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/targets/darwin/default.nix

@@ -43,6 +43,11 @@ in {
   };
 
   config = mkIf (activationCmds != [ ]) {
+    assertions = [
+      (hm.assertions.assertPlatform "targets.darwin.defaults" pkgs
+        platforms.darwin)
+    ];
+
     home.activation.setDarwinDefaults = hm.dag.entryAfter [ "writeBoundary" ] ''
       $VERBOSE_ECHO "Configuring macOS user defaults"
       ${concatStringsSep "\n" activationCmds}

modules/targets/darwin/fonts.nix

@@ -12,16 +12,18 @@ let
   fonts = "${fontsEnv}/share/fonts";
 in {
   # macOS won't recognize symlinked fonts
-  config.home.activation.copyFonts = hm.dag.entryAfter [ "writeBoundary" ] ''
-    copyFonts() {
-      rm -rf ${homeDir}/Library/Fonts/HomeManager || :
+  config = mkIf pkgs.hostPlatform.isDarwin {
+    home.activation.copyFonts = hm.dag.entryAfter [ "writeBoundary" ] ''
+      copyFonts() {
+        rm -rf ${homeDir}/Library/Fonts/HomeManager || :
 
-      local f
-      find -L "${fonts}" -type f -printf '%P\0' | while IFS= read -rd "" f; do
-        $DRY_RUN_CMD install $VERBOSE_ARG -Dm644 -T \
-          "${fonts}/$f" "${homeDir}/Library/Fonts/HomeManager/$f"
-      done
-    }
-    copyFonts
-  '';
+        local f
+        find -L "${fonts}" -type f -printf '%P\0' | while IFS= read -rd "" f; do
+          $DRY_RUN_CMD install $VERBOSE_ARG -Dm644 -T \
+            "${fonts}/$f" "${homeDir}/Library/Fonts/HomeManager/$f"
+        done
+      }
+      copyFonts
+    '';
+  };
 }

modules/targets/darwin/keybindings.nix

@@ -31,6 +31,11 @@ in {
   };
 
   config = mkIf (cfg.keybindings != { }) {
+    assertions = [
+      (hm.assertions.assertPlatform "targets.darwin.keybindings" pkgs
+        platforms.darwin)
+    ];
+
     # NOTE: just copy the files because symlinks won't be recognized by macOS
     home.activation.setCocoaKeybindings =
       hm.dag.entryAfter [ "writeBoundary" ] ''

modules/targets/darwin/search.nix

@@ -20,6 +20,11 @@ in {
   };
 
   config = mkIf (cfg.search != null) {
+    assertions = [
+      (hm.assertions.assertPlatform "targets.darwin.search" pkgs
+        platforms.darwin)
+    ];
+
     targets.darwin.defaults = {
       NSGlobalDomain.NSPreferredWebServices = {
         NSWebServicesProviderWebSearch = {

modules/targets/generic-linux.nix

@@ -27,6 +27,10 @@ in {
   };
 
   config = mkIf cfg.enable {
+    assertions = [
+      (hm.assertions.assertPlatform "targets.genericLinux" pkgs platforms.linux)
+    ];
+
     xdg.systemDirs.data = [
       # Nix profiles
       "\${NIX_STATE_DIR:-/nix/var/nix}/profiles/default/share"