⚡ Bolt: [performance improvement] Defer PathBuf allocations during graph traversals

[bashandbone]

💡 What: Defer PathBuf allocations in ensure_node (graph creation) and tarjan_dfs (graph cycle detection) by using borrowed &Path references for RapidMap lookups, allocating with to_path_buf() only when strictly inserting new values.

🎯 Why: Frequent to_path_buf() calls inside tight graph traversal loops and large node additions were causing massive O(E) redundant heap allocations for paths that already existed in the lookup maps, acting as a measurable CPU bottleneck.

📊 Impact: Improves the execution time of dense graph traversals (measured via find_affected_files_10000_nodes benchmark) by up to ~20%.

🔬 Measurement: Run cargo bench -p thread-flow --bench bench_graph_traversal to verify the performance improvements on graph queries.

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PR created automatically by Jules for task 4176612288963007239 started by @bashandbone

Summary by Sourcery

Improve performance of incremental dependency graph traversals by reducing redundant path allocations during node creation and cycle detection.

Enhancements:

• Avoid repeated PathBuf allocations in Tarjan DFS by reusing a single owned path per node and relying on borrowed lookups for lowlink updates.
• Change node creation in the dependency graph to check for existing entries with borrowed paths before allocating new PathBuf keys.
• Document the allocation deferral pattern for graph traversals in the Bolt performance notes.

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[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Optimizes graph traversal performance by removing redundant PathBuf allocations in Tarjan’s DFS and node creation, relying on borrowed &Path references for map lookups and only allocating PathBufs when inserting new entries, plus updates the Bolt notes with the new performance guideline.

File-Level Changes

Change	Details	Files
Reduce redundant PathBuf allocations in Tarjan’s strongly-connected components traversal.	Introduce a single PathBuf (v_buf) per tarjan_dfs invocation instead of multiple to_path_buf() calls Use borrowed &Path keys for lookups and mutations in indices and lowlinks maps where possible Keep stack and on_stack collections keyed by owned PathBuf while minimizing clones	crates/flow/src/incremental/invalidation.rs
Avoid unnecessary PathBuf allocation when ensuring a node exists in the dependency graph.	Replace entry(to_path_buf()).or_insert_with(...) with a contains_key check using &Path Allocate a PathBuf and insert a new AnalysisDefFingerprint only when the node is missing	crates/flow/src/incremental/graph.rs
Document the performance pattern of deferring PathBuf allocations during graph traversals.	Add a Bolt note describing the cost of repeated to_path_buf() in tight loops for map lookups Recommend using borrowed &Path references for RapidMap lookups and deferring allocations until insertion time	.jules/bolt.md

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[sourcery-ai]

Hey - I've left some high level feedback:

• In tarjan_dfs, you can avoid multiple v_buf.clone() calls by reusing ownership (e.g., clone once for one of the maps and move the original into stack/on_stack) to further cut down per-node overhead.
• The new .jules/bolt.md entry is dated 2024-05-24 but appears after a 2026 entry; consider reordering or updating the date so the log remains chronologically consistent.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `tarjan_dfs`, you can avoid multiple `v_buf.clone()` calls by reusing ownership (e.g., clone once for one of the maps and move the original into `stack`/`on_stack`) to further cut down per-node overhead.
- The new `.jules/bolt.md` entry is dated 2024-05-24 but appears after a 2026 entry; consider reordering or updating the date so the log remains chronologically consistent.

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[Copilot]

Pull request overview

This PR improves performance in the incremental dependency graph by reducing redundant PathBuf allocations during node creation and Tarjan DFS cycle detection, leaning on borrowed &Path lookups and deferring owned allocations until insertion is required.

Changes:

• Update Tarjan DFS to reuse borrowed &Path for RapidMap lookups (get_mut/get) instead of allocating via to_path_buf() per edge.
• Update DependencyGraph::ensure_node to check existence with a borrowed key before allocating a PathBuf for insertion.
• Add Bolt performance notes documenting the “defer PathBuf allocation” pattern.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
crates/flow/src/incremental/invalidation.rs	Reduce per-edge PathBuf allocations in Tarjan SCC DFS by using borrowed lookups for lowlink/index updates.
crates/flow/src/incremental/graph.rs	Defer PathBuf allocation in ensure_node by checking existence with a borrowed &Path first.
.jules/bolt.md	Document the allocation deferral approach for graph traversal hot paths.

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