nixos-rebuild error after nix flake update

[marvin-tr-81]

Hello,

I am on nixos-unstable branch.

I am using flakes. I have the following configuration for nixvim:

    nixvim = {
      url = "github:nix-community/nixvim";
     inputs.nixpkgs.follows = "nixpkgs";
    };

I normally import nixvim module make my configurations. It was working perfectly until I updated my flake.

When I nix flake update and nixos-rebuild test, I get the following error:

error:
       … while calling the 'head' builtin
         at /nix/store/gbvafd2vji5gy46210ri488asgn296hp-source/lib/attrsets.nix:1574:11:
         1573|         || pred here (elemAt values 1) (head values) then
         1574|           head values
             |           ^
         1575|         else

       … while evaluating the attribute 'value'
         at /nix/store/gbvafd2vji5gy46210ri488asgn296hp-source/lib/modules.nix:846:9:
          845|     in warnDeprecation opt //
          846|       { value = addErrorContext "while evaluating the option `${showOption loc}':" value;
             |         ^
          847|         inherit (res.defsFinal') highestPrio;

       … while evaluating the option `system.build.toplevel':

       … while evaluating definitions from `/nix/store/gbvafd2vji5gy46210ri488asgn296hp-source/nixos/modules/system/activation/top-level.nix':

       … while evaluating the option `assertions':

       … while evaluating definitions from `/nix/store/jq5wa4csd3rb6xzixhzqav327f9bsc1v-source/wrappers/_shared.nix':

       … while evaluating the option `programs.nixvim':

       … while evaluating definitions from `/nix/store/jq5wa4csd3rb6xzixhzqav327f9bsc1v-source/modules/top-level/files':

       … while evaluating the module argument `pkgs' in "/nix/store/jq5wa4csd3rb6xzixhzqav327f9bsc1v-source/modules/top-level/files":

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: assertion '(((builtins).isAttrs localSystem) -> (! ((localSystem) ? parsed)))' failed
       at /nix/store/gbvafd2vji5gy46210ri488asgn296hp-source/pkgs/top-level/default.nix:51:1:
           50| # of package sets.
           51| assert builtins.isAttrs localSystem -> !(localSystem ? parsed);
             | ^
           52| assert builtins.isAttrs crossSystem -> !(crossSystem ? parsed);

Removing inputs.nixpkgs.follows = "nixpkgs"; does not help.

What might be the problem?

Thanks in advance.

[khaneliman]

Looks like the error we are blocking our flake lock update for #2964. After removing the follows did you do a nix flake update again?

[marvin-tr-81]

Ah ok, this was it. I totally forgot to nix flake update again. Noob mistake.

Thank you so much.

BTW, should I add follow later, or is it better not to follow at all?

edit: typo

[MattSturgeon]

Ah ok, this was it. I totally forgot to nix flake update again. Noob mistake.

You can also do nix flake lock to apply changes to your lockfile without necessarily chacking for more updates.

BTW, should I add follow later, or is it better not to follow at all?

This is a personal choice. One that I'd like to be documented better in our FAQ.

Essentially using follows allows you to re-use the nixpkgs that you already downloaded for other flakes that also depend on it. This is often what you want, but it has one major downside you should be aware of:

Nixvim is tested against the nixpkgs specified in our lockfile. If you choose to "follow" a different nixpkgs revision, you don't benefit from our test suite because there may be subtle differences between the nixpkgs we test against and the nixpkgs you use.

If you're aware of this, then a follows isn't a big deal. Because you know you can temporarily remove it if you run into issues.

But if you want things to "just work," I'd recommend not having a nixvim follows, and instead letting nixvim use the nixpkgs revision specified in our lockfile.

[marvin-tr-81]

Not using "follows" for now sounds to be the safer option. It's good to know where to check when I encounter a problem.

Thanks for the advises. Have a nice day.