⚡ Bolt: [performance improvement] Defer PathBuf allocations in Tarjan's SCC DFS

[bashandbone]

💡 What: Replaced v.to_path_buf() with v (borrowed &Path) in state.lowlinks.get_mut and state.indices.get calls during the recursive tarjan_dfs graph traversal algorithm.

🎯 Why: In recursive DFS traversal functions over a dependency graph, repeatedly calling .to_path_buf() right before querying a map creates a substantial number of temporary heap allocations (one for every visited edge in SCC check). Using the borrowed reference eliminates O(E) unnecessary heap memory allocations inside RapidMap::get_mut() lookups.

📊 Impact: Reduces memory overhead and time spent allocating strings/paths on the heap significantly when computing invalidations for very large and deeply nested dependency graphs.

🔬 Measurement: Run cargo test -p thread-flow --test invalidation_tests to verify graph traversals still succeed, ensuring there are no regressions. Measure runtime impact with cargo test --test test_performance_large_graph or equivalent backend benchmarks.

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PR created automatically by Jules for task 7578039882070192528 started by @bashandbone

Summary by Sourcery

Enhancements:

• Use borrowed Path references instead of allocating new PathBufs for lowlink and index lookups in the Tarjan SCC invalidation logic.

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[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Replaces repeated PathBuf allocations in the Tarjan SCC DFS with borrowed &Path lookups to reduce heap allocations and improve performance during graph invalidation computations.

File-Level Changes

Change	Details	Files
Use borrowed &Path keys instead of allocating temporary PathBufs during Tarjan SCC state lookups.	Change lowlink updates to call state.lowlinks.get_mut(v) instead of allocating v.to_path_buf() for each lookup. Change on-stack successor handling to use state.lowlinks.get_mut(v) directly with the borrowed path. Change root-node detection to use state.indices.get(v) and state.lowlinks.get(v) without intermediate PathBuf allocations.	crates/flow/src/incremental/invalidation.rs

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[Copilot]

Pull request overview

This PR optimizes the Tarjan SCC DFS used by InvalidationDetector by removing repeated PathBuf allocations during map lookups, relying instead on borrowed &Path queries against RapidMap<PathBuf, _>.

Changes:

• Replace &v.to_path_buf() with v for indices/lowlinks get/get_mut lookups inside tarjan_dfs.
• Reduce per-edge temporary heap allocations during SCC traversal to improve performance on large graphs.

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