This is definitely bad diagnostics, and bad diagnostics should be issues in the rust repo. I’ve looked at existing issues and I can’t seem to find anything matching this? There are overarching issues with related topics, but I think making a separate issue would be more useful since this is a small diagnostics failure that can be fixed separately.
Whether you want to try and improve it is up to you! If you do make an issue, and note that you’d be willing to help, I’m sure people would be willing to mentor/give you some reference of where this would need to be changed. That is, if you aren’t already familiar with the area - if you are, I guess you could just submit an issue and pull request at the same time? Or just a pull request - I’m less familiar with the contributing requirements.
Regardless I think making an issue would be a good idea. If it ends up as “won’t fix” or “will fix some other way” (which I seriously doubt - this is a horrible message), then no harm is done, and anyone else having the same issue will be able to find it easier since it’ll be searchable in the rust repo.