nixos/nix-channel: Take care of NIX_PATH's non-empty default when disabled

nixos/modules/config/nix-channel.nix

@@ -9,6 +9,7 @@
 { config, lib, ... }:
 let
   inherit (lib)
+    mkDefault
     mkIf
     mkOption
     stringAfter
@@ -88,10 +89,14 @@ in
       rm $out/bin/nix-channel
     '';
 
-    environment.sessionVariables = mkIf (cfg.nixPath != []) {
+    # NIX_PATH has a non-empty default according to Nix docs, so we don't unset
+    # it when empty.
+    environment.sessionVariables = {
       NIX_PATH = cfg.nixPath;
     };
 
+    nix.settings.nix-path = mkIf (! cfg.channel.enable) (mkDefault "");
+
     system.activationScripts.nix-channel = mkIf cfg.channel.enable
       (stringAfter [ "etc" "users" ] ''
         # Subscribe the root user to the NixOS channel by default.

nixos/tests/installer.nix

@@ -338,6 +338,18 @@ let
       with subtest("nix-channel command is not available anymore"):
         machine.succeed("! which nix-channel")
 
+      # Note that the channel profile is still present on disk, but configured
+      # not to be used.
+      with subtest("builtins.nixPath is now empty"):
+        machine.succeed("""
+          [[ "[ ]" == "$(nix-instantiate builtins.nixPath --eval --expr)" ]]
+        """)
+
+      with subtest("<nixpkgs> does not resolve"):
+        machine.succeed("""
+          ! nix-instantiate '<nixpkgs>' --eval --expr
+        """)
+
       with subtest("Evaluate flake config in fresh env without nix-channel"):
         machine.succeed("nixos-rebuild switch --flake /root/my-config#xyz")