nixpkgs/nixos/doc/manual/development/assertions.section.md

Warnings and Assertions

When configuration problems are detectable in a module, it is a good idea to write an assertion or warning. Doing so provides clear feedback to the user and prevents errors after the build.

Although Nix has the abort and builtins.trace functions to perform such tasks, they are not ideally suited for NixOS modules. Instead of these functions, you can declare your warnings and assertions using the NixOS module system.

Warnings

This is an example of using warnings.

{ config, lib, ... }:
{
  config = lib.mkIf config.services.foo.enable {
    warnings =
      if config.services.foo.bar
      then [ ''You have enabled the bar feature of the foo service.
               This is known to cause some specific problems in certain situations.
               '' ]
      else [];
  };
}

Assertions

This example, extracted from the syslogd module shows how to use assertions. Since there can only be one active syslog daemon at a time, an assertion is useful to prevent such a broken system from being built.

{ config, lib, ... }:
{
  config = lib.mkIf config.services.syslogd.enable {
    assertions =
      [ { assertion = !config.services.rsyslogd.enable;
          message = "rsyslogd conflicts with syslogd";
        }
      ];
  };
}