Editions for the time being don't cover all language evolution scenarios, e.g. breaking changes on the standard library, and having incompatible crates on the same build talking to each other.
Is anybody contemplating nontrivial breaking changes to the standard library? When env::set_var and env::remove_var were marked unsafe, that was done across an edition boundary even though it's unsound to leave them as-is in older editions.
Do you have a particular scenario in mind for incompatible crates? This doesn't seem like a language evolution problem.
Do you have a particular scenario in mind for incompatible crates? This doesn't seem like a language evolution problem.