refactor: rewrite the whole pydantic dataclass logic

pydantic/fields.py

@@ -919,12 +919,13 @@ def is_complex(self) -> bool:
         """
         Whether the field is "complex" eg. env variables should be parsed as JSON.
         """
+        from .dataclasses import PydanticDataclass
         from .main import BaseModel  # noqa: F811
 
         return (
             self.shape != SHAPE_SINGLETON
+            or isinstance(self.type_, PydanticDataclass)
             or lenient_issubclass(self.type_, (BaseModel, list, set, frozenset, dict))
-            or hasattr(self.type_, '__pydantic_model__')  # pydantic dataclass
         )
 
     def _type_display(self) -> PyObjectStr:

pydantic/schema.py

@@ -363,13 +363,13 @@ def get_flat_models_from_field(field: ModelField, known_models: TypeModelSet) ->
     :param known_models: used to solve circular references
     :return: a set with the model used in the declaration for this field, if any, and all its sub-models
     """
-    from .dataclasses import dataclass, is_builtin_dataclass
+    from .dataclasses import PydanticDataclass, dataclass
     from .main import BaseModel  # noqa: F811
 
     flat_models: TypeModelSet = set()
 
     # Handle dataclass-based models
-    if is_builtin_dataclass(field.type_):
+    if isinstance(field.type_, PydanticDataclass):
         field.type_ = dataclass(field.type_)
     field_type = field.type_
     if lenient_issubclass(getattr(field_type, '__pydantic_model__', None), BaseModel):