def function_plain_schema(self, schema: core_schema.PlainValidatorFunctionSchema) -> JsonSchemaValue:
        """Generates a JSON schema that matches a function-plain schema.

        Args:
            schema: The core schema.

        Returns:
            The generated JSON schema.
        """
        if self.mode == 'validation' and (input_schema := schema.get('json_schema_input_schema')):
            return self.generate_inner(input_schema)

        return self.handle_invalid_for_json_schema(
            schema, f'core_schema.PlainValidatorFunctionSchema ({schema["function"]})'
        )
Generates a JSON schema that matches a function-plain schema.

        Args:
            schema: The core schema.

        Returns:
            The generated JSON schema.
        
pydantic/json_schema.py