Author here. In my idea, x..y would be like x+y: not fallible, but may panic. There would be a separate fallible function for creating ranges, analogous to checked_add.
(Of course unlike x+y, x..y would require checks in release builds too.)
Agree with you that there won't be any problems with panicking in this setup.
Although, it'll require some non-minor api adjustments: at the moment, Range's fields are public. To maintain the invariant, we would have to make them private and provide getters. Which we actually already do for RangeInclusive, because of that extra bool field. Which is an inconsistent mess :)
(Of course unlike x+y, x..y would require checks in release builds too.)