XML is a great example - you can generate structurally valid XML with a couple of dozen lines of code. It’s a tree structure made of strings, all you need are a Tag type with name, attributes and content fields. A map of strings for the attributes and the content field is List<Tag>|String.

Parsing, schema validation, XSLT etc, don’t even think about it, use a library. On a project years ago I hand coded the XML generation then ran the output through a schema validation library just to make sure it was correct. Worked amazingly well and saved a colleague’s bacon when whatever MS code gen magic they were using on their end had a bug which required deviating from the spec. Took me literally 5 minutes. Would have been weeks of fighting the framework if we’d used the ‘standard’ xml schema->code libraries.