Originally commented on #1800 (comment)

Looks like the way home-manager handles the "default" pkgs is to set the pkgsPath special arg, and use that to construct pkgs: defaultPkgs = import pkgsPath config.nixpkgs.

Meaning they use a fresh instance, with its own config/overlays, but matching the same locked channel.

Unless these nixos module options are used:

home-manager.useGlobalPkgs

Whether to enable using the system configuration’s pkgs argument in Home Manager. This disables the Home Manager options nixpkgs.*.

home-manager.useUserPackages

Whether to enable installation of user packages through the users.users.<name>.packages option.

I believe when home-manager is used standalone, it sets pkgsPath to <nixpkgs> when not using flakes or home-manager's inputs.nixpkgs otherwise.

It might do the same thing when it's a NixOS module, or it might use pkgs.path (I'm not sure).

I think this is the reason why it won't build successfully for me in my configuration tied to nix-darwin, nixos, home-manager.
Everytime I try to build it complains. Even when I remove everything from my nixvim config, it complains about something I haven't touched and have no control over. currently, it complains over nvim-cmp-ai

Everytime I try to build it complains. Even when I remove everything from my nixvim config, it complains about something I haven't touched and have no control over. currently, it complains over nvim-cmp-ai

It's hard to know exactly what you're saying without seeing the error messages you're referencing, but if you're saying that you've been having mismatched nixpkgs issues on your nix-darwin + nixvim setup I have some good news:

The initial stages of this feature were recently merged, you can now define programs.nixvim.nixpkgs.pkgs and assign a compatible pkgs instance from the nixos-unstable channel.

In the future we'd like to have more options for constructing a pkgs instance internally, and perhaps even defaulting to the channel lock from our flake inputs instead of our current behaviour of defaulting to the pkgs argument from the host module configuration.

The initial stages of this feature were recently merged, you can now define programs.nixvim.nixpkgs.pkgs and assign a compatible pkgs instance from the nixos-unstable channel.

Wait so does that mean I can specify:

inputs.nixvim = {
      url = "github:nix-community/nixvim";
      inputs.nixpkgs.follows = "nixpkgs"; # Use the stable channel for nixvim
};

now?

No, that can't be supported as we rely on packages present in unstable nixpkgs on our master. You still need to use nixvim's stable branch if you want to use a stable nixpkgs.
This change mostly allows to use an unstable nixpkgs for nixvim on a stable system, or a stable nixpkgs on an unstable system, but the pkgs used must match nixvim's branch

The initial stages of this feature were recently merged, you can now define programs.nixvim.nixpkgs.pkgs and assign a compatible pkgs instance from the nixos-unstable channel.

Awesome.

Nixvim now builds with my stable system, using unstable packages for nixvim. 👏👏