nix: catch reads of unmanaged defaults

When we’re not managing the Nix installation, these defaults
aren’t used out of the box and won’t accurately represent the
state of any unmanaged Nix or the desired Nix package, so reading
the option defaults is a bug.

This was previously a warning for `nix.package` and a silent failure
for all the others. Now that all the problematic accesses in nix-darwin
have been appropriately conditionalized, and since a throw gives a
backtrace where a warning doesn’t, give throwing defaults to all the
`nix.*` options that don’t reflect reality and that that modules
shouldn’t be reading when `nix.enable` is off.

I’m not in love with the implementation strategy here… ideally
we’d think of something better than this and then upstream it to
NixOS. `nix.nrBuildUsers` growing a fake default that is never used
is particularly unfortunate. But this should hopefully catch mistakes
in module code reasonably reliably.

(cherry picked from commit cd445c5)

modules/nix/default.nix

@@ -154,6 +154,14 @@ let
     })
   ];
 
+  managedDefault = name: default: {
+    default = if cfg.enable then default else throw ''
+      ${name}: accessed when `nix.enable` is off; this is a bug in
+      nix-darwin or a third‐party module
+    '';
+    defaultText = default;
+  };
+
 in
 
 {
@@ -211,9 +219,7 @@ in
 
       package = mkOption {
         type = types.package;
-        default = warnIf (!cfg.enable)
-          "nix.package: accessed when `nix.enable` is off; this is a bug"
-          pkgs.nix;
+        inherit (managedDefault "nix.package" pkgs.nix) default;
         defaultText = literalExpression "pkgs.nix";
         description = ''
           This option specifies the Nix package instance to use throughout the system.
@@ -242,7 +248,7 @@ in
 
       distributedBuilds = mkOption {
         type = types.bool;
-        default = false;
+        inherit (managedDefault "nix.distributedBuilds" false) default defaultText;
         description = ''
           Whether to distribute builds to the machines listed in
           {option}`nix.buildMachines`.
@@ -252,7 +258,7 @@ in
       # Not in NixOS module
       daemonProcessType = mkOption {
         type = types.enum [ "Background" "Standard" "Adaptive" "Interactive" ];
-        default = "Standard";
+        inherit (managedDefault "nix.daemonProcessType" "Standard") default defaultText;
         description = ''
           Nix daemon process resource limits class. These limits propagate to
           build processes. `Standard` is the default process type
@@ -267,7 +273,7 @@ in
       # Not in NixOS module
       daemonIOLowPriority = mkOption {
         type = types.bool;
-        default = false;
+        inherit (managedDefault "nix.daemonIOLowPriority" false) default defaultText;
         description = ''
           Whether the Nix daemon process should considered to be low priority when
           doing file system I/O.
@@ -395,7 +401,7 @@ in
             };
           };
         });
-        default = [ ];
+        inherit (managedDefault "nix.buildMachines" [ ]) default defaultText;
         description = ''
           This option lists the machines to be used if distributed builds are
           enabled (see {option}`nix.distributedBuilds`).
@@ -409,7 +415,7 @@ in
       envVars = mkOption {
         type = types.attrs;
         internal = true;
-        default = { };
+        inherit (managedDefault "nix.envVars" { }) default defaultText;
         description = "Environment variables used by Nix.";
       };
 
@@ -424,6 +430,7 @@ in
 
       nrBuildUsers = mkOption {
         type = types.int;
+        inherit (managedDefault "nix.nrBuildUsers" 0) default defaultText;
         description = ''
           Number of `nixbld` user accounts created to
           perform secure concurrent builds.  If you receive an error
@@ -451,11 +458,13 @@ in
       # Definition differs substantially from NixOS module
       nixPath = mkOption {
         type = nixPathType;
-        default = lib.optionals cfg.channel.enable [
-          # Include default path <darwin-config>.
-          { darwin-config = "${config.environment.darwinConfig}"; }
-          "/nix/var/nix/profiles/per-user/root/channels"
-        ];
+        inherit (managedDefault "nix.nixPath" (
+          lib.optionals cfg.channel.enable [
+            # Include default path <darwin-config>.
+            { darwin-config = "${config.environment.darwinConfig}"; }
+            "/nix/var/nix/profiles/per-user/root/channels"
+          ]
+        )) default;
 
         defaultText = lib.literalExpression ''
           lib.optionals cfg.channel.enable [
@@ -477,7 +486,7 @@ in
 
       checkConfig = mkOption {
         type = types.bool;
-        default = true;
+        inherit (managedDefault "nix.checkConfig" true) default defaultText;
         description = ''
           If enabled (the default), checks for data type mismatches and that Nix
           can parse the generated nix.conf.
@@ -538,15 +547,15 @@ in
             };
           }
         ));
-        default = { };
+        inherit (managedDefault "nix.registry" { }) default defaultText;
         description = ''
           A system-wide flake registry.
         '';
       };
 
       extraOptions = mkOption {
         type = types.lines;
-        default = "";
+        inherit (managedDefault "nix.extraOptions" "") default defaultText;
         example = ''
           keep-outputs = true
           keep-derivations = true
@@ -715,7 +724,7 @@ in
             };
           };
         };
-        default = { };
+        inherit (managedDefault "nix.settings" { }) default defaultText;
         description = ''
           Configuration for Nix, see
           <https://nixos.org/manual/nix/stable/#sec-conf-file>

tests/nix-enable.nix

@@ -2,7 +2,6 @@
 
 {
   nix.enable = false;
-  nix.package = throw "`nix.package` used when `nix.enable` is turned off";
 
   test = ''
     printf >&2 'checking for unexpected Nix binary in /sw/bin\n'