I'm developing a tool that generates code from some various data. The tool itself will be licensed with the MIT license, which strikes a good balance for me in terms of allowing the freedom to use and modify it, while still holding the copyright.

OK, but what is the legal status of the code generated by the tool? Who holds the copyright for code generated by a tool? Do I need to give users of the tool a license for the generated code, or do they already have that by virtue of it being generated by them?

What is different about this code generation system (which may be relevant) is that the source information about the code generation is provided by the system itself. The user doesn't feed source data in; the source data is bundled along with it. They simply have the means to transform it in various ways (filtering out parts of the data they don't want, etc). Obviously they could edit the bundled data. Does that affect anything about this?