borrowwill still be the default in almost all cases
Huh, maybe I have misunderstood all along. I didn't think borrow was currently the default; my understanding was that retains are omitted but for small types, bits might be copied. I guess that's not distinguishable from borrow for copiable types which I guess is what I was saying.
so you shouldn't have to write it much
So the only place you'd be writing it is, e.g. for initializer parameters that you don't want to consume?
That sounds pretty good!
to avoid aliasing writes to shared mutable state in the former case
Sorry, I don't quite see how that plays out. We're talking about copies implicitly generated by the compiler, but a borrowed variable is immutable, so you can only mutate a different variable; if there's a copy involved it seems like it would have to be made explicitly by the user. And if you mean it's sharing state behind-the-scenes, no amount of copying will avoid the aliasing. Could you explain?
to preserve the lifetime of a value the caller either still needs to keep around for itself, or doesn't have ownership of in the first place, in the latter
That makes total sense; it's a place we'd be telling the user to make an explicit copy in Val (unless they turn on @implicitcopy). Maybe the models are closer than I thought. /cc @Alvae