`nix build`ing the default output of a derivation

[ncfavier]

With the legacy CLI, running nix-build '<nixpkgs>' -A feh would build and print feh's default output.

With the new CLI (starting from 1ddabe1), nix build nixpkgs#feh --print-out-paths prints feh.man, then feh.out, so if you only want the default output you have to specify it explicitly: nixpkgs#feh^out. This is not ideal as you need to know the name of the default output.

My ideas for solving this, in decreasing order of preference:

1. only build the default output by default instead of outputsToInstall (i.e. revert 1ddabe1), at least for nix build
2. extend the ^ syntax to be able to specify only the default output (ideas: feh^, feh^default, feh^0)
3. make sure the outputs are kept in their original order so that you can just take the first line (and maybe use nlohmann::ordered_json when printing JSON)
4. in the spirit of nix-env: always print output names in JSON and XML #6242, add an outputName field to the output of nix build --json containing the default output

Crazier ideas that would require big changes to the ecosystem:

5. enforce the default output to always be out
6. rethink outputs as "parts of a whole", i.e. when building feh^out,man, merge out and man into a single output path

[tomberek]

nix build nixpkgs#"$attrPath"^"$(nix eval nixpkgs#"$attrPath".outputName --raw)" --print-out-paths

can work for the moment. Considering print-out-paths is an intermediate compromise in between the proper JSON output and nothing, I prefer option 4, if anything.