json_view: from_json() of the macros reads views directly (#5922)

get<T>() and get_to() of a view call T's from_json() with the view itself
when that from_json() accepts views but not an unrelated probe type, and
fall back to materialize() otherwise. The from_json() functions of the
NLOHMANN_DEFINE_TYPE_*, NLOHMANN_DEFINE_DERIVED_TYPE_* and
NLOHMANN_JSON_SERIALIZE_ENUM(_STRICT) macros accept basic_json values and
views; a derived type whose base reads only json reads the base part
through materialize(). The library's own from_json() overloads are not
called with views: from_json() of a view (and of the probe) only looks up
functions by ADL.

New public trait nlohmann::is_json_view<T>, and the member type
basic_json_type of basic_json_view. With duplicate keys, the members of
such a T are those the view's lookups find (the first ones).

bench_decode.cpp measures decoding into structs (4-6x faster than
materialize().get<T>() on twitter, citm_catalog, and synthetic records).

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

docs/mkdocs/docs/api/macros/nlohmann_define_derived_type.md

@@ -147,6 +147,13 @@ void to_json(BasicJsonType& j, const B& b) {
     See Implementation limits for [NLOHMANN_DEFINE_TYPE_INTRUSIVE](nlohmann_define_type_intrusive.md) and
     [NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE](nlohmann_define_type_non_intrusive.md), respectively.
 
+!!! info "Views"
+
+    The generated `from_json()` also reads the views of [`json_document`](../json_document.md)s
+    (`<nlohmann/json_view.hpp>`): [`get<T>()`](../basic_json_view/get.md) on a view looks the members up in the
+    view, without building a `json` value first. The base part is read from the view as well if the base's
+    `from_json()` accepts views, and from [`materialize()`](../basic_json_view/materialize.md) otherwise.
+
 ## Examples
 
 ??? example "Example: (1) NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE"
@@ -192,3 +199,5 @@ void to_json(BasicJsonType& j, const B& b) {
 6. Added in version 3.12.0.
 
 All six macros were changed to allow an empty member list in version 3.13.0.
+
+The generated `from_json()` reads the views of `json_document`s since version 3.13.0.

docs/mkdocs/docs/api/macros/nlohmann_define_type_intrusive.md

@@ -81,6 +81,12 @@ See the examples below for the concrete generated code.
       that serializes a struct's members positionally into a JSON array; for that, write `to_json`/`from_json` by
       hand, building/reading a `json::array()` of the members in order.
 
+!!! info "Views"
+
+    The generated `from_json()` also reads the views of [`json_document`](../json_document.md)s
+    (`<nlohmann/json_view.hpp>`): [`get<T>()`](../basic_json_view/get.md) on a view looks the members up in the
+    view, without building a `json` value first.
+
 ## Examples
 
 ??? example "Example: (1) NLOHMANN_DEFINE_TYPE_INTRUSIVE"
@@ -189,3 +195,5 @@ See the examples below for the concrete generated code.
 
 All three macros were changed to work with any `basic_json` specialization (not only `nlohmann::json`) in version 3.12.0,
 and to allow an empty member list in version 3.13.0.
+
+The generated `from_json()` reads the views of `json_document`s since version 3.13.0.

docs/mkdocs/docs/api/macros/nlohmann_define_type_non_intrusive.md

@@ -80,6 +80,12 @@ See the examples below for the concrete generated code.
       that serializes a struct's members positionally into a JSON array; for that, write `to_json`/`from_json` by
       hand, building/reading a `json::array()` of the members in order.
 
+!!! info "Views"
+
+    The generated `from_json()` also reads the views of [`json_document`](../json_document.md)s
+    (`<nlohmann/json_view.hpp>`): [`get<T>()`](../basic_json_view/get.md) on a view looks the members up in the
+    view, without building a `json` value first.
+
 ## Examples
 
 ??? example "Example: (1) NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE"
@@ -188,3 +194,5 @@ See the examples below for the concrete generated code.
 
 All three macros were changed to work with any `basic_json` specialization (not only `nlohmann::json`) in version 3.12.0,
 and to allow an empty member list in version 3.13.0.
+
+The generated `from_json()` reads the views of `json_document`s since version 3.13.0.

docs/mkdocs/docs/api/macros/nlohmann_json_serialize_enum.md

@@ -45,6 +45,12 @@ inline void from_json(const BasicJsonType& j, type& e);
       Define [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](json_use_objects_for_enum_keyed_maps.md) to store them as objects
       with the converted keys. Such maps can be read from both forms.
 
+!!! info "Views"
+
+    The generated `from_json()` also reads the views of [`json_document`](../json_document.md)s
+    (`<nlohmann/json_view.hpp>`), comparing the view with the entries of the mapping, without building a `json` value
+    first.
+
 ## Examples
 
 ??? example "Example: Basic usage"
@@ -88,3 +94,5 @@ inline void from_json(const BasicJsonType& j, type& e);
 ## Version history
 
 Added in version 3.4.0.
+
+The generated `from_json()` reads the views of `json_document`s since version 3.13.0.

docs/mkdocs/docs/api/macros/nlohmann_json_serialize_enum_strict.md

@@ -48,6 +48,12 @@ inline void from_json(const BasicJsonType& j, type& e);
       Define [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](json_use_objects_for_enum_keyed_maps.md) to store them as objects
       with the converted keys. Such maps can be read from both forms.
 
+!!! info "Views"
+
+    The generated `from_json()` also reads the views of [`json_document`](../json_document.md)s
+    (`<nlohmann/json_view.hpp>`), comparing the view with the entries of the mapping, without building a `json` value
+    first.
+
 ## Examples
 
 ??? example "Example: Basic usage"