Add versioned, ABI-tagged inline namespace and namespace macros (#3590)

* Add versioned inline namespace

Add a versioned inline namespace to prevent ABI issues when linking code
using multiple library versions.

* Add namespace macros

* Encode ABI information in inline namespace

Add _diag suffix to inline namespace if JSON_DIAGNOSTICS is enabled, and
_ldvcmp suffix if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON is enabled.

* Move ABI-affecting macros into abi_macros.hpp

* Move std_fs namespace definition into std_fs.hpp

* Remove std_fs namespace from unit test

* Format more files in tests directory

* Add unit tests

* Update documentation

* Fix GDB pretty printer

* fixup! Add namespace macros

* Derive ABI prefix from NLOHMANN_JSON_VERSION_*

include/nlohmann/detail/input/binary_reader.hpp

@@ -32,8 +32,7 @@
 #include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 
@@ -2969,5 +2968,6 @@ class binary_reader
     /// the SAX parser
     json_sax_t* sax = nullptr;
 };
+
 }  // namespace detail
-}  // namespace nlohmann
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/detail/input/input_adapters.hpp

@@ -26,10 +26,10 @@
 #include <nlohmann/detail/iterators/iterator_traits.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
+
 /// the supported input formats
 enum class input_format_t { json, cbor, msgpack, ubjson, bson, bjdata };
 
@@ -410,7 +410,7 @@ struct container_input_adapter_factory< ContainerType,
 }
        };
 
-} // namespace container_input_adapter_factory_impl
+}  // namespace container_input_adapter_factory_impl
 
 template<typename ContainerType>
 typename container_input_adapter_factory_impl::container_input_adapter_factory<ContainerType>::adapter_type input_adapter(const ContainerType& container)
@@ -489,5 +489,6 @@ class span_input_adapter
   private:
     contiguous_bytes_input_adapter ia;
 };
+
 }  // namespace detail
-}  // namespace nlohmann
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/detail/input/json_sax.hpp

@@ -17,8 +17,7 @@
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/string_concat.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 
 /*!
 @brief SAX interface
@@ -724,6 +723,6 @@ class json_sax_acceptor
         return false;
     }
 };
-}  // namespace detail
 
-}  // namespace nlohmann
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/detail/input/lexer.hpp

@@ -22,10 +22,10 @@
 #include <nlohmann/detail/input/position_t.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
+
 ///////////
 // lexer //
 ///////////
@@ -1627,5 +1627,6 @@ scan_number_done:
     /// the decimal point
     const char_int_type decimal_point_char = '.';
 };
+
 }  // namespace detail
-}  // namespace nlohmann
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/detail/input/parser.hpp

@@ -24,8 +24,7 @@
 #include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 ////////////
@@ -505,4 +504,4 @@ class parser
 };
 
 }  // namespace detail
-}  // namespace nlohmann
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/detail/input/position_t.hpp

@@ -10,10 +10,12 @@
 
 #include <cstddef> // size_t
 
-namespace nlohmann
-{
+#include <nlohmann/detail/abi_macros.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
+
 /// struct to capture the start position of the current token
 struct position_t
 {
@@ -31,5 +33,5 @@ struct position_t
     }
 };
 
-} // namespace detail
-} // namespace nlohmann
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END