Add json_document and json_view: node index, parser, and document

Add json_document and json_view, a read-only, zero-copy index of a
JSON text, as the first public slice of the zero-copy view (#5295).

A parse produces a flat array of 16-byte nodes in document order,
one per value and one per object key. Strings stay in the source
text; escaped strings are decoded into an arena. Integers are
converted while their digits are in the cache; floats keep only
their digit layout and are converted on read. Containers store the
size of their subtree, so a reader can step over one in constant
time. A document makes a handful of allocations, however many
values it has.

The parser accepts exactly what json::parse accepts, with every
combination of ignore_comments and ignore_trailing_commas, with and
without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is
portable C++11 and does not depend on byte order.

basic_json_document adds parse, parse_copy, accept, read (reuses a
document's memory), root, is_discarded, source, owns_source,
node_count, memory_usage, and shrink_to_fit. basic_json_view adds
type, the is_* queries, operator bool, size, empty, materialize,
and source_offset. A parse error throws the same exception
basic_json::parse would throw for the same input, message and
position included.

detail::abi_config keeps JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp
undefines them, in the ABI namespace so they always match the
basic_json in use.

A NUL byte that ends a // comment is the end of the input, as in
parse() since #5696.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view_macros.cpp

@@ -0,0 +1,47 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+// All other tests keep the library's macros (JSON_TEST_KEEP_MACROS). This one
+// includes json_view.hpp as users do, so that json.hpp undefines its macros
+// (JSON_HAS_CPP_17, JSON_STRICT_NUL_HANDLING, ...) before the view is compiled.
+#undef JSON_TEST_KEEP_MACROS
+#include <nlohmann/json_view.hpp>
+
+#include <string>
+#include <type_traits>
+
+#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+    #include <string_view>
+    #define JSON_VIEW_TEST_HAS_STRING_VIEW 1
+#else
+    #define JSON_VIEW_TEST_HAS_STRING_VIEW 0
+#endif
+
+// the view's own macros do not leak
+#if defined(NLOHMANN_VIEW_LIKELY) || defined(NLOHMANN_VIEW_UNLIKELY) || defined(NLOHMANN_VIEW_ALWAYS_INLINE) || defined(NLOHMANN_VIEW_NOINLINE) \
+    || defined(NLOHMANN_VIEW_NODISCARD) || defined(NLOHMANN_VIEW_THROW) || defined(NLOHMANN_VIEW_HAS_CPP_17) || defined(NLOHMANN_VIEW_LITTLE_ENDIAN) \
+    || defined(NLOHMANN_VIEW_REPEAT16)
+    #error "json_view.hpp leaks a macro"
+#endif
+
+TEST_CASE("json_view without the library's macros")
+{
+    // (this file also gets C++17 builds: it mentions JSON_HAS_CPP_17)
+#if JSON_VIEW_TEST_HAS_STRING_VIEW
+    CHECK(std::is_same<nlohmann::json_view::string_view_t, std::string_view>::value);
+#endif
+    const std::string text = "[1, 2.5, \"x\"]";
+    const nlohmann::json_document d = nlohmann::json_document::parse(text);
+    CHECK(d.root().size() == 3);
+    CHECK(d.root().materialize() == nlohmann::json::parse(text));
+    // the NUL handling of the library's configuration
+    const std::string with_nul("[1]\0x", 5);
+    CHECK(nlohmann::json_document::accept(with_nul) == nlohmann::json::accept(with_nul));
+}