perf(json): write escapes and indentation directly into the builder

[bobzhang]

Json::stringify allocated a throwaway String for every string value and object key: escape() built a fresh StringBuilder, returned a String, and the caller copied it into the outer buffer — with no fast path for the overwhelmingly common case of strings that need no escaping.

Change

• escape_into(buf, str, escape_slash~) replaces escape(): scans code units for the first escapable character (the escape set is all-ASCII, so code-unit scanning is exact even around surrogate pairs). Clean strings — most keys and values — become a single write_string. Strings containing escapes take the original char-by-char loop, now writing directly into the outer builder. The fallback path is byte-identical to the old loop, including for malformed UTF-16 (unpaired surrogates copy through verbatim; a regression test built with unsafe_substring pins both paths).
• write_indent replaces indent_str, writing the newline and spaces directly instead of allocating "\n" + " ".repeat(n) per separator beyond the old 8-space cache. Non-positive indent now explicitly means compact output (previously a negative indent hit an accidental abort inside String::repeat).

Benchmarks (native, release; committed as json/stringify_bench_test.mbt)

bench	before	after
string-heavy n=1000	3.89 ms	1.16 ms	~3.4×
pretty indent=4 n=200	1.11 ms	0.37 ms	~3.0×
escape-heavy n=500	320 µs	220 µs	~1.5×

Review

Reviewed by Codex CLI (codex-cli 0.144.1) in two rounds. Round 1 rejected the initial version, catching an abort on unpaired surrogates adjacent to escape characters (view-slice boundary validation) and a negative-indent behavior change — both real. The revised version avoids view slicing entirely; round 2: "Approved — no blocking findings. Both concerns are resolved, and well-formed output remains equivalent... Public interface is unchanged."

Signed-off-by: Codex CLI codex@openai.com

Validation

• moon check clean, moon fmt applied, no .mbti changes
• moon test: 6716 passed, 0 failed (json 185/185, incl. new surrogate regression test)

🤖 Generated with Claude Code

[coveralls]

Coverage Report for CI Build 5182

Coverage decreased (-0.009%) to 91.259%

Details

• Coverage decreased (-0.009%) from the base build.
• Patch coverage: 17 of 17 lines across 1 file are fully covered (100%).
• 1 coverage regression across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
json/json.mbt	1	98.99%

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Coverage Stats

Relevant Lines:	17206
Covered Lines:	15702
Line Coverage:	91.26%
Coverage Strength:	198928.07 hits per line

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💛 - Coveralls