circular reasoning is a potential source of unsoundness

Oh, I see your point! That's an interesting thought.

You're right; I'm not sure how that ought to be handled. I don't think the list of considerations for unsafe code to be aware of have changed very much over time.

Perhaps it would still be possible to make it an edition change by tagging each trait implementation with which edition it comes from, and disallowing coinductive cycles that dip into previous editions, but that's pure speculation on my part.