Read iterator pairs of std::string and std::vector as pointer ranges (#6029)

* Read iterator pairs of std::string and std::vector as pointer ranges

Before C++20, a pair of iterators of a contiguous container cannot be told
from any other random-access iterator, so json::parse(s.begin(), s.end())
on a std::string or std::vector<char> was read one character at a time,
without the bulk scans of strings, numbers and indentation that the
container itself gets (the container overload of input_adapter() already
maps it to a pointer range). The iterators of std::string and of a
std::vector of 1-byte integral elements are now matched exactly and mapped
to the pointer range they cover; an empty range is not dereferenced. The
character type stays the same, so the results and error positions do not
change.

parse(first, last) of a std::string gets 1.7-2.0x as fast in C++17 on an
Apple M1 Max, accept(first, last) 2.7-3.1x, and from_cbor(first, last) of a
std::vector 8-23% faster.

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

* Fix CI: clang-tidy, unused result, Infer false positive in tests

- unit-deserialization.cpp: raw string literals, and assign the result
  of parse() in CHECK_THROWS_AS (-Wunused-result)
- unit-bson.cpp: mark Infer's NULLPTR_DEREFERENCE on from_bson() with an
  iterator pair as a false positive

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

* Fix implicit char conversion in iterator-pair test

The test built std::vector<unsigned char> and <signed char> from a char
range, which -fsanitize=integer reports as an implicit conversion.

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

* Tests: sweep_allocator::construct() accepts pointers to const

Recent libc++ (emsdk latest) probes construct(const T*, const T&) in an
unevaluated context while checking for trivial copies; the static_cast to
void* in the noexcept specification then fails to compile.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasEsrzHhSNouxSoEwqz
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Tests: guard sweep_allocator::construct() against const types instead of const_cast

The const_cast added in the previous commit is rejected by
cppcoreguidelines-pro-type-const-cast (clang-tidy). Make construct() not
viable for const U instead, which is what libc++ probes for.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasEsrzHhSNouxSoEwqz
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix CI: astyle and clang-tidy for sweep_allocator::construct()

astyle 3.4.13 reformats the enable_if<!...> default template argument
(Check amalgamation). Delete the const overload instead, which makes the
libc++ probe for const objects fail in the SFINAE context just the same
and needs no <type_traits>.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasEsrzHhSNouxSoEwqz
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Iterator ranges: astyle-safe trait members; reserve in the test helper

The byte check and the container match are static constexpr members
instead of integral_constant arguments and a default template argument
with comparisons, which astyle spaced out.

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

include/nlohmann/detail/input/input_adapters.hpp

@@ -1348,6 +1348,64 @@ struct iterator_input_adapter_factory<IteratorType, SentinelType, enable_if_t<is
     }
 };
 
+/*!
+@brief whether @a IteratorType is an iterator of std::string or of a
+       std::vector of 1-byte elements
+
+Their elements are contiguous, but before C++20 that cannot be detected from
+the iterator type (std::contiguous_iterator), so an iterator pair of them would
+be read one character at a time through the iterators. They are read as the
+pointer range they are instead, as the containers themselves are (see the
+container overload of input_adapter()). The types are matched exactly, so no
+other iterator can be mistaken for one. (std::basic_string is only matched for
+char: other character types need std::char_traits specializations that the
+standard library need not have.)
+*/
+template<typename CharType>
+struct is_byte_char_type
+{
+    // a member, not an integral_constant argument, which astyle misformats
+    static constexpr bool value = sizeof(CharType) == 1 && std::is_integral<CharType>::value && !std::is_same<CharType, bool>::value;
+};
+
+template<typename IteratorType, typename CharType, bool IsByte = is_byte_char_type<CharType>::value>
+struct is_std_contiguous_byte_iterator_impl : std::false_type {};
+
+// (the containers are only named for byte types)
+template<typename IteratorType, typename CharType>
+struct is_std_contiguous_byte_iterator_impl<IteratorType, CharType, true>
+{
+    static constexpr bool value = !std::is_pointer<IteratorType>::value
+                                  && (std::is_same<IteratorType, typename std::vector<CharType>::iterator>::value
+                                      || std::is_same<IteratorType, typename std::vector<CharType>::const_iterator>::value
+                                      || std::is_same<IteratorType, std::string::iterator>::value
+                                      || std::is_same<IteratorType, std::string::const_iterator>::value);
+};
+
+template<typename IteratorType>
+struct is_std_contiguous_byte_iterator
+    : is_std_contiguous_byte_iterator_impl<IteratorType, typename std::remove_cv<typename std::iterator_traits<IteratorType>::value_type>::type> {};
+
+template<typename IteratorType>
+struct iterator_input_adapter_factory<IteratorType, IteratorType, enable_if_t<is_std_contiguous_byte_iterator<IteratorType>::value>>
+{
+    using iterator_type = IteratorType;
+    using sentinel_type = IteratorType;
+    using char_type = typename std::remove_cv<typename std::iterator_traits<iterator_type>::value_type>::type;
+    using adapter_type = iterator_input_adapter<const char_type*>;
+
+    static adapter_type create(IteratorType first, IteratorType last)
+    {
+        if (first == last)
+        {
+            // (an empty range must not be dereferenced)
+            return adapter_type(nullptr, nullptr);
+        }
+        const char_type* const begin = std::addressof(*first);
+        return adapter_type(begin, begin + (last - first));
+    }
+};
+
 // General purpose iterator-based input (iterator+sentinel pair; SentinelType
 // defaults to IteratorType for the common same-type case, but may differ for
 // C++20 ranges-style iterator+sentinel pairs). Only enable for types that can