experiment with StringBuilder to guard strings under construction

[davidhewitt]

WIP, pushing to see pipeline feedback.

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Summary by cubic

Introduce StringBuilder to enforce resource-tracked string growth and prevent OOMs from unbounded intermediates. Adopt it in string padding and tab expansion so oversized builds fail fast with MemoryError.

• New Features

  • Added string_builder::StringBuilder that reserves bytes with the ResourceTracker as it grows, and releases on drop/finish; 2× growth reduces tracker calls.
  • Implements fmt::Write; tracker errors surface via finish. Added TODOs in py_repr/py_str to route repr/str building through StringBuilder.

• Bug Fixes

  • str.expandtabs, center, ljust, rjust, and zfill now build via StringBuilder, with exact-capacity reservations for bounded paths (s.len() + pad) and safe 2× growth for unbounded expandtabs.
  • Added str_expandtabs_memory_limit test; expanded CLAUDE.md with builder guidance.

^{Written for commit 6aeb9cd. Summary will update on new commits.}

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 15 skipped benchmarks¹

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_{Comparing dh/expandtabs (6aeb9cd) with main (fbdb2e1)}

Footnotes

1. 15 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. ↩

[github-actions]

Codecov Results 📊

❌ Patch coverage is 0.93%. Project has 24223 uncovered lines.
❌ Project coverage is 66.1%. Comparing base (base) to head (head).

Files with missing lines (3)

File	Patch %	Lines
crates/monty/src/string_builder.rs	0.00%	⚠️ 73 Missing
crates/monty/src/types/str.rs	0.00%	⚠️ 33 Missing
crates/monty/src/types/py_trait.rs	50.00%	⚠️ 1 Missing

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
- Coverage    66.19%    66.10%    -0.09%
==========================================
  Files          279       281        +2
  Lines        71052     71447      +395
  Branches    152853    153474      +621
==========================================
+ Hits         47030     47224      +194
- Misses       24022     24223      +201
- Partials      3993      4044       +51

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Generated by Codecov Action

[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 30 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/monty/src/string_builder.rs	69.86%	19 Missing and 3 partials ⚠️
crates/monty/src/types/str.rs	78.78%	3 Missing and 4 partials ⚠️
crates/monty/src/types/py_trait.rs	50.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[cubic-dev-ai]

2 issues found across 5 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>