I don't really care what format the cargo files use (that's just an inconsequential bikeshed as far as I'm concerned) but in your example "comment" is really not a comment. I would expect to be able to put free-form comments anywhere I want and have them thrown away by the parser.

Your solution works fine for docstrings, but comments and docstrings are not the same thing (although many languages that don't support docstrings in the syntax hack them together using comments, admittedly).

But beyond that, what's the argument for switching to json? Is there some kind of intercompatibility with npm/Node.js to be gained?