Add error code

docs/errors/usage_errors.md

@@ -1117,4 +1117,20 @@ except PydanticUserError as exc_info:
     assert exc_info.code == 'model-config-invalid-field-name'
 ```
 
+## [`with_config`][pydantic.config.with_config] is used on a `BaseModel` subclass {#with-config-on-model}
+
+This error is raised when the [`with_config`][pydantic.config.with_config]  decorator is used on a class which is already a Pydantic model (use the `model_config` attribute instead).
+
+```py
+from pydantic import BaseModel, PydanticUserError, with_config
+
+try:
+
+    @with_config({"allow_inf_nan": True})
+    class Model(BaseModel):
+        bar: str
+
+except PydanticUserError as exc_info:
+    assert exc_info.code == 'with-config-on-model'
+
 {% endraw %}

pydantic/config.py

@@ -1026,7 +1026,7 @@ def inner(class_: _TypeT, /) -> _TypeT:
         if is_model_class(class_):
             raise PydanticUserError(
                 f'Cannot use `with_config` on {class_.__name__} as it is a Pydantic model',
-                code=None,
+                code='with-config-on-model',
             )
         class_.__pydantic_config__ = config
         return class_

pydantic/errors.py

@@ -62,6 +62,7 @@
     'dataclass-init-false-extra-allow',
     'clashing-init-and-init-var',
     'model-config-invalid-field-name',
+    'with-config-on-model',
 ]