Generally if I do a
nix build and it ends up, for example, building ghc rather than fetching it from a binary cache, It’s because I’ve done something wrong in my
flake.nix, and I need to edit it. I never want to be doing that, it takes too long.
So what I want to know is, how do I know in advance whether I’ve screwed it up? I don’t want to be 10 minutes into the build, and then I glance over and find the sneaky bastard trying to get away with building ghc behind my back again.
Is there some general way of asking how much of a build will come from binary caches, and how much I will be building myself, in advance of actually performing it?