Because there was absolutely no way of setting this without rewriting parts of the module otherwise.

Motivation for this change

When using https://github.com/DBCDK/morph with NixOS machines specifying documentation.nixos.includeAllModules = true;, the manuals of NixOS instantations were impure & repeatedly rebuilding due to both morph's use of temporary directories & Nixpkgs modules and the normal location of the morph network's repository being in a user's home directory along with our definition of Nixpkgs modules. Some examples from these manual builds:

<attrs>
  <attr name="declarations">
    <list>
      <string value="/tmp/morph-974414533/options.nix" />
    </list>
  <attr name="default">

<attrs>
  <attr name="declarations">
    <list>
      <string value="/home/bb010g/Documents/sysadmin/contoso/modules/nix-substituters.nix" />
    </list>
  <attr name="default">

NixOS's documentation machinery has a configuration option to deal with this, extraSources, but it's not exposed, and the expressions aren't structured in a way to allow sneaky, underhanded configuration either. You can't get around this without patching Nixpkgs. (This PR has been tested so far on our repository via applying pkgs.applyPatches { src = nixpkgs.outPath; patches = [ ./nixpkgs-nixos-expose-extrasources.patch ]; } before importing Nixpkgs. (Whee.))

With this change in place, the repository-directory impurity demonstrated can be eliminated with nixos.extraSources = [ ./. ]; from the repository root, and the morph impurity with an (approximate) patch to /data/eval-machines.nix of:

--- a/data/eval-machines.nix
+++ b/data/eval-machines.nix
@@ -26,6 +26,8 @@ rec {
             [ ({ config, lib, options, ... }: {
                 key = "deploy-stuff";
                 imports = [ ./options.nix ];
+                # Make doc builds determinisitic, even with our tempdir module imports
+                documentation.nixos.extraSources = [ ../. ];
                 # Provide a default hostname and deployment target equal
                 # to the attribute name of the machine in the model.
                 networking.hostName = lib.mkDefault machineName;

Sorry for the delay on PRing this; I know this should have been in way earlier for 20.03's sake. If possible, I'd like to get this backported to 20.03 for sanity in situtations like morph when

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

This change is