I don’t think we should expect Xcode to do a good job with formatting of syntax that does not yet exist in the language. Further, Xcode does not do that great a job of formatting in general. Proposals should not be expected to look nice given any specific formatter, only to be amenable to clear formatting.
If formatters struggle with the language they should be improved. That is (currently) outside the scope of SE. Anyone interested in improving formatting that comes out of the box should review the epic discussion that was had on the topic as well as the core team’s feedback and start new discussions on the topic in the development section of the forum (as the core team requested).