I happen to have written up a pre-RFC covering this feature:
I haven't gotten much feedback about it, and I'm less sure if I'll proceed with the RFC process myself.
Possible, yes, but not necessarily feasible. Doing this fully transparently would require the compiler to scan the entire graph of types to determine which enums are nested in which, probably use heuristics to decide how frequently does this happen and whether some cases may be worth sacrificing.
Unless you're thinking of having a single global counter and by default assigning discriminants by reading and then incrementing it. But that feels like a rather wasteful solution.
In any case, compact fields as described by myself get you most of the way there, are explicit and much cheaper to implement.