Support passing Config subclasses to SubConfig and ConfigItems

They are validated by default

mkdocs/config/base.py

@@ -279,7 +279,7 @@ class LegacyConfig(Config):
     """
 
     def __init__(self, schema: PlainConfigSchema, config_file_path: Optional[str] = None):
-        self._schema = schema
+        self._schema = tuple((k, v) for k, v in schema)  # Re-create just for validation
         super().__init__(config_file_path)
 
 

mkdocs/config/config_options.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import functools
 import ipaddress
 import os
 import string
@@ -25,26 +26,54 @@
 from mkdocs.exceptions import ConfigurationError
 
 T = TypeVar('T')
+SomeConfig = TypeVar('SomeConfig', bound=Config)
 
 
-class SubConfig(BaseConfigOption[Config]):
+class SubConfig(Generic[SomeConfig], BaseConfigOption[SomeConfig]):
     """
     Subconfig Config Option
 
+    New: If targeting MkDocs 1.4+, please pass a subclass of Config to the
+    constructor, instead of the old style of a sequence of ConfigOption instances.
+    Validation is then enabled by default.
+
     A set of `config_options` grouped under a single config option.
     By default, validation errors and warnings resulting from validating
     `config_options` are ignored (`validate=False`). Users should typically
     enable validation with `validate=True`.
     """
 
-    def __init__(self, *config_options: PlainConfigSchemaItem, validate: bool = False):
+    @overload
+    def __init__(
+        self: SubConfig[SomeConfig], config_class: t.Type[SomeConfig], *, validate: bool = True
+    ):
+        """Create a sub-config in a type-safe way, using fields defined in a Config subclass."""
+
+    @overload
+    def __init__(
+        self: SubConfig[LegacyConfig],
+        *config_options: PlainConfigSchemaItem,
+        validate: bool = False,
+    ):
+        """Create an untyped sub-config, using directly passed fields."""
+
+    def __init__(self, *config_options, validate=None):
         super().__init__()
         self.default = {}
-        self.config_options = config_options
-        self._do_validation = validate
+        if (
+            len(config_options) == 1
+            and isinstance(config_options[0], type)
+            and issubclass(config_options[0], Config)
+        ):
+            if validate is None:
+                validate = True
+            (self._make_config,) = config_options
+        else:
+            self._make_config = functools.partial(LegacyConfig, config_options)
+        self._do_validation = bool(validate)
 
     def run_validation(self, value):
-        config = LegacyConfig(self.config_options)
+        config = self._make_config()
         try:
             config.load_dict(value)
             failed, warnings = config.validate()
@@ -152,25 +181,34 @@ def run_validation(self, value):
         return [fake_config[k] for k in fake_keys]
 
 
-class ConfigItems(ListOfItems[Config]):
+class ConfigItems(Generic[SomeConfig], ListOfItems[SomeConfig]):
     """
     Config Items Option
 
     Validates a list of mappings that all must match the same set of
     options.
+
+    New: If targeting MkDocs 1.4+, please pass a subclass of Config to the
+    constructor, instead of the old style of a sequence of ConfigOption instances.
+    Validation is then enabled by default.
     """
 
     @overload
-    def __init__(self, *config_options: PlainConfigSchemaItem, validate: bool = False):
-        ...
+    def __init__(
+        self: ConfigItems[SomeConfig], config_class: t.Type[SomeConfig], *, validate: bool = True
+    ):
+        """Create a config for a list of sub-configs in a type-safe way, using fields defined in a Config subclass."""
 
     @overload
     def __init__(
-        self, *config_options: PlainConfigSchemaItem, required: bool, validate: bool = False
+        self: ConfigItems[LegacyConfig],
+        *config_options: PlainConfigSchemaItem,
+        validate: bool = False,
+        required: bool = False,
     ):
-        ...
+        """Create a config for a list of untyped sub-configs, using directly passed fields."""
 
-    def __init__(self, *config_options, required=None, validate=False):
+    def __init__(self, *config_options, required=None, validate=None):
         super().__init__(SubConfig(*config_options, validate=validate))
         self._legacy_required = required
         self.required = bool(required)