Build long strings straight into their value: one allocation each (0151)

[rmorozov]

Requested by Ruslan · project thread

Before: since 0140 P2 (#462) a string longer than 15 characters lived in a refcounted StringObject that held a std::string, so every long string a render made cost two allocations (the string's buffer, then the object), and join rebuilt its result once per item. Render allocations on string-heavy cases rose 14-32% (targets in docs/tasks/0151).

After: a long string's characters follow its StringObject in one allocation, and the code that makes strings writes them there once. All 0151 targets are met, and Render instructions drop on every string-heavy case.

join of an empty sequence is now '', as in Python (it was undefined; parity case filters.join_empty).

How. StringObject::Make(size) allocates the object plus its characters (class operator new(size_t, CharCount), matching operator deletes). Producers use InternalValue::MakeText(size, writer) (writer gets a boost::span<CharT>), InternalValue::FromText(views...), or StringBuilder (new src/string_builder.h, fmt::basic_memory_buffer with 500 characters on the stack) when the length is unknown. Converted: join, escape/forceescape, ~ on two strings, +/* on strings, slices and indexing, trim, title, upper/lower/capitalize, truncate, tojson, Value strings taken by value, string literals at load (escaped ones on the stack in template_parser.cpp). Visitors now get a heap string as a std::basic_string_view of the object (short strings are still materialised as std::basic_string); IntValue2Value copies owned text into the Value, so a user callable can keep its arguments (test CallableKeepsBuiltStringArguments). TakeString always copies now.

Two layout-only tweaks hold instruction counts at base: ApplyNewlineSequence out of line (it otherwise pulls MakeRawTextRenderer out of FineParseBlock, Load/large_static +0.8%) and ~WorkspaceMaps forced inline (GCC outlined it in Render, Render/plain_text +5 Ir).

Local counts (count.py, ce547f1 → this branch; allocations per iteration)

case	target	before	after	instructions
Render/strings	975	1,285	449	-36.8%
Render/config_file	1,120	1,281	1,071	-6.2%
Render/html_autoescape	932	1,094	748	-10.5%
Render/chat_mistral	416	491	360	-10.8%
Render/chat_llama	350	421	262	-16.6%
Render/chat_qwen	318	395	249	-16.0%
Render/filters	35	51	29	-26.9%
Load/chat_llama	59	76	59	-3.9%
Load/chat_qwen	45	57	45	-4.5%
Load/mitsuhiko_table_wide	45	51	45	-4.5%

No benchmark above base by more than +0.05% (Load/for_filter_if). Performance track verdict on CI job 114577037925: approved, no red gate cases.

Left at two allocations (none shows in the suite): indent, urlize, center, wordwrap, striptags, replace, format, ~ with a non-string operand. Listed in docs/tasks/0151.

Roles: main session; verifier 1 round (ASan+UBSan, clang C++20, ~10k differential templates vs base and Python: no behaviour change except the empty join; its blocking items were clang-tidy hits, fixed before the first push). Red pushes: 1 (461ba2d: CodeQL suspicious-add-sizeof on the characters' offset, added to a std::byte pointer; 8fe177a counts it on an unsigned char pointer).

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