perf(string): single-alloc join/repeat/replace_once with safety fixes

[xcb3d]

Summary

Single-allocation fast paths for the boa_string hot paths (join / repeat / replace_once / index_of), plus the safety fixes found during maintainer review.

It changes the following:

• Add JsString::repeat/repeat_str with exponential doubling and checked overflow handling; repeat(1) shares via clone
• Add JsString::join with single allocation, empty-neutral Latin1 detection, zero-len early return and hoisted separator variant
• Add JsString::replace_once/replace_once_at reusing index_of, checked total_len and empty-neutral encoding
• Dedupe JsStr::index_of into shared same/mixed helpers with first-byte skip and debug asserts
• Fix Array.prototype.join self-check to object identity only (join.call('a') no longer returns empty)
• Fix unsafe pointer writes via &raw mut, document nan-boxed as_str lifetime, harden build_as_latin1 truncation docs
• Intern trim results back to static strings; hoist pad filler variant; reuse replace position to avoid double search
• Add coverage for join/index_of/replace_once/repeat/trim

Performance

Criterion boa_benches --bench scripts -- strings (release), same machine, baseline (upstream HEAD 665f0392) vs patched — 2 runs each, median:

Benchmark	Scenario	Before	After	Delta
strings/split	Split 26k-char string, 1-char separator	3.65 ms	0.039 ms (39 µs)	🟢 ~93x faster
strings/replace	10,000 substring replacements	6.53 ms	5.69 ms	🟢 ~13% faster
strings/concat	10,000 += concatenations	2.87 ms	2.66 ms	🟢 ~7% faster
strings/slice	10,000 slices over 260 MB string	2.70 ms	2.55 ms	🟢 ~6% faster

Why: split("a") calls the 1-char index_of fast path (position() directly instead of windows().position()) thousands of times; replace benefits from the single-alloc replace_once + no double search (replace_once_at); concat/slice gain indirectly via zero-copy as_str and doubling repeat in setup.

Targeted JS micro-bench (debug binary, Date.now()): repeat-100k 3 ms → 1 ms (~3x), replace-match-10k 97 ms → 73 ms (~25%), concat-5k 44 ms → 38 ms (~14%).

Verification

• cargo test -p boa_string --lib: 30/30 ✅ (4 new test fns)
• cargo test -p boa_engine --lib: 1120/0 ✅
• cargo clippy -p boa_string --all-targets, cargo clippy -p boa_engine --lib: clean ✅
• cargo fmt --check, git diff --check: clean ✅
• cargo +nightly miri test -p boa_string --lib: 30/30 ✅
• cargo make run-ci (fmt + lint all/no-features): pass ✅

Machine config

• CPU: 13th Gen Intel Core i7-13700H (14 cores / 20 threads, up to 5.0 GHz)
• RAM: 31 GiB (+ 8 GiB swap)
• OS: Ubuntu 24.04.4 LTS, kernel 7.0.0-30-generic x86_64
• Toolchain: rustc 1.98.0 / cargo 1.98.0, Miri on nightly 1.100.0
• Note: benches run back-to-back on the same machine; single-run noise is ±5–10% (criterion outliers observed), medians of 2 runs reported above.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,578	53,578	0
Passed	51,426	51,426	0
Ignored	1,648	1,648	0
Failed	504	504	0
Panics	0	0	0
Conformance	95.98%	95.98%	0.00%

Tested main commit: 665f03924a54e5162be227e7e909612e36f6e35a
Tested PR commit: a63a92f4cd5e691928c54355e98c55463f89c2cf
Compare commits: 665f039...a63a92f

[codecov]

Codecov Report

❌ Patch coverage is 79.04509% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.81%. Comparing base (6ddc2b4) to head (a63a92f).
⚠️ Report is 1050 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/string/mod.rs	51.51%	48 Missing ⚠️
core/string/src/lib.rs	91.61%	14 Missing ⚠️
core/string/src/str.rs	85.36%	12 Missing ⚠️
core/engine/src/value/inner/legacy.rs	0.00%	4 Missing ⚠️
core/string/src/builder.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5506       +/-   ##
===========================================
+ Coverage   47.24%   62.81%   +15.57%     
===========================================
  Files         476      536       +60     
  Lines       46892    60616    +13724     
===========================================
+ Hits        22154    38077    +15923     
+ Misses      24738    22539     -2199     

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[jedel1043]

This is fixing too many things at once; 800+ lines of new code (some of them being unsafe). Please split your optimizations into multiple PRs.