nixpkgs/nixos/doc/manual/development/assertions.xml

<section xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        xml:id="sec-assertions">
 <title>Warnings and Assertions</title>

 <para>
  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.
 </para>

 <para>
  Although Nix has the <literal>abort</literal> and <literal>builtins.trace</literal> <link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">functions</link> 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.
 </para>

 <section xml:id="sec-assertions-warnings">
  <title>Warnings</title>

  <para>
   This is an example of using <literal>warnings</literal>.
  </para>

<programlisting>
<![CDATA[
{ 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 [];
  }
}
]]>
</programlisting>
 </section>

 <section xml:id="sec-assertions-assertions">
  <title>Assertions</title>

  <para>
   This example, extracted from the <link xlink:href="https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix"> <literal>syslogd</literal> module </link> shows how to use <literal>assertions</literal>. 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.
  </para>

<programlisting>
<![CDATA[
{ config, lib, ... }:
{
  config = lib.mkIf config.services.syslogd.enable {
    assertions =
      [ { assertion = !config.services.rsyslogd.enable;
          message = "rsyslogd conflicts with syslogd";
        }
      ];
  }
}
]]>
</programlisting>
 </section>
</section>