Nix installable resolution is actually impure for supposedly pure attributes like nixosConfigurations

[roberth]

Describe the bug

Let me be clear. We usually don't want to ignore the context of a Nix CLI invocation. nix run should not require users to specify their platform.

However, in some cases we do want to ignore the CLI context. For instance, nixosConfigurations is not behind a system attribute set, because a machine configuration should be built in one way only. Reproducibly.

It would seem that the documented schema enforces this. No system attribute means one way to do it, but this is actually not the case, and users are finding out. I'll try not to proliferate the bad idea, but it involves installable resolution. So I'll jump to the solution instead.

Steps To Reproduce

Intentionally omitted.

Expected behavior

Nix implements a flag so that currentSystem is not used during installable resolution. I believe this comes down to treating the fragment as a literal attribute path without any lookup trickery.
nixos-rebuild passes this flag.

nix-env --version output

Additional context

Even nixpkgs.buildPlatform would be a massive impurity not to be overlooked by a flake's git history.

Priorities

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[nixos-discourse]

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

https://discourse.nixos.org/t/2023-07-14-nix-team-meeting-minutes-71/30472/1

[gytis-ivaskevicius]

If the proposal is to change nixosConfigurations schema (like doing nixosConfigurations.<system>.<hostname>) then I'd appreciate it if this was to be implemented as well #6257

[blaggacao]

More Context on home-manager

Home Manager users are fundamentally system-portable. Advancement here could trigger evolution there.

[roberth]

This doesn't change the schema. The schema can be discussed at #6257.