brokk_bifrost_core/analyzer/

capabilities.rs

use crate::analyzer::code_unit_index::CodeUnitIndex;
use crate::analyzer::model::{CodeUnit, ImportInfo, ProjectFile};
use crate::analyzer::pool_memo::{KeyedPoolSafeMemo, PoolSafeMemo};
use crate::analyzer::query_token::QueryToken;
use crate::cancellation::CancellationToken;
use crate::compact_graph::{CompactRows, CompactRowsBuilder};
use crate::hash::{HashMap, HashSet};
use std::any::Any;
use std::collections::{BTreeSet, VecDeque};
use std::sync::Arc;

use rayon::prelude::*;

pub trait CapabilityProvider: Any {
    fn as_any(&self) -> &dyn Any;
}

impl<T: Any> CapabilityProvider for T {
    fn as_any(&self) -> &dyn Any {
        self
    }
}

pub trait ImportAnalysisProvider: CapabilityProvider + Send + Sync {
    /// Shared, not owned: every memoizing implementation already stores the set
    /// behind an `Arc` in its per-file cache, and the hottest consumers only
    /// read it (a membership test in candidate discovery, a projection to
    /// source files in the reverse-import index). Returning the `Arc` removes a
    /// whole-set clone per cache hit and per insert.
    fn imported_code_units_of(&self, file: &ProjectFile) -> Arc<HashSet<CodeUnit>>;
    fn referencing_files_of(&self, file: &ProjectFile) -> HashSet<ProjectFile>;

    /// Return the union of files that reference any `target`, restricted to
    /// the caller's candidate universe.
    ///
    /// A depth-bounded caller already knows both sets and must not be forced to
    /// initialize a provider's whole-workspace reverse index once per target.
    /// Providers with a structured batch representation can override this.
    /// The default preserves the file-at-a-time result for every existing
    /// provider.
    fn referencing_files_of_targets(
        &self,
        targets: &HashSet<ProjectFile>,
        candidates: &[ProjectFile],
        cancellation: &CancellationToken,
    ) -> HashSet<ProjectFile> {
        let candidate_set: HashSet<ProjectFile> = candidates.iter().cloned().collect();
        let mut referencing = HashSet::default();
        for target in targets {
            if cancellation.is_cancelled() {
                break;
            }
            referencing.extend(
                self.referencing_files_of(target)
                    .into_iter()
                    .filter(|file| candidate_set.contains(file)),
            );
        }
        referencing
    }

    /// Return import facts for a group of files without requiring each caller
    /// to hydrate a complete per-file analyzer state. `None` preserves the
    /// existing file-at-a-time behavior for providers without a bulk read model.
    fn import_infos_for_files(
        &self,
        _files: &[ProjectFile],
    ) -> Option<HashMap<ProjectFile, Vec<ImportInfo>>> {
        None
    }

    /// The file's import facts, which fall through to the import-tier store
    /// read when nothing retained answers. Crossing that tier is only cheap
    /// under an open request scope, so the caller proves one is open by
    /// passing a [`QueryToken`] (issue #2423).
    fn import_info_of(&self, _token: QueryToken<'_>, _file: &ProjectFile) -> Vec<ImportInfo> {
        Vec::new()
    }

    /// Resolve imported source units from already-loaded import facts. Providers
    /// that cannot do this cheaply return `None` and use `imported_code_units_of`.
    fn imported_code_units_from_infos(
        &self,
        _file: &ProjectFile,
        _imports: &[ImportInfo],
    ) -> Option<Arc<HashSet<CodeUnit>>> {
        None
    }

    /// Resolve directly imported project files from already-loaded import facts.
    /// Providers that do not expose file-level edges return `None` and callers
    /// can derive a conservative approximation from imported declarations.
    fn imported_files_from_infos(
        &self,
        _file: &ProjectFile,
        _imports: &[ImportInfo],
    ) -> Option<HashSet<ProjectFile>> {
        None
    }

    fn relevant_imports_for(&self, _code_unit: &CodeUnit) -> HashSet<String> {
        HashSet::default()
    }

    /// Whether `source_file` can reference a declaration of `target`. A `true`
    /// is an answer; a `false` is only an absence of evidence, which is why
    /// callers still run the expansion backstop after one. Providers that can
    /// prove the negative override [`Self::import_reachability`] instead and
    /// derive this from it.
    fn could_import_file(
        &self,
        _source_file: &ProjectFile,
        _imports: &[ImportInfo],
        _target: &ProjectFile,
    ) -> bool {
        false
    }

    /// Resolve, in one batch, whatever [`Self::could_import_file`] would look
    /// up once per candidate.
    ///
    /// The shared import-graph candidate walk visits every workspace file, so
    /// a provider that answers each visit with its own store lookup pays that
    /// lookup once per import statement in the workspace -- 397k to 662k
    /// `definition_candidates` round trips inside a single `scan_usages`
    /// query on a 35k-file Rust workspace (#1748). The walk knows its whole
    /// candidate set before it inspects any of it, so this hook lets the
    /// provider enumerate its keys up front and collapse them into batched
    /// reads.
    ///
    /// Doing nothing is always correct: the per-candidate path still answers
    /// exactly as before, just without the shared warm result. Providers whose
    /// per-candidate answer is already file-local (Python, JS/TS, Go, C++)
    /// keep the default.
    fn prefetch_import_targets(
        &self,
        _files: &[ProjectFile],
        _import_infos: Option<&HashMap<ProjectFile, Vec<ImportInfo>>>,
        _cancellation: &crate::cancellation::CancellationToken,
    ) {
    }

    /// The three-valued form of [`Self::could_import_file`], which lets a
    /// provider that has a completeness proof retire the caller's backstop.
    ///
    /// The default preserves the historical contract exactly by mapping the
    /// bool spelling's `true` to [`ImportReachability::Reaches`] and its
    /// `false` to [`ImportReachability::Unknown`] -- never to `DoesNotReach`,
    /// because the bool contract never distinguished "no" from "I did not
    /// find one".
    ///
    /// Bridging runs in this direction only. A provider overrides this method
    /// and defines `could_import_file` over it, so the two spellings cannot
    /// drift apart.
    fn import_reachability(
        &self,
        source_file: &ProjectFile,
        imports: &[ImportInfo],
        target: &ProjectFile,
    ) -> ImportReachability {
        if self.could_import_file(source_file, imports, target) {
            ImportReachability::Reaches
        } else {
            ImportReachability::Unknown
        }
    }
}

/// How completely a provider can answer "can `source_file` reference a
/// declaration of `target`?".
///
/// Candidate discovery asks this per (candidate, target) pair and, when it
/// gets no positive answer, materializes every declaration the candidate
/// imports as a recall backstop. For a namespace-import language that
/// expansion is the whole workspace's top-level types per using directive,
/// which is the shape that burned #1194. The backstop exists only because the
/// bool contract could not distinguish a proven "no" from an unproven one.
///
/// A provider must return `DoesNotReach` only from a proof that covers every
/// way its language can name a declaration without importing it. Anything less
/// is `Unknown`: the cost of an unnecessary expansion is time, the cost of a
/// wrong `DoesNotReach` is a missing usage.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ImportReachability {
    /// The candidate can reference the target. Accept it without expanding.
    Reaches,
    /// The candidate provably cannot. Reject it without expanding.
    DoesNotReach,
    /// Undecided. The caller runs its own backstop, exactly as before this
    /// verdict existed.
    Unknown,
}

/// Resolve direct project-file edges from structured import facts. Prefer a
/// provider's file-level resolver so imports whose target has no declarations
/// remain visible; otherwise conservatively project resolved declaration
/// identities back to their source files.
pub fn resolve_imported_files_from_infos(
    provider: &dyn ImportAnalysisProvider,
    file: &ProjectFile,
    imports: &[ImportInfo],
) -> HashSet<ProjectFile> {
    provider
        .imported_files_from_infos(file, imports)
        .unwrap_or_else(|| {
            provider
                .imported_code_units_from_infos(file, imports)
                .unwrap_or_else(|| provider.imported_code_units_of(file))
                .iter()
                .map(|unit| unit.source().clone())
                .collect()
        })
}

pub fn build_reverse_import_index<F>(
    files: &[ProjectFile],
    resolve_imported: F,
    parallel: bool,
) -> HashMap<ProjectFile, Arc<HashSet<ProjectFile>>>
where
    F: Fn(&ProjectFile) -> Arc<HashSet<CodeUnit>> + Sync,
{
    build_reverse_file_index(
        files,
        |file| {
            resolve_imported(file)
                .iter()
                .map(|code_unit| code_unit.source().clone())
                .collect::<Vec<_>>()
        },
        parallel,
    )
}

pub type ReverseFileIndex = HashMap<ProjectFile, Arc<HashSet<ProjectFile>>>;

pub fn memoized_reverse_import_index<F, Files>(
    memo: &PoolSafeMemo<ReverseFileIndex>,
    files: Files,
    resolve_imported: F,
) -> Arc<ReverseFileIndex>
where
    F: Fn(&ProjectFile) -> Arc<HashSet<CodeUnit>> + Sync + Copy,
    Files: Fn() -> Vec<ProjectFile> + Copy,
{
    memoized_reverse_file_index(memo, files, |file| {
        resolve_imported(file)
            .iter()
            .map(|code_unit| code_unit.source().clone())
            .collect::<Vec<_>>()
    })
}

pub fn memoized_reverse_file_index<F, I, Files>(
    memo: &PoolSafeMemo<ReverseFileIndex>,
    files: Files,
    resolve_targets: F,
) -> Arc<ReverseFileIndex>
where
    F: Fn(&ProjectFile) -> I + Sync + Copy,
    I: IntoIterator<Item = ProjectFile>,
    Files: Fn() -> Vec<ProjectFile> + Copy,
{
    memo.get_or_build(
        || {
            let files = files();
            build_reverse_file_index(&files, resolve_targets, true)
        },
        || {
            let files = files();
            build_reverse_file_index(&files, resolve_targets, false)
        },
    )
}

pub fn build_reverse_file_index<F, I>(
    files: &[ProjectFile],
    resolve_targets: F,
    parallel: bool,
) -> HashMap<ProjectFile, Arc<HashSet<ProjectFile>>>
where
    F: Fn(&ProjectFile) -> I + Sync,
    I: IntoIterator<Item = ProjectFile>,
{
    let collect_edges = |file: &ProjectFile| {
        let source = file.clone();
        resolve_targets(file)
            .into_iter()
            .filter_map(move |target| (target != source).then(|| (target, source.clone())))
            .collect::<Vec<_>>()
    };
    let edges: Vec<_> = if parallel {
        files.par_iter().flat_map(collect_edges).collect()
    } else {
        files.iter().flat_map(collect_edges).collect()
    };

    let mut reverse: HashMap<ProjectFile, HashSet<ProjectFile>> = HashMap::default();
    for (target, source) in edges {
        reverse.entry(target).or_default().insert(source);
    }
    reverse
        .into_iter()
        .map(|(file, refs)| (file, Arc::new(refs)))
        .collect()
}

/// `Send + Sync` like its three sibling capabilities: every implementor is an
/// analyzer, and a parallel whole-workspace scan holds the provider across its
/// fan-out (`kotlin_graph`'s edge builder does exactly that).
pub trait TypeAliasProvider: CapabilityProvider + Send + Sync {
    fn is_type_alias(&self, _code_unit: &CodeUnit) -> bool {
        false
    }
}

pub trait TestDetectionProvider: CapabilityProvider {}

/// Which slice of the workspace a descendant index covers.
///
/// This is the memo key, and two values is the whole range: the excluded set is
/// a pure function of the analyzer and the file, so every request that asks to
/// leave test files out describes the same index.
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub enum DescendantIndexVariant {
    /// Every class-like declaration the analyzer indexes.
    WholeWorkspace,
    /// The declarations whose source file the caller's predicate rejected are
    /// left out. In practice that is a `scan_usages` request with
    /// `include_tests: false`, whose test files the answer would discard anyway.
    ProductionOnly,
}

/// The request state a whole-workspace descendant-index build needs: the
/// deadline it polls, and the slice of the workspace it covers.
///
/// The two travel together because one loop consumes both -- the per-class loop
/// in [`build_direct_descendant_index_from_candidates`] and, for C++, the
/// per-file include-closure walk beneath it -- and because the slice is the
/// memo key that the deadline's complete-or-nothing rule is applied to.
///
/// The exclusion predicate is supplied by the caller rather than named here.
/// Test classification needs an analyzer (`file_is_test_only` reads a language
/// module index), which this crate must not know about; the closure crossing is
/// the same one the Rust usage walks already use for their `keep_going`
/// predicate. **The predicate must be a pure function of the analyzer and the
/// file.** That is the condition that makes [`DescendantIndexVariant`] a
/// complete key: two requests that both exclude test files must describe the
/// same index, or one would be served the other's.
#[derive(Clone, Copy)]
pub struct DescendantIndexScope<'a> {
    cancellation: &'a CancellationToken,
    excluded_source: Option<&'a dyn Fn(&ProjectFile) -> bool>,
}

impl<'a> DescendantIndexScope<'a> {
    /// Every class in the workspace, stopping when `cancellation` says to.
    pub fn whole_workspace(cancellation: &'a CancellationToken) -> Self {
        Self {
            cancellation,
            excluded_source: None,
        }
    }

    /// Every class whose source file `excluded` rejects is left out of the
    /// index entirely -- it is never handed to `get_direct_ancestors`, so the
    /// per-file resolution work behind that call is never charged.
    pub fn excluding_sources(
        cancellation: &'a CancellationToken,
        excluded: &'a dyn Fn(&ProjectFile) -> bool,
    ) -> Self {
        Self {
            cancellation,
            excluded_source: Some(excluded),
        }
    }

    pub fn cancellation(&self) -> &'a CancellationToken {
        self.cancellation
    }

    pub fn variant(&self) -> DescendantIndexVariant {
        match self.excluded_source {
            Some(_) => DescendantIndexVariant::ProductionOnly,
            None => DescendantIndexVariant::WholeWorkspace,
        }
    }

    /// The poll predicate for a builder that would rather take a plain
    /// `Fn() -> bool` than a token.
    pub fn keep_going(&self) -> impl Fn() -> bool + use<'_> {
        || !self.cancellation.is_cancelled()
    }

    /// Whether `declaration` belongs in this index.
    pub fn admits(&self, declaration: &CodeUnit) -> bool {
        self.excluded_source
            .is_none_or(|excluded| !excluded(declaration.source()))
    }
}

pub trait TypeHierarchyProvider: CapabilityProvider + Send + Sync {
    fn get_direct_ancestors(&self, code_unit: &CodeUnit) -> Vec<CodeUnit>;
    fn get_direct_descendants(&self, code_unit: &CodeUnit) -> HashSet<CodeUnit>;

    fn supports_type_hierarchy(&self, code_unit: &CodeUnit) -> bool {
        code_unit.is_class()
    }

    fn get_ancestors(&self, code_unit: &CodeUnit) -> Vec<CodeUnit> {
        traverse_hierarchy(code_unit, |next| self.get_direct_ancestors(next))
    }

    fn get_descendants(&self, code_unit: &CodeUnit) -> Vec<CodeUnit> {
        traverse_hierarchy(code_unit, |next| {
            self.get_direct_descendants(next).into_iter().collect()
        })
    }

    /// [`Self::get_direct_ancestors`] under a caller's deadline.
    ///
    /// `None` means the resolution stopped short; it is never an empty answer,
    /// which stays the honest way to say "no resolvable base types". The
    /// default checks the deadline once and delegates, which is right for a
    /// provider whose ancestor resolution is a lookup. C++ overrides it because
    /// resolving one class's bases means walking that file's whole transitive
    /// `#include` closure.
    fn get_direct_ancestors_within(
        &self,
        code_unit: &CodeUnit,
        scope: &DescendantIndexScope<'_>,
    ) -> Option<Vec<CodeUnit>> {
        (!scope.cancellation().is_cancelled()).then(|| self.get_direct_ancestors(code_unit))
    }

    /// [`Self::get_direct_descendants`] under a caller's deadline and workspace
    /// slice.
    ///
    /// `None` means the answer is not available because the build stopped
    /// short. It is never a partial answer, and a stopped build publishes
    /// nothing: a truncated index memoized as complete would be served to every
    /// later caller as the truth (the rule
    /// `cancelled_cold_candidate_discovery_does_not_publish_partial_index`
    /// pins for the Rust walk caches).
    ///
    /// The default checks the deadline once and delegates, which is the correct
    /// behaviour for a provider whose descendant answer is bounded by
    /// construction. Every provider that inverts the ancestor relation over the
    /// whole workspace overrides this; a provider that also honours
    /// [`DescendantIndexScope::admits`] builds one index per
    /// [`DescendantIndexVariant`]. Ignoring `admits` is sound -- the index is
    /// then a superset and the caller's own file filter still applies -- so
    /// only the providers where the prune pays for itself implement it.
    fn get_direct_descendants_within(
        &self,
        code_unit: &CodeUnit,
        scope: &DescendantIndexScope<'_>,
    ) -> Option<HashSet<CodeUnit>> {
        (!scope.cancellation().is_cancelled()).then(|| self.get_direct_descendants(code_unit))
    }

    /// [`Self::get_descendants`] under a caller's deadline and workspace slice,
    /// polling once per node of the walk. `None` carries the same meaning as in
    /// [`Self::get_direct_descendants_within`].
    fn get_descendants_within(
        &self,
        code_unit: &CodeUnit,
        scope: &DescendantIndexScope<'_>,
    ) -> Option<Vec<CodeUnit>> {
        traverse_hierarchy_while(code_unit, scope.cancellation(), |next| {
            self.get_direct_descendants_within(next, scope)
                .map(|descendants| descendants.into_iter().collect())
        })
    }

    fn get_polymorphic_matches<T: CodeUnitIndex>(
        &self,
        target: &CodeUnit,
        analyzer: &T,
    ) -> Vec<CodeUnit>
    where
        Self: Sized,
    {
        if !target.is_function() {
            return Vec::new();
        }

        let Some(parent) = analyzer.parent_of(target) else {
            return Vec::new();
        };

        self.get_descendants(&parent)
    }
}

/// Exact declaration identities plus compact ancestor-to-descendant rows.
pub struct DirectDescendantIndex {
    nodes: Box<[CodeUnit]>,
    row_by_ancestor: HashMap<CodeUnit, u32>,
    descendants: CompactRows<u32>,
}

impl DirectDescendantIndex {
    pub fn from_indexed_nodes(
        nodes: Vec<CodeUnit>,
        index_by_node: HashMap<CodeUnit, u32>,
        mut edges: Vec<(u32, u32)>,
    ) -> Self {
        assert_eq!(nodes.len(), index_by_node.len());
        assert!(nodes.iter().enumerate().all(|(index, node)| {
            index_by_node.get(node).copied()
                == Some(
                    u32::try_from(index).expect("hierarchy index declarations must fit in a u32"),
                )
        }));
        assert!(edges.iter().all(|(ancestor, descendant)| {
            (*ancestor as usize) < nodes.len() && (*descendant as usize) < nodes.len()
        }));
        edges.sort_unstable();
        edges.dedup();

        let row_count = usize::from(!edges.is_empty())
            + edges
                .windows(2)
                .filter(|pair| pair[0].0 != pair[1].0)
                .count();
        let mut row_by_ancestor = HashMap::default();
        let mut descendants = CompactRowsBuilder::with_capacity(row_count, edges.len());
        let mut cursor = 0;
        while cursor < edges.len() {
            let ancestor = edges[cursor].0;
            let start = cursor;
            while cursor < edges.len() && edges[cursor].0 == ancestor {
                cursor += 1;
            }
            let row =
                u32::try_from(descendants.rows()).expect("hierarchy index rows must fit in a u32");
            row_by_ancestor.insert(nodes[ancestor as usize].clone(), row);
            descendants.push_row(
                edges[start..cursor]
                    .iter()
                    .map(|(_, descendant)| *descendant),
            );
        }
        Self {
            nodes: nodes.into_boxed_slice(),
            row_by_ancestor,
            descendants: descendants.finish(),
        }
    }

    pub fn descendants(&self, ancestor: &CodeUnit) -> HashSet<CodeUnit> {
        let Some(row) = self.row_by_ancestor.get(ancestor).copied() else {
            return HashSet::default();
        };
        self.descendants
            .row(row as usize)
            .iter()
            .map(|descendant| self.nodes[*descendant as usize].clone())
            .collect()
    }
}

/// Answer a descendant query from a variant-keyed index cell, building the
/// variant the caller asked for if it is missing.
///
/// Every analyzer that memoizes a whole-workspace descendant index repeats this
/// same dance: pick the cell for the scope's variant, build while the deadline
/// holds, publish nothing if it does not, project the ancestor's row. It lives
/// here so the complete-or-nothing rule is stated once instead of copied into
/// each language module.
pub fn descendants_from_variant_index(
    index: &KeyedPoolSafeMemo<DescendantIndexVariant, DirectDescendantIndex>,
    scope: &DescendantIndexScope<'_>,
    code_unit: &CodeUnit,
    build: impl Fn() -> Option<DirectDescendantIndex>,
) -> Option<HashSet<CodeUnit>> {
    Some(
        index
            .cell(&scope.variant())
            // The builders are serial, so the same closure serves both memo
            // arms; the memo's value here is the non-blocking claim protocol.
            .get_or_build_while(&scope.keep_going(), &build, &build)?
            .descendants(code_unit),
    )
}

/// Invert the ancestor relation over every class the analyzer indexes.
///
/// `scope` decides two things. Its predicate drops declarations before they are
/// ever handed to `get_direct_ancestors`, which is where the per-declaration
/// resolution cost is charged (for C++ that is a whole transitive `#include`
/// closure per declaring file). Its token bounds the loop: `None` means the
/// build stopped short and must not be published.
pub fn build_direct_descendant_index<A, P>(
    analyzer: &A,
    provider: &P,
    scope: &DescendantIndexScope<'_>,
) -> Option<DirectDescendantIndex>
where
    A: CodeUnitIndex,
    P: TypeHierarchyProvider + ?Sized,
{
    build_direct_descendant_index_from_candidates(
        analyzer
            .all_declarations()
            .filter(|candidate| candidate.is_class() && scope.admits(candidate))
            .collect(),
        |candidate| provider.get_direct_ancestors_within(candidate, scope),
        &scope.keep_going(),
    )
}

/// The edge-building half of [`build_direct_descendant_index`], for providers
/// that assemble their candidate list some other way.
///
/// `keep_going` is polled once per candidate. That granularity is the natural
/// checkpoint: one candidate is one `direct_ancestors` call, which is the unit
/// of work whose tens of thousands of repetitions made this loop unbounded in
/// the first place (issue #1748). `direct_ancestors` answers `None` when its
/// own work stopped short, which stops this loop too -- a candidate whose bases
/// were half-resolved must not contribute a half-populated edge set.
pub fn build_direct_descendant_index_from_candidates<F>(
    mut candidates: Vec<CodeUnit>,
    mut direct_ancestors: F,
    keep_going: &dyn Fn() -> bool,
) -> Option<DirectDescendantIndex>
where
    F: FnMut(&CodeUnit) -> Option<Vec<CodeUnit>>,
{
    candidates.sort();
    candidates.dedup();
    let mut types_by_fq_name: HashMap<String, Vec<CodeUnit>> = HashMap::default();
    for candidate in &candidates {
        types_by_fq_name
            .entry(candidate.fq_name())
            .or_default()
            .push(candidate.clone());
    }
    let mut nodes = candidates.clone();
    let mut index_by_node: HashMap<_, _> = nodes
        .iter()
        .enumerate()
        .map(|(index, node)| {
            (
                node.clone(),
                u32::try_from(index).expect("hierarchy index declarations must fit in a u32"),
            )
        })
        .collect();
    let mut edges = Vec::new();
    for candidate in candidates {
        if !keep_going() {
            return None;
        }
        let descendant = index_by_node[&candidate];
        for ancestor in direct_ancestors(&candidate)? {
            let ancestor = types_by_fq_name
                .get(&ancestor.fq_name())
                .and_then(|same_name| {
                    let mut same_source = same_name
                        .iter()
                        .filter(|unit| unit.source() == candidate.source());
                    let exact = same_source.next()?;
                    same_source.next().is_none().then(|| exact.clone())
                })
                .unwrap_or(ancestor);
            let ancestor = *index_by_node.entry(ancestor.clone()).or_insert_with(|| {
                let index = u32::try_from(nodes.len())
                    .expect("hierarchy index declarations must fit in a u32");
                nodes.push(ancestor);
                index
            });
            edges.push((ancestor, descendant));
        }
    }
    Some(DirectDescendantIndex::from_indexed_nodes(
        nodes,
        index_by_node,
        edges,
    ))
}

/// [`traverse_hierarchy`] under a deadline: polls once per node popped and
/// propagates a stopped step as `None` rather than returning the nodes it had
/// already collected. A partial subtype set read as complete would let a caller
/// conclude that a class has no further subclasses.
fn traverse_hierarchy_while<F>(
    root: &CodeUnit,
    cancellation: &CancellationToken,
    mut next: F,
) -> Option<Vec<CodeUnit>>
where
    F: FnMut(&CodeUnit) -> Option<Vec<CodeUnit>>,
{
    let direct = next(root)?;
    if direct.is_empty() {
        return Some(Vec::new());
    }

    let mut seen = BTreeSet::new();
    let mut result = Vec::new();
    let mut queue = VecDeque::new();

    for item in direct {
        if seen.insert(item.fq_name()) {
            queue.push_back(item.clone());
            result.push(item);
        }
    }

    while let Some(current) = queue.pop_front() {
        if cancellation.is_cancelled() {
            return None;
        }
        for item in next(&current)? {
            if seen.insert(item.fq_name()) {
                queue.push_back(item.clone());
                result.push(item);
            }
        }
    }

    Some(result)
}

fn traverse_hierarchy<F>(root: &CodeUnit, mut next: F) -> Vec<CodeUnit>
where
    F: FnMut(&CodeUnit) -> Vec<CodeUnit>,
{
    let direct = next(root);
    if direct.is_empty() {
        return Vec::new();
    }

    let mut seen = BTreeSet::new();
    let mut result = Vec::new();
    let mut queue = VecDeque::new();

    for item in direct {
        if seen.insert(item.fq_name()) {
            queue.push_back(item.clone());
            result.push(item);
        }
    }

    while let Some(current) = queue.pop_front() {
        for item in next(&current) {
            if seen.insert(item.fq_name()) {
                queue.push_back(item.clone());
                result.push(item);
            }
        }
    }

    result
}