>>> Also, I might want to use some binary cache, but when I am offline Nix
>>> builds wait for a long time for a reply from a binary cache. It's not so
>>> bad if I am intentionally building something and can pass an empty value
>>> for binary-caches via the command line, but doing it for each new 
>>> command I execute sounds excessive.
>>
>>Yes, this is true, although I consider it a Nix bug.  I shouldn't have
>>to remember to say --option substitute false for trivial offline
>>rebuilds.
>
> But there are cases where it would be hard for Nix to get right (the
> ??network won't become reachable?? part ??? the notion of triviality of the
> build is obviously hopeless even with manual declaration)

IMO it should start builds in parallel with trying to pull them from the
network.  It can abort the other after one finishes.  But that's OT.