perf: add nearest_tsconfig cache to avoid redundant directory walks

[Brooooooklyn]

Problem

find_tsconfig_auto walks from a file up to / checking each directory. Without nearest_tsconfig, sibling files repeat the same walk:

src/d0/f0.jsx  →  walk d0/ → src/ → project/ → /  →  None   (12 hops)
src/d0/f1.jsx  →  walk d0/ → src/ → project/ → /  →  None   (12 hops, identical)
src/d0/f2.jsx  →  walk d0/ → src/ → project/ → /  →  None   (12 hops, identical)

Solution

After a walk completes, propagate the result to every path visited. Future walks short-circuit when they hit a cached path.

Call 1: find_tsconfig("src/d0/f0.jsx")

  Visit f0.jsx → d0/ → src/ → project/ → ... → /   (full walk, 12 hops)
  Result: None
  Propagate: set nearest_tsconfig = None on f0.jsx, d0/, src/, project/, ...

Call 2: find_tsconfig("src/d0/f1.jsx")

  Visit f1.jsx → d0/ has nearest_tsconfig = None → STOP   (2 hops)

Call 3: find_tsconfig("src/d1/f0.jsx")

  Visit f0.jsx → d1/ → src/ has nearest_tsconfig = None → STOP   (3 hops)

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Note

Medium Risk
Touches find_tsconfig_auto resolution/caching behavior; while intended as a pure perf optimization, incorrect propagation or caching of None could cause hard-to-debug mis-resolution across paths.

Overview
Speeds up auto tsconfig.json discovery by adding a per-CachedPath nearest_tsconfig cache and propagating the final walk result (including None) to every visited directory, allowing sibling lookups to short-circuit instead of re-walking ancestors.

Also moves the find_tsconfig tracing span inside the resolved_tsconfig get_or_try_init closure so tracing only runs when the value is first computed.

^{Written by Cursor Bugbot for commit 220da99. This will update automatically on new commits. Configure here.}

[Brooooooklyn]

• perf: add nearest_tsconfig cache to avoid redundant directory walks #1071 👈 (View in Graphite)
• main

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.37%. Comparing base (c8ea103) to head (220da99).
⚠️ Report is 27 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1071      +/-   ##
==========================================
+ Coverage   92.35%   92.37%   +0.02%     
==========================================
  Files          21       21              
  Lines        4079     4092      +13     
==========================================
+ Hits         3767     3780      +13     
  Misses        312      312              

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[codspeed-hq]

Merging this PR will improve performance by 4.62%

⚡ 1 improved benchmark
✅ 12 untouched benchmarks
⏩ 5 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	resolver_real[multi-thread]	462.3 µs	441.8 µs	+4.62%

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_{Comparing perf/reduce-tsconfig-lookup (220da99) with main (ccd85ea)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩