Move platform check into modules

Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.

Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.

This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.

Fixes #1906

modules/programs/abook.nix

@@ -27,7 +27,11 @@ in {
   };
 
   config = mkIf cfg.enable {
+    assertions =
+      [ (hm.assertions.assertPlatform "programs.abook" pkgs platforms.linux) ];
+
     home.packages = [ pkgs.abook ];
+
     xdg.configFile."abook/abookrc" = mkIf (cfg.extraConfig != "") {
       text = ''
         # Generated by Home Manager.

modules/programs/foot.nix

@@ -49,6 +49,9 @@ in {
   };
 
   config = mkIf cfg.enable {
+    assertions =
+      [ (hm.assertions.assertPlatform "programs.foot" pkgs platforms.linux) ];
+
     home.packages = [ cfg.package ];
 
     xdg.configFile."foot/foot.ini" = mkIf (cfg.settings != { }) {

modules/programs/getmail.nix

@@ -55,6 +55,10 @@ in {
   };
 
   config = mkIf getmailEnabled {
+    assertions = [
+      (hm.assertions.assertPlatform "programs.getmail" pkgs platforms.linux)
+    ];
+
     home.file = foldl' (a: b: a // b) { }
       (map (a: { "${renderConfigFilepath a}".text = renderAccountConfig a; })
         accounts);

modules/programs/i3status-rust.nix

@@ -249,6 +249,11 @@ in {
   };
 
   config = mkIf cfg.enable {
+    assertions = [
+      (hm.assertions.assertPlatform "programs.i3status-rust" pkgs
+        platforms.linux)
+    ];
+
     home.packages = [ cfg.package ];
 
     xdg.configFile = mapAttrs' (cfgFileSuffix: cfg:

modules/programs/mangohud.nix

@@ -82,24 +82,25 @@ in {
     };
   };
 
-  config = mkIf cfg.enable (mkMerge [
-    {
-      home.packages = [ cfg.package ];
+  config = mkIf cfg.enable {
+    assertions = [
+      (hm.assertions.assertPlatform "programs.mangohud" pkgs platforms.linux)
+    ];
 
-      home.sessionVariables = mkIf cfg.enableSessionWide {
-        MANGOHUD = 1;
-        MANGOHUD_DLSYM = 1;
-      };
+    home.packages = [ cfg.package ];
 
-      xdg.configFile."MangoHud/MangoHud.conf" =
+    home.sessionVariables = mkIf cfg.enableSessionWide {
+      MANGOHUD = 1;
+      MANGOHUD_DLSYM = 1;
+    };
+
+    xdg.configFile = {
+      "MangoHud/MangoHud.conf" =
         mkIf (cfg.settings != { }) { text = renderSettings cfg.settings; };
-    }
-    {
-      xdg.configFile = mapAttrs'
-        (n: v: nameValuePair "MangoHud/${n}.conf" { text = renderSettings v; })
-        cfg.settingsPerApplication;
-    }
-  ]);
+    } // mapAttrs'
+      (n: v: nameValuePair "MangoHud/${n}.conf" { text = renderSettings v; })
+      cfg.settingsPerApplication;
+  };
 
   meta.maintainers = with maintainers; [ zeratax ];
 }

modules/programs/rofi.nix

@@ -402,12 +402,15 @@ in {
   };
 
   config = mkIf cfg.enable {
-    assertions = [{
-      assertion = cfg.theme == null || cfg.colors == null;
-      message = ''
-        Cannot use the rofi options 'theme' and 'colors' simultaneously.
-      '';
-    }];
+    assertions = [
+      (hm.assertions.assertPlatform "programs.rofi" pkgs platforms.linux)
+      {
+        assertion = cfg.theme == null || cfg.colors == null;
+        message = ''
+          Cannot use the rofi options 'theme' and 'colors' simultaneously.
+        '';
+      }
+    ];
 
     lib.formats.rasi.mkLiteral = value: {
       _type = "literal";

modules/programs/terminator.nix

@@ -63,7 +63,12 @@ in {
   };
 
   config = mkIf cfg.enable {
+    assertions = [
+      (hm.assertions.assertPlatform "programs.terminator" pkgs platforms.linux)
+    ];
+
     home.packages = [ cfg.package ];
+
     xdg.configFile."terminator/config" =
       mkIf (cfg.config != { }) { text = toConfigObject cfg.config; };
   };

modules/programs/waybar.nix

@@ -353,16 +353,26 @@ in {
     in allWarnings;
 
   in mkIf cfg.enable (mkMerge [
-    { home.packages = [ cfg.package ]; }
+    {
+      assertions = [
+        (lib.hm.assertions.assertPlatform "programs.waybar" pkgs
+          lib.platforms.linux)
+      ];
+
+      home.packages = [ cfg.package ];
+    }
+
     (mkIf (cfg.settings != [ ]) {
       # Generate warnings about defined but unreferenced modules
       inherit warnings;
 
       xdg.configFile."waybar/config".source = configSource;
     })
+
     (mkIf (cfg.style != null) {
       xdg.configFile."waybar/style.css".text = cfg.style;
     })
+
     (mkIf cfg.systemd.enable {
       systemd.user.services.waybar = {
         Unit = {