Installables with empty outputsToInstall are installed, and do not use default output

[roberth]

I've tracked the cause down to 1ddabe1, which just adds the out behavior without any explanation.

It see multiple issues with it:

• out is not necessarily the default output
• we're ignoring the author's intent not to install anything
• this is derivation-based, not output based - unnecessary coupling we want to move away from (Redefine outputs in terms of language-level "package", not necessarily store-level derivation (RFC-92, and multi-drv packages, docs) #6507)
• not well tested or documented

Originally posted by @roberth in #10825 (comment)

[fricklerhandwerk]

Triaged in Nix team meeting:

• @tomberek: not sure if it makes sense to give the empty list an additional meaning, that would be adding a new convention
• @fricklerhandwerk: find it questionable if we should even spend time on this, since that would be a Nixpkgs convention we'd be enforcing "from below" (we already have enough coupling)
• @roberth: just proposing to make it more regular; if it's specifically set to the empty list, we should not install anything
  • @edolstra: agreed, possibly could throw a warning
    • @fricklerhandwerk: throwing a warning for users of nix profile may not help if they are consumers of Nixpkgs, which they either won't (be able to) change on the fly anyway, or we'd have to tell them how to do it (I don't want Nix to be aware of Nixpkgs)
• @edolstra: if we want to change the behavior, we should be quick about it, since now people will start depending on that code path not crashing
• @roberth: by the way, nix build also observes this attribute
• decision: change it as proposed

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-06-12-nix-team-meeting-minutes-152/46958/1