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Resolve the parse/emit string-representation split (mach-std#340) with a decode-on-demand accessor: the parser keeps storing raw wire bytes (zero-copy), and value_string_decode unescapes them into logical bytes at the point of consumption -- the inverse of the emit escaper. Handles " \ \/ \b\f\n\r\t and \uXXXX including surrogate-pair astral code points decoded to UTF-8, erroring on malformed escapes. Documents the raw-vs-decoded contract on the storage field and both accessors.
Escape matrix (short escapes, \uXXXX BMP incl. uppercase hex, surrogate-pair astral -> UTF-8, verbatim UTF-8 pass-through), malformed-escape rejection (bad escape, truncated/non-hex \u, unpaired/lone/bad surrogate, trailing backslash), the emit size-then-fill buffer protocol, and the reframed round-trip decode(parse(emit(make_string(L)))) == L.
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Closes #340.
Problem
std.data.jsonstored two incompatible meanings in one field: parse yields theraw on-wire bytes between the quotes (escapes intact, zero-copy), while emit
treats
str_valas logical bytes and re-escapes. Aparse -> emitpipelinedouble-escaped every wire escape.
Decision
Per the issue (2026-07-03, delegated): decode-on-demand accessor. Zero-copy
raw parse stays; a decoding accessor gives correct string semantics at the
consumption point without making every parse pay decode cost.
Change
value_string_decode(v, buf, len) -> Result[usize, str]: resolves the raw wireescapes of a string value into logical bytes in a caller buffer — the inverse
of the emit escaper. Handles
\" \\ \/ \b \f \n \r \tand\uXXXX(a high+lowsurrogate pair combines into one astral code point, encoded to UTF-8 via the
validated
std.text.utf8encoder). Errors on malformed escapes (unknownescape,
\uwithout four hex digits, ill-formed surrogate pair). Uses the samecounting-
BufWritersize-then-fill protocol asemit.str_val,value_string, the newaccessor, and the file header.
Emit semantics are unchanged — this PR adds the decode path only.
Tests
Escape matrix (short escapes,
\uXXXXBMP, surrogate-pair astral, verbatimUTF-8 pass-through), malformed-escape errors, buffer-sizing protocol, and the
reframed round-trip
decode(parse(emit(make_string(L)))) == L.