How to tell in advance which deps will come from binary cache

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1

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?

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2

--dry-run should do the trick for you

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3

The stable CLI also usually prints “These paths will be built” and “These paths will be fetched” before starting to do just that…

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Nice, thanks! It looks like that flag doesn’t exist for nix develop, is there some equivalent? Maybe there’s some nix build devShell --dry-run command?