nixfiles/modules/features.nix

# Feature-flag option declarations shared by every NixOS host (imported via
# baseModules in flake.nix). Declaring the flags here -- rather than inside the
# module that implements them -- means a host can read or set a flag without
# importing the (often large) implementation module. In particular,
# features.swayDesktop.enable is read by modules/users.nix on every host, but a
# headless host (e.g. the Pi) must be able to leave it at its default without
# pulling in modules/sway.nix. The implementation lives in modules/sway.nix,
# gated on this flag.
#
# The file also carries the host capability facts those flags derive from
# (features.cpu.*). features.claudeCode.enable is such a derived flag: it is
# computed from the declared CPU level here and read by home/claude.nix through
# home-manager's osConfig, so a machine that cannot run the tool never installs
# it, on any host, without per-host opt-outs.
{
  config,
  lib,
  pkgs,
  ...
}:
let
  cfg = config.features;

  # Claude Code runs on Node, whose V8 build requires SSE4.2 and POPCNT -- the
  # x86-64-v2 feature set. On an older x86_64 CPU it does not run (illegal
  # instruction), so it must not be installed there.
  claudeCodeMinLevel = 2;
  claudeCodeSupported =
    !pkgs.stdenv.hostPlatform.isx86_64 || cfg.cpu.microarchLevel >= claudeCodeMinLevel;
in
{
  options.features = {
    swayDesktop.enable = lib.mkEnableOption "the Sway desktop";

    cpu.microarchLevel = lib.mkOption {
      type = lib.types.ints.between 1 4;
      default = 2;
      example = 1;
      description = ''
        The x86-64 psABI microarchitecture level the host CPU implements:
        1 = the original baseline, 2 = SSE4.2/POPCNT (Nehalem, 2008+),
        3 = AVX2, 4 = AVX-512.

        Nix cannot detect this (evaluation is pure and hosts are often built
        elsewhere), so a machine older than the default declares its own level
        and the flags below derive from it. Ignored on non-x86_64 hosts.
      '';
    };

    claudeCode.enable = lib.mkOption {
      type = lib.types.bool;
      default = claudeCodeSupported;
      defaultText = lib.literalMD ''
        `true`, unless the host declares an x86-64 microarchitecture level
        below ${toString claudeCodeMinLevel}
      '';
      description = ''
        Whether to install Claude Code in this host's home-manager profiles
        (implemented in home/claude.nix). Defaults off on CPUs below
        x86-64-v${toString claudeCodeMinLevel}, which cannot run it; forcing it
        on such a host is an evaluation error.
      '';
    };
  };

  config.assertions = [
    {
      assertion = cfg.claudeCode.enable -> claudeCodeSupported;
      message = ''
        features.claudeCode.enable is on, but this host declares
        features.cpu.microarchLevel = ${toString cfg.cpu.microarchLevel}.
        Claude Code needs x86-64-v${toString claudeCodeMinLevel}
        (SSE4.2/POPCNT) and will not run on an older CPU.
      '';
    }
  ];
}