps the allocation, so a loop that can
    /// hoist one cursor pays for it once however many nodes it visits
    /// (#1112).
    ///
    /// Worth the plumbing only where a loop visits many nodes. Measured
    /// on the corpus slice, a full metric walk reaches [`children`] on
    /// 3-6 % of nodes in C++, Rust, JavaScript, and Java, 16 % in C#,
    /// and 60 % in Python, where one scan — the instance-attribute walk
    /// in `metrics::npa::python` — was 92 % of the total, and
    /// [`preorder`] plus the suppression DFS are the rest. Crate-wide
    /// there is one more per-node consumer, `output::dump`'s renderer,
    /// which no metric walk runs and so is absent from that total.
    /// Predicates
    /// that hold a bare `&Node` and scan one node's children keep
    /// [`children`]: threading a cursor to them would cross the
    /// `Checker` / `Getter` trait surface for one allocation per call.
    ///
    /// [`preorder`]: Self::preorder
    ///
    /// [`children`]: Self::children
    #[inline]
    pub fn children_with<'c>(&self, cursor: &'c mut Cursor<'a>) -> ChildrenWith<'c, 'a> {
        let scan = ChildScan::seed(self, cursor);
        ChildrenWith { cursor, scan }
    }

    /// The children after `child`, nearest first: what
    /// `children().skip_while(|c| c.id() != child.id()).skip(1)` yields,
    /// and nothing when `child` is not one of this node's children.
    ///
    /// That scan costs `child`'s index, so a walk that asks it of every
    /// child of a wide list is quadratic in the list (#1545). This seeks
    /// `child` by byte instead, which is `O(log n)` in a list the grammar
    /// stores as a `repeat()` and still `O(index)` in a flat one, an
    /// `ERROR` above all; see [`Cursor::goto_first_child_for_byte`]. A
    /// walk that asks of every child of a node that may be flat goes
    /// through [`Ancestors::siblings_after`] (#1584).
    pub fn children_after(&self, child: &Node<'a>) -> impl Iterator<Item = Node<'a>> + use<'a> {
        let mut cursor = self.cursor();
        let mut more =
            self.seat_on_child(child, &mut cursor).is_some() && cursor.goto_next_sibling();
        std::iter::from_fn(move || {
            let next = more.then(|| cursor.node())?;
            more = cursor.goto_next_sibling();
            Some(next)
        })
    }

    /// The children before `child`, nearest first, and nothing when
    /// `child` is not one of this node's children.
    ///
    /// `child`'s index comes from [`Cursor::goto_first_child_for_byte`]
    /// and each sibling from [`Node::child`], both `O(log n)` in the
    /// child count where the list is a `repeat()`, stored as a balanced
    /// tree of hidden nodes, and both `O(index)` where it is flat, as an
    /// `ERROR` built by skipping tokens is. A walk that asks of every
    /// child of such a node goes through [`Ancestors::siblings_before`]
    /// (#1584).
    pub fn children_before(&self, child: &Node<'a>) -> impl Iterator<Item = Node<'a>> + use<'a> {
        let node = *self;
        let index = self.seat_on_child(child, &mut self.cursor()).unwrap_or(0);
        (0..index).rev().filter_map(move |i| node.child(i))
    }

    // Seats `cursor`, which sits on this node, on `child` and returns its
    // index, or `None` when `child` is not one of this node's children.
    fn seat_on_child(&self, child: &Node<'a>, cursor: &mut Cursor<'a>) -> Option<usize> {
        if let Some(index) = cursor.goto_first_child_for_byte(child.start_byte())
            && cursor.node().id() == child.id()
        {
            return Some(index);
        }
        // The lookup passes over a zero-width child, which ends at the
        // byte it starts at; only error recovery inserts one.
        cursor.reset(self);
        let mut index = 0;
        let mut found = cursor.goto_first_child();
        while found && cursor.node().id() != child.id() {
            found = cursor.goto_next_sibling();
            index += 1;
        }
        found.then_some(index)
    }

    /// A fresh cursor positioned on this node, for the traversals that
    /// reuse one cursor across many child scans
    /// ([`children_with`](Self::children_with)).
    #[inline]
    #[must_use]
    pub fn cursor(&self) -> Cursor<'a> {
        Cursor(self.0.walk())
    }

    /// Counts this node's ancestors satisfying `check`, walking upward
    /// from the parent and stopping at (and excluding) the first
    /// ancestor satisfying `stop`. An ancestor that is the `if` of an
    /// `else if` chain never counts — it is a continuation of the
    /// branch above it, not a new enclosing one.
    ///
    /// `ancestors` is the chain the caller descended through. Passing
    /// [`Ancestors::unknown`] is always correct and answers identically;
    /// it just pays `O(depth)` per step instead of `O(1)`.
    pub fn count_specific_ancestors<C: Checker>(
        &self,
        ancestors: Ancestors<'a, '_>,
        check: fn(&Node) -> bool,
        stop: fn(&Node) -> bool,
    ) -> usize {
        let mut count = 0;
        for (parent, above_parent) in ancestors.iter(self) {
            if stop(&parent) {
                break;
            }
            if check(&parent) && !C::is_else_if(&parent, above_parent) {
                count += 1;
            }
        }
        count
    }

    /// Returns `true` iff this node's parent satisfies `parent_pred`
    /// AND that parent's own parent (this node's grandparent)
    /// satisfies `grand_pred`. Returns `false` as soon as either link
    /// is absent or its predicate fails, so a misordered predicate
    /// cannot silently degrade to a single-predicate check.
    ///
    /// `ancestors` is the chain the caller descended through. Passing
    /// [`Ancestors::unknown`] is always correct and answers
    /// identically; it just pays [`Node::parent`]'s `O(depth)` for each
    /// of the two links, which on a per-node metric arm is quadratic in
    /// nesting depth (#1096).
    pub fn parent_grandparent_match(
        &self,
        ancestors: Ancestors<'a, '_>,
        parent_pred: fn(&Node) -> bool,
        grand_pred: fn(&Node) -> bool,
    ) -> bool {
        let mut climb = ancestors.iter(self);
        let Some((parent, _)) = climb.next() else {
            return false;
        };
        if !parent_pred(&parent) {
            return false;
        }
        let Some((grand, _)) = climb.next() else {
            return false;
        };
        grand_pred(&grand)
    }

    /// Returns a pre-order iterator over this node and all of its
    /// descendants (this node first, then each child subtree left to
    /// right).
    ///
    /// The traversal is allocation-light: it reuses one work stack *and
    /// one cursor*, and visits each node exactly once, so a full walk is
    /// O(n) in the subtree size and allocates only the stack's growth.
    /// Building a fresh cursor per visited node instead would cost a
    /// `malloc`/`free` pair per node (#1112). Every yielded [`Node`]
    /// carries the underlying tree lifetime `'a`, so callers may collect
    /// or retain the handles.
    ///
    /// This is the Rust counterpart of the Python `Node.walk()` binding
    /// (issue #728): the binding wraps each yielded node, so Rust and
    /// Python share one traversal order.
    #[must_use]
    pub fn preorder(&self) -> Preorder<'a> {
        Preorder {
            stack: vec![*self],
            cursor: self.cursor(),
        }
    }

    /// Collects every node in this subtree (this node included) whose
    /// [`kind`](tree_sitter::Node::kind) is listed in `kinds`, in
    /// pre-order.
    ///
    /// Membership is an exact match against the raw grammar kind — the
    /// same unaltered vocabulary `big_code_analysis::Ast::root_node` exposes, not
    /// the `Alterator`-curated kinds `big_code_analysis::Ast::dump` emits. This is
    /// the Rust counterpart of the Python `Node.descendants_by_kind()`
    /// binding (issue #728).
    #[must_use]
    pub fn descendants_by_kind(&self, kinds: &[&str]) -> Vec<Node<'a>> {
        self.preorder()
            .filter(|node| kinds.contains(&node.kind()))
            .collect()
    }
}

/// Pre-order iterator over a node and its descendants, returned by
/// [`Node::preorder`].
///
/// Holds a single work stack of not-yet-visited nodes, and a single
/// cursor to enumerate each node's children with. Each step pops the
/// next node, pushes its children so the leftmost is visited first, and
/// yields the popped node — so the sequence is the node, then each child
/// subtree in order. Both are reused across steps (children are pushed
/// then the freshly-pushed slice is reversed in place), so the walk
/// allocates only the stack's growth: no fresh buffer per node, and no
/// fresh `TreeCursor` either (#1112).
pub struct Preorder<'a> {
    stack: Vec<Node<'a>>,
    cursor: Cursor<'a>,
}

impl<'a> Iterator for Preorder<'a> {
    type Item = Node<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        let node = self.stack.pop()?;
        // Push children in document order, then reverse just the slice we
        // appended so the leftmost child ends up on top of the stack and
        // is visited next — pre-order without a per-node temporary.
        let first_child = self.stack.len();
        // Destructured so the stack and the cursor are borrowed as the
        // disjoint fields they are.
        let Self { stack, cursor } = self;
        stack.extend(node.children_with(cursor));
        stack[first_child..].reverse();
        Some(node)
    }
}

/// An `AST` cursor.
#[derive(Clone)]
pub struct Cursor<'a>(TreeCursor<'a>);

impl<'a> Cursor<'a> {
    /// Re-seats the cursor on `node`.
    #[inline]
    pub fn reset(&mut self, node: &Node<'a>) {
        self.0.reset(node.0);
    }

    /// Moves to the next sibling; `false` when there is none.
    #[inline]
    pub fn goto_next_sibling(&mut self) -> bool {
        self.0.goto_next_sibling()
    }

    /// Moves to the first child; `false` when there is none.
    #[inline]
    pub fn goto_first_child(&mut self) -> bool {
        self.0.goto_first_child()
    }

    /// Moves to the first child that ends after `byte` and returns its
    /// index in [`Node::children`] order, or `None` when no child does.
    ///
    /// "Ends after" is strict, so a **zero-width** child at `byte` — a
    /// MISSING token the parser inserted during error recovery — is
    /// skipped and the cursor lands on its successor. That is the one way
    /// this differs from `children().position(..)`, which finds such a
    /// node; a caller resolving a node's own index must therefore check
    /// [`Cursor::node`] against it, as both call sites here do.
    ///
    /// The index is what makes this worth having over a sibling scan:
    /// tree-sitter stores a `repeat()` child list under balanced hidden
    /// nodes and skips each by its cached visible-child count, so this
    /// is `O(log n)` in the sibling count where `children().position(..)`
    /// is `O(n)` — the difference between a linear and a quadratic walk
    /// when every child of a wide list asks for its own index.
    ///
    /// Only for such a list. The seek steps through the children in
    /// order and can skip only a *hidden* subtree, so over a flat child
    /// array — an `ERROR` that recovery built by skipping tokens has no
    /// hidden levels — it is the same `O(index)` scan (#1584).
    #[inline]
    pub fn goto_first_child_for_byte(&mut self, byte: usize) -> Option<usize> {
        self.0.goto_first_child_for_byte(byte)
    }

    /// The node the cursor currently sits on.
    #[inline]
    #[must_use]
    pub fn node(&self) -> Node<'a> {
        Node(self.0.node())
    }
}

/// Position of a child scan, independent of who owns the cursor driving
/// it.
///
/// [`Children`] and [`ChildrenWith`] differ only in that — one owns its
/// cursor, the other borrows the caller's — and `children_with` exists
/// to save an allocation, not to answer differently. Keeping the
/// termination rule here rather than in each iterator is what stops the
/// two from drifting.
struct ChildScan {
    done: bool,
    remaining: usize,
}

impl ChildScan {
    /// Seats `cursor` on `node`'s first child.
    ///
    /// `goto_first_child` returns false when the node has no children,
    /// in which case the scan is exhausted from the outset. Termination
    /// is then driven entirely by the cursor (see [`ChildScan::step`]),
    /// so the iterator stops exactly when the tree reports no further
    /// siblings — it can never pad the sequence with duplicate nodes if
    /// `child_count` and the cursor walk ever disagree.
    ///
    /// `child_count` is the authoritative length for the
    /// `ExactSizeIterator` contract; for well-formed trees it equals the
    /// cursor sibling walk, so the reported length and the emitted data
    /// agree. A childless node reports `0` so the empty iterator's
    /// length matches its (lack of) data.
    fn seed<'a>(node: &Node<'a>, cursor: &mut Cursor<'a>) -> Self {
        cursor.reset(node);
        Self::descend(node, cursor)
    }

    /// [`ChildScan::seed`], for a cursor already seated on `node` —
    /// which is what [`Node::cursor`] hands back, and `ts_node_walk` and
    /// `ts_tree_cursor_reset` run the same `ts_tree_cursor_init`. Only
    /// [`Node::children`] may skip the reset; every other caller reuses
    /// a cursor left wherever the previous scan ended.
    fn descend<'a>(node: &Node<'a>, cursor: &mut Cursor<'a>) -> Self {
        let done = !cursor.goto_first_child();
        Self {
            done,
            remaining: if done { 0 } else { node.child_count() },
        }
    }

    /// Yields the cursor's current child and advances past it.
    fn step<'a>(&mut self, cursor: &mut Cursor<'a>) -> Option<Node<'a>> {
        if self.done {
            return None;
        }
        let result = cursor.node();
        // The cursor is the single source of truth for termination:
        // once there is no next sibling this yield is the last one.
        self.done = !cursor.goto_next_sibling();
        // Keep the advertised length consistent with termination: when
        // the cursor stops, nothing remains. For well-formed trees this
        // equals `child_count - emitted`; if the cursor walk and
        // `child_count` ever disagree, this still honors the
        // `ExactSizeIterator` contract (`len() == 0` exactly at
        // exhaustion) rather than reporting a phantom remainder.
        self.remaining = if self.done {
            0
        } else {
            self.remaining.saturating_sub(1)
        };
        Some(result)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.remaining, Some(self.remaining))
    }
}

/// Iterator over a node's direct children, returned by
/// [`Node::children`]. Owns the cursor it walks with.
///
/// Termination is driven by the cursor alone: each step yields the
/// cursor's current node, then advances with `goto_next_sibling`,
/// stopping the moment that returns false. This makes the cursor the
/// single source of truth for both the emitted data and when to stop, so
/// the sequence can never be padded with duplicates if `child_count` and
/// the actual sibling walk disagree.
///
/// The `ExactSizeIterator` length is reported from `child_count` (tracked
/// in `ChildScan`). For well-formed trees the cursor walk and
/// `child_count` agree, so the advertised length matches the data.
pub struct Children<'a> {
    cursor: Cursor<'a>,
    scan: ChildScan,
}

impl<'a> Iterator for Children<'a> {
    type Item = Node<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        self.scan.step(&mut self.cursor)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.scan.size_hint()
    }
}

impl ExactSizeIterator for Children<'_> {}

/// Iterator over a node's direct children, returned by
/// [`Node::children_with`]. Borrows the caller's cursor rather than
/// building one, which is the whole of the difference: it yields exactly
/// what [`Children`] yields, through the same `ChildScan`.
pub struct ChildrenWith<'c, 'a> {
    cursor: &'c mut Cursor<'a>,
    scan: ChildScan,
}

impl<'a> Iterator for ChildrenWith<'_, 'a> {
    type Item = Node<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        self.scan.step(self.cursor)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.scan.size_hint()
    }
}

impl ExactSizeIterator for ChildrenWith<'_, '_> {}

impl<'a> Search<'a> for Node<'a> {
    fn act_on_node(&self, action: &mut dyn FnMut(&Node<'a>, Ancestors<'a, '_>)) {
        let mut cursor = self.cursor();
        let mut stack = Vec::new();
        // Ancestor chain of the node being visited, root first. Kept by
        // the same truncate/push rule as the metric walk, so a predicate
        // the action applies can read an ancestor as a slice index
        // rather than through the `O(depth)` `Node::parent` (#1088).
        //
        // Seeded with this subtree root's own ancestry rather than left
        // empty: `Ancestors` reads an empty chain as "this node is the
        // tree root", so on a subtree an empty seed would report no
        // parent for `*self` — silently costing e.g. the JS getters the
        // binding a `function_expression` takes its name from. One
        // climb, and none at all for the tree root this is called on
        // today.
        let mut chain: Vec<Node<'a>> = std::iter::successors(self.parent(), Node::parent).collect();
        chain.reverse();
        let depth = chain.len();
        // As in the metric walk: the children of each wide node on
        // `chain`, so a sibling lookup in a flat `ERROR` is a binary
        // search rather than a seek from its start (#1584). Perl's
        // `call` filter reads the token after every bareword (#1580).
        // The seeded ancestors are recorded too, since the walk never
        // pushes them.
        let mut siblings = SiblingCache::default();
        for (at, ancestor) in chain.iter().enumerate() {
            siblings.record(ancestor, at, ancestor.children());
        }

        stack.push((*self, depth));

        while let Some((node, depth)) = stack.pop() {
            chain.truncate(depth);
            action(
                &node,
                Ancestors::checked(&chain, &node).with_siblings(&siblings),
            );
            chain.push(node);
            // Source order in, tail reversed in place, so the LIFO
            // `stack` yields the leftmost child first — pre-order with
            // no staging buffer.
            let first_child = stack.len();
            stack.extend(
                node.children_with(&mut cursor)
                    .map(|child| (child, depth + 1)),
            );
            siblings.record(
                &node,
                depth,
                stack[first_child..].iter().map(|(child, _)| *child),
            );
            stack[first_child..].reverse();
        }
    }

    fn first_child(&self, pred: fn(u16) -> bool) -> Option<Node<'a>> {
        self.children().find(|&child| pred(child.kind_id()))
    }

    fn act_on_child(&self, action: &mut dyn FnMut(&Node<'a>)) {
        for child in self.children() {
            action(&child);
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::langs::MozjsCode;
    use crate::test_support::for_each_node_with_chain;

    /// The `child(0)` + `next_sibling()` chain [`Node::wraps_any`] used
    /// between #217 and #1088, kept here as the reference the cursor
    /// walk that replaced it is checked against.
    ///
    /// The swap was made for cost, not for behaviour: a sibling step
    /// resolves its parent, and `tree_sitter` resolves a parent by
    /// descending from the root, so the chain was `O(children × depth)`
    /// where the cursor is `O(children)`. Nothing about the *set* of
    /// children was supposed to change, and this is what says so —
    /// node-by-node over a real tree, same order and same short-circuit,
    /// without hardcoding grammar `kind_id`s.
    #[cfg(feature = "mozjs")]
    fn sibling_chain_has_sibling(node: OtherNode, id: u16) -> bool {
        node.parent().is_some_and(|parent| {
            let mut cur = parent.child(0);
            while let Some(c) = cur {
                if c.kind_id() == id {
                    return true;
                }
                cur = c.next_sibling();
            }
            false
        })
    }

    /// `utf8_text` answers `None` for a span outside the buffer instead
    /// of aborting the process.
    ///
    /// The only way to reach that branch is to read against a buffer
    /// other than the one the tree was parsed from, which no caller does
    /// today — so this is the whole coverage the guard can have, and it
    /// is a real one: against the delegating implementation
    /// (`self.0.utf8_text(data).ok()`) this test panics at
    /// `str::from_utf8(&source[start..end])` rather than failing, which
    /// is precisely the behaviour the guard removes. Verified by revert.
    #[test]
    #[cfg(feature = "rust")]
    fn utf8_text_is_none_for_a_span_past_the_buffer() {
        let code = b"fn f() {}\n";
        let tree = Tree::new::<crate::langs::RustCode>(code);
        let root = tree.get_root();

        // Sanity: the span is in range against its own buffer, so a
        // `None` below means the bound fired and not that the node is
        // empty or the text is not UTF-8.
        assert_eq!(root.utf8_text(code), Some("fn f() {}\n"));

        // The same node, read against a buffer that ends mid-span.
        assert_eq!(root.utf8_text(&code[..4]), None);
    }

    #[cfg(feature = "mozjs")]
    #[test]
    fn has_sibling_matches_the_retired_sibling_chain() {
        // Arrow functions exercise the `check_if_arrow_func!` call site
        // that motivated #521 (PropertyIdentifier siblings on the JS/TS
        // closure-classification hot path).
        let code = b"const o = { m: (a) => a + 1, n: function () {} }; foo.bar();";
        let tree = Tree::new::<MozjsCode>(code);
        let ts_tree = tree.as_ts_tree();

        // Collect the grammar kinds that actually occur, so the
        // equivalence check covers present-sibling (true) cases.
        let mut kinds = std::collections::BTreeSet::new();
        let mut stack = vec![ts_tree.root_node()];
        while let Some(n) = stack.pop() {
            kinds.insert(n.kind_id());
            let mut child = n.child(0);
            while let Some(c) = child {
                stack.push(c);
                child = c.next_sibling();
            }
        }
        // Include an id that does not occur anywhere for absent-sibling
        // (false) coverage.
        let absent_id = u16::MAX;

        let mut stack = vec![ts_tree.root_node()];
        let mut matched = 0;
        while let Some(n) = stack.pop() {
            let wrapped = Node(n);
            for &id in kinds.iter().chain(std::iter::once(&absent_id)) {
                let found = wrapped.has_sibling(Ancestors::unknown(), id);
                assert_eq!(
                    found,
                    sibling_chain_has_sibling(n, id),
                    "has_sibling diverged from the retired sibling chain at node kind {} for id {id}",
                    n.kind(),
                );
                matched += usize::from(found);
            }
            let mut child = n.child(0);
            while let Some(c) = child {
                stack.push(c);
                child = c.next_sibling();
            }
        }
        // The comment above claims the collected kinds cover the
        // present-sibling case; this enforces it. Both sides answering
        // `false` everywhere would agree without either scan ever
        // running to a match.
        assert!(
            matched > 0,
            "every answer was `false`, so the sibling scan was never exercised"
        );

        // No-parent node (root) always reports no sibling.
        let root = Node(ts_tree.root_node());
        assert!(!root.has_sibling(Ancestors::unknown(), absent_id));
        for &id in &kinds {
            assert!(
                !root.has_sibling(Ancestors::unknown(), id),
                "root node has no parent → no sibling"
            );
        }
    }

    /// `children()` must yield exactly the node's direct children, in
    /// order, for every node in a real tree — including the empty
    /// (leaf) and single-child cases. Termination is cursor-driven, so
    /// the emitted set is compared node-by-node against the raw
    /// tree-sitter `child(i)` walk (the ground truth for both order and
    /// count). This pins the no-duplicate-padding property: a desync
    /// between `child_count` and the cursor walk would surface here as
    /// extra trailing duplicates or a length mismatch.
    #[cfg(feature = "mozjs")]
    #[test]
    fn children_matches_tree_sitter_child_walk() {
        // Mix of leaf nodes (no children), single-child wrappers, and
        // multi-child constructs to cover all arities.
        let code = b"const o = { m: (a) => a + 1 }; foo(); ;";
        let tree = Tree::new::<MozjsCode>(code);
        let ts_tree = tree.as_ts_tree();

        let mut stack = vec![ts_tree.root_node()];
        while let Some(n) = stack.pop() {
            let wrapped = Node(n);

            // Ground truth: walk children by index off the raw node.
            let expected: Vec<_> = (0..n.child_count() as u32)
                .filter_map(|i| n.child(i))
                .map(|c| (c.id(), c.kind_id()))
                .collect();

            let actual =
                drain_checking_exact_size(wrapped.children(), expected.len(), "children", n.kind());
            assert_eq!(
                actual,
                expected,
                "children() diverged from child(i) walk at kind {}",
                n.kind(),
            );

            for i in 0..n.child_count() as u32 {
                if let Some(c) = n.child(i) {
                    stack.push(c);
                }
            }
        }
    }

    /// `child_by_field_name` (issue #786) must return the child at the
    /// underlying tree lifetime `'a`, not the method-call borrow of
    /// `&self`. The proof is a helper whose return type *requires* the
    /// child to outlive an intermediate `&Node` borrow: under the old
    /// `Option<Node<'_>>` signature the returned node would be tied to
    /// `parent`'s borrow and this would fail to compile. Binding the
    /// child to a variable that outlives the `&parent` reborrow inside
    /// the helper exercises the widened lifetime.
    #[cfg(feature = "cpp")]
    #[test]
    fn child_by_field_name_outlives_self_borrow() {
        // `find_named_child` takes the parent by value, reborrows it
        // through a `&` reference to call `child_by_field_name`, and
        // returns the child. The returned `Node<'a>` must survive past
        // that inner `&parent` borrow — only possible because the child
        // carries the tree lifetime, not the borrow of `&parent`.
        fn find_named_child<'a>(parent: Node<'a>) -> Option<Node<'a>> {
            let borrowed: &Node<'a> = &parent;
            borrowed.child_by_field_name("declarator")
        }

        let code = b"int answer = 42;";
        let tree = Tree::new::<crate::langs::CppCode>(code);
        let root = tree.get_root();

        // Walk to the `declaration` node, then pull its `declarator`
        // child out and hold it after the producing borrow has ended.
        let mut held: Option<Node> = None;
        let mut stack = vec![root];
        while let Some(n) = stack.pop() {
            if n.kind() == "declaration" {
                // `find_named_child` consumes a copy of `n`; the result
                // must remain valid here, well past the inner borrow.
                held = find_named_child(n);
                break;
            }
            for child in n.children() {
                stack.push(child);
            }
        }

        let declarator = held.expect("C declaration has a `declarator` field");
        // The held node is still usable: it kept its tree linkage rather
        // than dangling at the end of the producing borrow.
        assert_eq!(declarator.kind(), "init_declarator");
    }

    /// `Node::as_tree_sitter` (issue #556) must hand back the *same*
    /// underlying `tree_sitter::Node` the wrapper holds: identical
    /// `kind()` / `kind_id()` and a usable tree-sitter API. Obtaining
    /// the wrapper through the public `CppParser` + `ParserTrait::root`
    /// path (rather than the in-module `Tree::new`) proves the accessor
    /// is the public seam that replaced the former `pub` `.0` field.
    #[cfg(feature = "cpp")]
    #[test]
    fn as_tree_sitter_round_trips_wrapper_kind() {
        use crate::{CppParser, ParserTrait};
        use std::path::Path;

        let source = b"int main() { return 0; }";
        let parser = CppParser::new(source.to_vec(), Path::new("example.cpp"), None);
        let root = parser.root();

        let ts_root = root.as_tree_sitter();

        // A well-formed C++ translation unit roots at `translation_unit`.
        assert_eq!(ts_root.kind(), "translation_unit");
        // The accessor must agree with the wrapper's own kind views.
        assert_eq!(ts_root.kind(), root.kind());
        assert_eq!(ts_root.kind_id(), root.kind_id());
        // The returned node is usable as a tree-sitter node, not a copy
        // that has lost its tree linkage: the parse is error-free and
        // the root has children.
        assert!(!ts_root.has_error());
        assert!(ts_root.child_count() > 0);
    }

    /// Ground-truth pre-order walk over the raw tree-sitter node, by
    /// document order (`child(0..child_count)`). [`Node::preorder`] must
    /// emit exactly this sequence of node ids — node first, then each
    /// child subtree left to right.
    #[cfg(feature = "cpp")]
    fn ground_truth_preorder(node: OtherNode) -> Vec<usize> {
        let mut out = vec![node.id()];
        for i in 0..node.child_count() as u32 {
            if let Some(child) = node.child(i) {
                out.extend(ground_truth_preorder(child));
            }
        }
        out
    }

    #[cfg(feature = "cpp")]
    #[test]
    fn preorder_matches_recursive_document_order() {
        // A nested construct (function holding a declaration and a call)
        // gives the walk real depth and sibling fan-out to order.
        let code = b"int main() { int x = 1; foo(x); return 0; }";
        let tree = Tree::new::<crate::langs::CppCode>(code);
        let root = tree.get_root();

        let actual: Vec<usize> = root.preorder().map(|n| n.id()).collect();
        let expected = ground_truth_preorder(root.as_tree_sitter());

        assert_eq!(
            actual, expected,
            "preorder diverged from recursive child(0..n) document order"
        );
        // Sanity: a non-trivial tree, and the root is visited first.
        assert!(actual.len() > 5, "expected a multi-node tree");
        assert_eq!(actual[0], root.id(), "root must be yielded first");
    }

    #[cfg(feature = "cpp")]
    #[test]
    fn descendants_by_kind_collects_matching_subtree_nodes() {
        // `x` is declared once and used twice, so three `identifier`
        // nodes exist under the function; `main` is an identifier too.
        let code = b"int main() { int x = 1; return x + x; }";
        let tree = Tree::new::<crate::langs::CppCode>(code);
        let root = tree.get_root();

        let found = root.descendants_by_kind(&["identifier"]);
        // Cross-check against an independent pre-order count so the helper
        // cannot pass by matching everything or nothing.
        let expected: Vec<usize> = root
            .preorder()
            .filter(|n| n.kind() == "identifier")
            .map(|n| n.id())
            .collect();
        let actual: Vec<usize> = found.iter().map(Node::id).collect();
        assert_eq!(actual, expected);
        assert!(
            found.len() >= 3,
            "expected at least the `main`, `x` decl, and `x` uses"
        );
        assert!(
            found.iter().all(|n| n.kind() == "identifier"),
            "every collected node must match the requested kind"
        );

        // An absent kind yields nothing; a multi-kind filter unions.
        assert!(root.descendants_by_kind(&["no_such_kind"]).is_empty());
        assert!(
            root.descendants_by_kind(&["identifier", "number_literal"])
                .len()
                > found.len(),
            "adding `number_literal` must widen the match set"
        );
    }

    /// Drains `iter`, holding it to the `ExactSizeIterator` contract at
    /// every step, and returns the `(id, kind_id)` of each child yielded.
    ///
    /// `len()` must equal the node's `child_count` before the first step,
    /// fall by exactly one per yield, and be zero at exhaustion. Both
    /// child iterators are checked against it, so the contract is stated
    /// once — `children_with` exists to save an allocation, and a
    /// separate copy of this is how the two would come to disagree.
    #[cfg(feature = "mozjs")]
    fn drain_checking_exact_size<'a>(
        mut iter: impl ExactSizeIterator<Item = Node<'a>>,
        child_count: usize,
        what: &str,
        kind: &str,
    ) -> Vec<(usize, u16)> {
        assert_eq!(
            iter.len(),
            child_count,
            "{what}().len() disagreed with child_count at kind {kind}"
        );
        let mut remaining = child_count;
        let mut drained = Vec::with_capacity(remaining);
        while let Some(child) = iter.next() {
            remaining -= 1;
            assert_eq!(
                iter.len(),
                remaining,
                "{what}() size_hint drifted mid-iteration at kind {kind}"
            );
            drained.push((child.id(), child.kind_id()));
        }
        assert_eq!(
            iter.len(),
            0,
            "{what}() was not drained to zero len at kind {kind}"
        );
        drained
    }

    /// Ancestor ids yielded by `ancestors`, nearest first.
    #[cfg(any(
        feature = "c",
        feature = "elixir",
        feature = "java",
        feature = "javascript",
        feature = "kotlin",
        feature = "python",
        feature = "ruby",
        feature = "rust",
    ))]
    fn ancestor_ids(ancestors: Ancestors<'_, '_>, node: &Node<'_>) -> Vec<usize> {
        ancestors.iter(node).map(|(a, _)| a.id()).collect()
    }

    /// A known chain must answer every ancestor question exactly as
    /// climbing with `Node::parent` does — that equivalence is the whole
    /// premise of #1084, and it is what lets the predicates keep their
    /// original logic while dropping the `O(depth)` lookup.
    ///
    /// Checked node-by-node over one fixture per grammar family that
    /// actually consults an ancestor: C-family (`is_else_if` via the
    /// parent clause, `loc`'s declaration gate), JVM-family
    /// (`is_else_if` via the preceding `else` token), Python (the
    /// grandparent shape), and Elixir (`quote` templates).
    #[cfg(all(
        feature = "c",
        feature = "elixir",
        feature = "java",
        feature = "javascript",
        feature = "kotlin",
        feature = "python",
        feature = "ruby",
        feature = "rust",
    ))]
    #[test]
    fn a_known_chain_answers_exactly_what_climbing_answers() {
        /// `must_nest` names kinds that have to appear *inside another
        /// node of the same kind* in the fixture. `visited > 20` alone
        /// does not keep a fixture honest: a grammar bump that flattened
        /// the nesting a row was added for would leave a large,
        /// clean-parsing tree that no longer exercises the shape, and
        /// the parity assertions would keep passing over it.
        #[cfg(any(
            feature = "c",
            feature = "elixir",
            feature = "java",
            feature = "javascript",
            feature = "kotlin",
            feature = "python",
            feature = "ruby",
            feature = "rust",
        ))]
        fn assert_parity<L: LanguageInfo>(label: &str, code: &[u8], must_nest: &[&str]) {
            let mut nested_seen = vec![false; must_nest.len()];
            let visited = for_each_node_with_chain::<L>(code, |node, chain| {
                for (slot, kind) in nested_seen.iter_mut().zip(must_nest) {
                    *slot |= node.kind() == *kind
                        && chain.iter().any(|ancestor| ancestor.kind() == *kind);
                }
                let known = Ancestors::known(chain);
                let climbing = Ancestors::unknown();
                assert_eq!(
                    known.parent(node).map(|p| p.id()),
                    climbing.parent(node).map(|p| p.id()),
                    "{label}: parent of {} disagrees",
                    node.kind()
                );
                assert_eq!(
                    known.previous_sibling(node).map(|p| p.id()),
                    climbing.previous_sibling(node).map(|p| p.id()),
                    "{label}: previous sibling of {} disagrees",
                    node.kind()
                );
                assert_eq!(
                    ancestor_ids(known, node),
                    ancestor_ids(climbing, node),
                    "{label}: ancestor chain of {} disagrees",
                    node.kind()
                );
                // Each ancestor is handed *its* own chain, so a
                // predicate applied one level up stays as cheap and as
                // correct as one applied to the node itself.
                for (ancestor, above) in known.iter(node) {
                    assert_eq!(
                        above.parent(&ancestor).map(|p| p.id()),
                        ancestor.parent().map(|p| p.id()),
                        "{label}: sub-chain handed to {} is not its own",
                        ancestor.kind()
                    );
                }
            });
            assert!(visited > 20, "{label}: fixture is too small to prove much");
            for (found, kind) in nested_seen.iter().zip(must_nest) {
                assert!(
                    found,
                    "{label}: no `{kind}` sits inside another `{kind}`, so the \
                     fixture no longer exercises the nesting it was added for"
                );
            }
        }

        assert_parity::<crate::langs::CCode>(
            "c",
            b"int main() { if (a) { int x; } else if (b) { for (int i = 0; i < 2; i++) x; } }",
            &[],
        );
        assert_parity::<crate::langs::JavaCode>(
            "java",
            b"class A { void m() { if (a) {} else if (b) {} else {} for (int i = 0; i < 2; i++) {} } }",
            &[],
        );
        assert_parity::<crate::langs::PythonCode>(
            "python",
            b"def f(a, b):\n    if a:\n        pass\n    else:\n        if b:\n            pass\n    return a and b or a\n",
            &[],
        );
        assert_parity::<crate::langs::ElixirCode>(
            "elixir",
            b"defmodule M do\n  def g do\n    :ok\n  end\n  quote do\n    def f do\n      :ok\n    end\n  end\nend\n",
            &["call"],
        );

        // The shapes #1062 added as consumers, which the four fixtures
        // above do not contain: a function nested inside a function
        // (every language's `increment_function_depth` arm walks the
        // chain looking for one) and the two default-arm checks that
        // now read `Ancestors::parent` — Kotlin's `else ->` inside a
        // `when` and Ruby's `else` inside a `case`. Parity over the
        // machinery is not parity over the shape a caller asks about.
        assert_parity::<crate::langs::RustCode>(
            "rust",
            b"fn f(a: bool) { if a { } else if a { } fn g(b: bool) { if b { } } }\n",
            &["function_item"],
        );
        assert_parity::<crate::langs::KotlinCode>(
            "kotlin",
            b"fun f(x: Int) {\n    when (x) {\n        1 -> {}\n        else -> {}\n    }\n    fun g() {\n        if (x > 0) {}\n    }\n}\n",
            &["function_declaration"],
        );
        assert_parity::<crate::langs::RubyCode>(
            "ruby",
            b"def f(x)\n  case x\n  when 1 then 1\n  else 2\n  end\n  def g\n    if x\n    end\n  end\nend\n",
            &["method"],
        );

        // The shape #1088 added as a consumer: the JS-family
        // `Checker::is_func` / `is_closure` walk upward from an
        // `arrow_function` / `function_expression` looking for the
        // binding that names it, and end on `Ancestors::previous_sibling`
        // through `has_sibling`. None of the fixtures above contains
        // either node.
        assert_parity::<crate::langs::JavascriptCode>(
            "javascript",
            b"const f = a => { a => { g(() => 1); }; };\nconst o = { m: function () { return 1; } };\n",
            &["arrow_function"],
        );
    }

    /// The traversals #1112 moved onto [`Node::children_with`] must scan
    /// a whole tree on one cursor, not one per node.
    ///
    /// Nothing in the output says so: `children_with` yields exactly
    /// what `children` yields, so every metric, marker, and pre-order
    /// assertion in the suite holds just as well with a fresh
    /// `TreeCursor` built and freed per visited node. The counter is the
    /// only observable, which is why reverting one of these loops has to
    /// be a test failure rather than a silent allocation per node. The
    /// metric and suppression walks are guarded the same way from
    /// `big-code-analysis`, where they live.
    ///
    /// Seeding a real scan first is what makes it falsifiable: compared
    /// against zero these assertions would also pass with `record()`
    /// never wired up at all.
    // `python` and `rust` were stale: the body builds only `CCode` and
    // `MozjsCode`, so naming them kept the test out of builds that could
    // run it.
    #[cfg(all(feature = "c", feature = "mozjs"))]
    #[test]
    fn the_converted_traversals_scan_a_tree_on_one_cursor() {
        let seed_tree = Tree::new::<crate::langs::CCode>(b"int main() { int a; }");
        let _ = seed_tree.get_root().children().count();
        assert!(
            child_scan_cursors::observed() > 0,
            "the seed scan must be counted"
        );

        // `preorder` over a tree far larger than any per-call constant,
        // so "one per node" and "one per walk" cannot be confused.
        let tree = Tree::new::<MozjsCode>(
            b"const o = { m: (a) => a + 1, n: function () { return [1, 2, 3]; } };\nfoo(o);\n",
        );
        let before = child_scan_cursors::observed();
        let visited = tree.get_root().preorder().count();
        assert!(visited > 40, "fixture is too small to prove much");
        assert_eq!(
            child_scan_cursors::observed(),
            before,
            "preorder built a cursor per node; it holds one for the walk (#1112)"
        );

        // The `Search` walk, `act_on_node`. The counter records in
        // `children()`, the allocating form, so a walk that hoists its
        // cursor records nothing at all and a per-node one records once
        // per interior node. Asserting the exact zero is what tells a
        // hoisted cursor from a per-node one; a bound like `< nodes / 2`
        // would hold for either on a small fixture.
        let tree = Tree::new::<MozjsCode>(
            b"function f(a) { return { g: (b) => b + 1, h: [1, 2, 3] }; }\nf(2);\n",
        );
        let root = tree.get_root();
        let nodes = root.preorder().count();
        assert!(nodes > 30, "fixture is too small to prove much");

        let before = child_scan_cursors::observed();
        let mut seen = 0_usize;
        root.act_on_node(&mut |_, _| seen += 1);
        let scans = child_scan_cursors::observed() - before;
        assert_eq!(seen, nodes, "act_on_node must visit every node");
        assert_eq!(
            scans, 0,
            "act_on_node built {scans} cursors over {nodes} nodes; it holds one \
             for the walk (#1112)"
        );
    }

    /// [`Node::parent_grandparent_match`] must answer `false` when
    /// either link is missing, rather than degrading to a
    /// single-predicate check.
    ///
    /// Its doc states that invariant and Python's `Cyclomatic` `else`
    /// arm depends on it, but nothing exercised either absent-link
    /// return: every call in the suite runs on a node that has both a
    /// parent and a grandparent. Both predicates answer `true` here, so
    /// a `false` result can only come from the missing link — an
    /// implementation that skipped the second `climb.next()` and
    /// returned `parent_pred`'s answer would pass every other test and
    /// fail this one.
    ///
    /// Checked through both `Ancestors` constructors: the chain and the
    /// climb reach the end by different code paths (`split_last` on an
    /// empty slice, versus `Node::parent` returning `None`).
    #[cfg(feature = "c")]
    #[test]
    fn parent_grandparent_match_is_false_when_either_link_is_absent() {
        let tree = Tree::new::<crate::langs::CCode>(b"int main() { int a; }");
        let root = tree.get_root();
        let child = root.children().next().expect("the file has an item");
        let grandchild = child
            .children()
            .next()
            .expect("the function definition has children");
        let yes: fn(&Node) -> bool = |_| true;

        // No parent at all: the root, reached either way.
        assert!(!root.parent_grandparent_match(Ancestors::unknown(), yes, yes));
        assert!(!root.parent_grandparent_match(Ancestors::known(&[]), yes, yes));

        // A parent but no grandparent: a direct child of the root.
        assert!(!child.parent_grandparent_match(Ancestors::unknown(), yes, yes));
        assert!(!child.parent_grandparent_match(Ancestors::known(&[root]), yes, yes));

        // Both links present, so the same predicates now answer `true`.
        // Without this the assertions above would also hold for a
        // function that always returned `false`.
        assert!(grandchild.parent_grandparent_match(Ancestors::unknown(), yes, yes));
        assert!(grandchild.parent_grandparent_match(Ancestors::known(&[root, child]), yes, yes));
    }

    /// [`Node::children_with`] must yield exactly what
    /// [`Node::children`] yields — same nodes, same order, same
    /// `ExactSizeIterator` length at every step — for every node of a
    /// real tree.
    ///
    /// Checked against the raw `child(i)` walk rather than against
    /// `children()`: the two iterators share `ChildScan`, so a
    /// comparison between them would pass just as happily if the shared
    /// step were wrong. It also covers the reuse itself — one cursor
    /// drives every node's scan here, so a `reset` that failed to rewind
    /// would show as the second node inheriting the first's position.
    #[cfg(feature = "mozjs")]
    #[test]
    fn children_with_yields_exactly_what_children_does() {
        let code = b"const o = { m: (a) => a + 1, n: function () {} }; foo(); ;";
        let tree = Tree::new::<MozjsCode>(code);
        let root = tree.get_root();

        let mut cursor = root.cursor();
        let mut leaves = 0;
        let mut widest = 0;
        for node in root.preorder() {
            // Ground truth is the raw `child(i)` walk, not `children()`.
            // The two iterators share `ChildScan`, so checking one
            // against the other would pass just as happily if the shared
            // step were wrong.
            let raw = node.as_tree_sitter();
            let expected: Vec<_> = (0..raw.child_count() as u32)
                .filter_map(|i| raw.child(i))
                .map(|c| (c.id(), c.kind_id()))
                .collect();

            let actual = drain_checking_exact_size(
                node.children_with(&mut cursor),
                expected.len(),
                "children_with",
                node.kind(),
            );
            assert_eq!(
                actual,
                expected,
                "children_with diverged from the child(i) walk at kind {}",
                node.kind()
            );

            leaves += usize::from(expected.is_empty());
            widest = widest.max(expected.len());
        }
        // Both ends of the arity range, else the comparison could hold
        // over nothing but one-child wrappers.
        assert!(leaves > 0, "fixture must contain childless nodes");
        assert!(widest > 2, "fixture must contain a multi-child node");
    }

    /// `children_after` and `children_before` must yield, for every child
    /// of every node, the raw `child(i)` walk on either side of it — and
    /// nothing for a node that is not a child.
    ///
    /// The Kotlin fixture's recovery inserts a zero-width parameter name
    /// (`vararg: Int`), which `goto_first_child_for_byte` passes over, so
    /// it is the only input that reaches the scan fallback in
    /// `seat_on_child`; the count of such children pins that it does.
    #[cfg(feature = "kotlin")]
    #[test]
    fn children_after_and_before_match_the_child_walk() {
        let code = b"fun f(vararg: Int, /* c */ b: Int) { g(1, 2, 3) }";
        let tree = Tree::new::<crate::langs::KotlinCode>(code);
        let root = tree.get_root();

        // Each row is (parent kind, child index, after ids, before ids), and
        // a non-child row is (kind, after count, before count) against the
        // root, so a failing diff names the node without a message argument.
        let mut zero_width = 0;
        let (mut observed, mut expected) = (Vec::new(), Vec::new());
        let (mut stray_observed, mut stray_expected) = (Vec::new(), Vec::new());
        for node in root.preorder() {
            let raw = node.as_tree_sitter();
            let ids: Vec<_> = (0..raw.child_count() as u32)
                .filter_map(|i| raw.child(i))
                .map(|c| c.id())
                .collect();
            for (index, child) in node.children().enumerate() {
                zero_width += usize::from(child.start_byte() == child.end_byte());
                observed.push((
                    node.kind(),
                    index,
                    node.children_after(&child)
                        .map(|c| c.id())
                        .collect::<Vec<_>>(),
                    node.children_before(&child)
                        .map(|c| c.id())
                        .collect::<Vec<_>>(),
                ));
                expected.push((
                    node.kind(),
                    index,
                    ids[index + 1..].to_vec(),
                    ids[..index].iter().rev().copied().collect(),
                ));
            }
            if node.id() != root.id() {
                stray_observed.push((
                    node.kind(),
                    root.children_after(&node).count(),
                    root.children_before(&node).count(),
                ));
                stray_expected.push((node.kind(), 0, 0));
            }
        }
        assert_eq!(observed, expected);
        assert_eq!(stray_observed, stray_expected);
        assert!(
            zero_width > 0,
            "fixture must hold an inserted zero-width child"
        );
    }

    /// The `O(1)` guard [`Ancestors::checked`] keeps on by default must
    /// accept every chain a walker really builds — over several grammar
    /// families, not just the one fixture a failure would surface in.
    ///
    /// The assertion that an equal-span pair was seen is what makes the
    /// containment non-strict on purpose rather than by luck: a
    /// single-child wrapper (`expression_statement` over its expression,
    /// say) spans exactly what its child spans, so tightening either
    /// bound to `<` would reject a correct chain on most real input.
    #[cfg(all(feature = "c", feature = "javascript", feature = "python"))]
    #[test]
    fn checked_accepts_the_chains_the_walkers_build() {
        let mut equal_span_pairs = 0;
        let mut check = |node: &Node<'_>, chain: &[Node<'_>]| {
            let _ = Ancestors::checked(chain, node);
            if let Some(parent) = chain.last()
                && parent.start_byte() == node.start_byte()
                && parent.end_byte() == node.end_byte()
            {
                equal_span_pairs += 1;
            }
        };
        let visited = for_each_node_with_chain::<crate::langs::CCode>(
            b"int main() { if (a) { int x; } else { f(a, b); } }",
            &mut check,
        ) + for_each_node_with_chain::<crate::langs::JavascriptCode>(
            b"const o = { m: (a) => a + 1 };\nfoo.bar();\n",
            &mut check,
        ) + for_each_node_with_chain::<crate::langs::PythonCode>(
            b"def f(a):\n    if a:\n        return [x for x in a]\n",
            &mut check,
        );

        assert!(visited > 60, "fixtures are too small to prove much");
        assert!(
            equal_span_pairs > 0,
            "no parent spans exactly what its child does, so this fixture set \
             cannot tell non-strict containment from strict"
        );
    }

    /// A `push` moved ahead of the per-node computes leaves the node
    /// itself as `chain.last()`. Spans alone cannot see that — a node
    /// contains itself — so the identity half of the guard is what
    /// catches it.
    ///
    /// Debug-gated because `debug_assert!` compiles out under
    /// `--release`, where `checked` degrades to `known` by design.
    #[cfg(feature = "c")]
    #[test]
    #[cfg(debug_assertions)]
    #[should_panic(expected = "ancestor chain desynchronised")]
    fn checked_rejects_a_chain_ending_in_the_node_itself() {
        let tree = Tree::new::<crate::langs::CCode>(b"int main() { int a; }");
        let body = tree
            .get_root()
            .preorder()
            .find(|n| n.kind() == "compound_statement")
            .expect("fixture has a function body");
        let _ = Ancestors::checked(std::slice::from_ref(&body), &body);
    }

    /// A dropped `truncate` leaves the previous subtree's path in place,
    /// so the next node up gets a `chain.last()` from a sibling subtree —
    /// disjoint from it in bytes. That is the containment half.
    #[cfg(feature = "c")]
    #[test]
    #[cfg(debug_assertions)]
    #[should_panic(expected = "ancestor chain desynchronised")]
    fn checked_rejects_a_chain_from_a_disjoint_subtree() {
        let tree = Tree::new::<crate::langs::CCode>(b"int main() { int a; int b; }");
        let body = tree
            .get_root()
            .preorder()
            .find(|n| n.kind() == "compound_statement")
            .expect("fixture has a function body");
        let declarations: Vec<Node<'_>> = body
            .children()
            .filter(|n| n.kind() == "declaration")
            .collect();
        assert_eq!(declarations.len(), 2, "fixture has two declarations");
        // `int a;` neither contains nor equals `int b;`.
        let _ = Ancestors::checked(&declarations[..1], &declarations[1]);
    }

    /// [`Node::has_sibling`] must answer the same whether its parent
    /// comes off a known chain or from `Node::parent`.
    ///
    /// The parent lookup is the only thing #1088 changed here, and it is
    /// the half a caller cannot see: `check_if_arrow_func!` folds the
    /// answer into a disjunction, so a wrong parent would silently
    /// reclassify an arrow function rather than fail. Checked for every
    /// node against every kind the fixture contains, plus one that never
    /// occurs so the absent-sibling answer is covered too.
    #[cfg(feature = "javascript")]
    #[test]
    fn has_sibling_agrees_between_known_and_climbing() {
        // Object-literal methods and an arrow bound to a property are
        // the shapes whose `PropertyIdentifier` sibling the JS closure
        // check asks about.
        let code = b"const o = { m: (a) => a + 1, n: function () {} };\nconst p = a => a;\n";
        let mut kinds = std::collections::BTreeSet::new();
        for_each_node_with_chain::<crate::langs::JavascriptCode>(code, |node, _| {
            kinds.insert(node.kind_id());
        });
        // An id no node in the fixture carries, so the `false` answer is
        // exercised as well as the `true` one.
        let absent = u16::MAX;
        let mut agreed_true = 0;
        let visited =
            for_each_node_with_chain::<crate::langs::JavascriptCode>(code, |node, chain| {
                for &id in kinds.iter().chain(std::iter::once(&absent)) {
                    let known = node.has_sibling(Ancestors::known(chain), id);
                    let climbing = node.has_sibling(Ancestors::unknown(), id);
                    assert_eq!(
                        known,
                        climbing,
                        "has_sibling({id}) on {} disagrees between chain and climb",
                        node.kind()
                    );
                    agreed_true += usize::from(known);
                }
            });
        assert!(visited > 20, "fixture is too small to prove much");
        assert!(
            agreed_true > 0,
            "every answer was `false`, so the sibling scan never ran to a match"
        );
    }

    /// [`Search::act_on_node`] must hand each node its true ancestry
    /// even when the walk starts below the tree root.
    ///
    /// The seed is what decides this. [`Ancestors`] reads an empty chain
    /// as "this node is the root", so seeding empty — which is correct
    /// for every caller today (`bca function`, `find` and `count`, whose
    /// walks all start at the root) — would report no parent for the
    /// subtree root and shift every answer beneath it. For the JS getters
    /// that means
    /// losing the `variable_declarator` a `function_expression` takes
    /// its name from, so the space would silently be named
    /// `<anonymous>`.
    ///
    /// No caller passes a subtree yet, so nothing else would catch this;
    /// the fixture below is the guard, and it fails against an empty
    /// seed both here and through `Ancestors::checked`'s debug
    /// assertion.
    #[cfg(feature = "mozjs")]
    #[test]
    fn act_on_node_hands_a_subtree_its_real_ancestry() {
        let code = b"var outer = function () { return 1; };\n";
        let tree = Tree::new::<MozjsCode>(code);
        let root = tree.get_root();
        let subtree = root
            .preorder()
            .find(|n| n.kind() == "variable_declarator")
            .expect("fixture has a variable_declarator");
        assert!(
            subtree.parent().is_some(),
            "the walk must start below the root, else the seed is vacuous"
        );

        let mut visited = 0;
        subtree.act_on_node(&mut |node, ancestors| {
            assert_eq!(
                ancestors.parent(node).map(|p| p.id()),
                node.parent().map(|p| p.id()),
                "parent of {} disagrees with the tree",
                node.kind()
            );
            visited += 1;
        });
        assert!(visited > 3, "subtree is too small to prove much");
    }

    /// A walk started below the root records its seeded ancestors'
    /// children too, so a sibling lookup off a wide one is answered from
    /// the cache. Without them the lookup's debug assertion fails: the
    /// cache is lent, and the wide parent on the chain was never pushed
    /// by this walk (#1580).
    #[cfg(feature = "mozjs")]
    #[test]
    fn act_on_node_records_a_wide_seeded_ancestor() {
        // Discriminates only through `Ancestors::lookup`'s `debug_assert!`:
        // meaningful under `make pre-commit`'s debug build, vacuous under
        // `--release` (lesson #80).
        let code = "var a = 1;\n".repeat(WIDE_PARENT + 6);
        let tree = Tree::new::<MozjsCode>(code.as_bytes());
        let root = tree.get_root();
        assert!(root.child_count() > WIDE_PARENT, "the root must be wide");
        let subtree = root.child(3).expect("fixture has a fourth statement");

        let mut after = None;
        subtree.act_on_node(&mut |node, ancestors| {
            if node.id() == subtree.id() {
                after = Some(ancestors.siblings_after(node).count());
            }
        });
        assert_eq!(after, Some(root.child_count() - 4));
    }

    /// `previous_sibling` must not answer "no previous sibling" when the
    /// chain it was handed belongs to a different node.
    ///
    /// The known path finds the answer by scanning the chain's last
    /// entry for `node`; a miss means the caller paired the two wrongly.
    /// Reporting `None` there would be a wrong answer dressed as a
    /// legitimate one, so the fallback re-asks the tree.
    #[cfg(feature = "c")]
    #[test]
    fn previous_sibling_falls_back_on_a_chain_that_is_not_this_nodes() {
        let code = b"int main() { int a; int b; }";
        let tree = Tree::new::<crate::langs::CCode>(code);
        let root = tree.get_root();
        let body = root
            .preorder()
            .find(|n| n.kind() == "compound_statement")
            .expect("fixture has a function body");
        let declarations: Vec<Node<'_>> = body
            .children()
            .filter(|n| n.kind() == "declaration")
            .collect();
        assert_eq!(declarations.len(), 2, "fixture has two declarations");

        let second = declarations[1];
        let expected = second
            .previous_sibling()
            .map(|p| p.id())
            .expect("the second declaration has a previous sibling");
        // A chain ending in the *root* does not describe `second`, whose
        // parent is the function body.
        let foreign = [root];
        assert_eq!(
            Ancestors::known(&foreign)
                .previous_sibling(&second)
                .map(|p| p.id()),
            Some(expected),
            "a mismatched chain must fall back, not report `None`"
        );
        assert!(
            Ancestors::known(&[]).previous_sibling(&second).is_none(),
            "an empty chain means `second` is the root, which has no siblings"
        );
    }

    /// `count_specific_ancestors` must return the same count whichever
    /// way it reaches the ancestors. Uses `loc`'s real C predicate pair
    /// (`while`/`for`/`if` header, stopping at the enclosing block), so
    /// the fixture exercises both the counted case (the `for`-header
    /// declaration) and the stopped case (the block-scoped ones).
    #[cfg(feature = "c")]
    #[test]
    fn count_specific_ancestors_agrees_between_known_and_climbing() {
        let code =
            b"int main() { int a; if (x) { int b; } for (int i = 0; i < 2; i++) { int c; } }";
        let mut counted = 0;
        let mut nonzero = 0;
        let visited = for_each_node_with_chain::<crate::langs::CCode>(code, |node, chain| {
            if node.kind() != "declaration" {
                return;
            }
            let check: fn(&Node) -> bool = |n| {
                matches!(
                    n.kind(),
                    "while_statement" | "for_statement" | "if_statement"
                )
            };
            let stop: fn(&Node) -> bool = |n| n.kind() == "compound_statement";
            let known = node.count_specific_ancestors::<crate::langs::CCode>(
                Ancestors::known(chain),
                check,
                stop,
            );
            let climbing = node.count_specific_ancestors::<crate::langs::CCode>(
                Ancestors::unknown(),
                check,
                stop,
            );
            assert_eq!(
                known,
                climbing,
                "declaration at row {}: known chain counted {known}, climbing counted {climbing}",
                node.start_row()
            );
            counted += 1;
            nonzero += usize::from(known > 0);
        });
        assert!(visited > 20);
        assert_eq!(counted, 4, "fixture must hold four declarations");
        assert_eq!(
            nonzero, 1,
            "only the `for`-header declaration sits under a header with no block between"
        );
    }
}