I like it, but hard 👎 on actually preventing evaluation.

We could try to get some logging primitive into nix (see NixOS/nix#1197)

You could block evaluation by default but have it allowed with setting some option iKnowIAmRunningOldVersion = "16.09"; or something.

I don't like Nix evaluation to print huge messages (or in fact any messages) because these end up spamming my console every time I run a Nix command. (In fact, commands like nixos-rebuild may well show the same message multiple times.)

As to the implementation, why not just put the check in /default.nix? There's already a version check there. We don't need a hidden file plus a library function that isn't actually generic enough to put in the library.

It may also be more appropriate to put the check in the NixOS evaluation (where we already have an assertion / warnings mechanism). After all, we have stable NixOS channels, not Nixpkgs channels.

I also disagree with aborting evaluation of and old release, but warning people about it is great!

@edolstra I can be on board with not Nix evals printing messages. Wouldn't moving it to default.nix do the same thing? Do you have a different suggestion on how to get the word out? Example: we could make nixos-rebuild itself do this.

I've looked into the assertion / warning mechanism in the module system. It suffers from the same problem as using builtins.trace itself; the warning is completely hidden by the build output that comes after this.

(which is followed by many pages of build output)

I do like the suggestion to put it in the nixos module system.
What about putting it in nixos/modules/misc/version.nix?

The previous few commits implement a new approach.

The warnings are now collected into both a warnings.json file and a warnings.txt.raw file. The latter one is printed in the activation script.

I've added the warnings.json as well, so that other tools might be able to inspect them.

We probably should make nixos-rebuild print these warnings, and then pass a flag to the activation script to not print those warnings again.
This would make it possible to see the warning every time you'd switch to a problematic profile, but still prevent printing the same warning twice.

I included a screenshot of how it would look like below.

As you see, the warning is not outputted during the nix evaluation phase, but outputted in the activation script.

Reproduce it by:

1. echo 'deprecated' > .version-unsupported-state
2. $(NIX_PATH=nixpkgs=$(pwd):nixos-config=$(pwd)/configuration.nix nix-build nixos -A system)/bin/switch-to-configuration dry-activate
3. Creating a configuration.nix with the following content.

{ config, pkgs, ... }: {
  fileSystems."/" = {
    device = "/dev/sda1";
    fsType = "ext4";
  };

  boot.loader.grub.enable = true;
  boot.loader.grub.device = "/dev/sda";
}

One suggestion I have, is that any message which tells the user that something is wrong should also provide enough information such that the user can fix the issue such as a link to some documentation.

@nbp a very valid point. I've incorporated that.

I'll split the PR in two separate ones; one containing the NixOS state part, and the other will contain the change in reporting warnings.

Ah, I guess then I can scrap the code I started writing a few days ago. :)