buildGoPackage: updated docs for goPackages after rework

[kamilchm]

Motivation for this change

Stale docs for goPackages on master after #16017

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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CC: @rushmorem @zimbatm

[mention-bot]

By analyzing the blame information on this pull request, we identified @vcunat and @chris-martin to be potential reviewers

[rushmorem]

Shouldn't we also explain how to build binaries especially now that we can no longer install by the attribute path in all-packages.nix? Say nix-build -A deis.bin for example...

[kamilchm]

As it turned out packages can be installed with nix-env -i to have bin in the profile. It all about differences of multiple outputs in nix-build and nix-env. #16103 (comment)

[rushmorem]

Yes, @vcunat's explanation helped make things clearer. I don't think this is an issue any more. But the documentation around this is seriously lacking IMHO.

[kamilchm]

But is it lacking here or in somewhere between nix-build and multiple outputs? I can mention it here anyway.

[rushmorem]

Re-reading nix-build shows that this is in fact documented there. It says "If a derivation has multiple outputs, nix-build will build the default (first) output." How about we make "bin" the default instead (https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/go-modules/generic/default.nix#L198)?

[rushmorem]

On second thought the docs say that by convention the default should be dev or out. Thanks again @vcunat!

[kamilchm]

So maybe we should leave the decision what needs to be build to developer who want to use nix-build and rely on nix-env default for installation?

[rushmorem]

Yes, that's what I think we should do.

[kamilchm]

Should we remove all .bin attributes from all-packages then?

[rushmorem]

Yes please, I hadn't realised that we added them for other packages besides go2nix.

[vcunat]

Note that changing the order of outputs has impact on the way things work when the package is added into buildInputs. It's described in nixpkgs manual, multiple-output section (not on stable yet).