nix: warn on the ignored swarm-ui switches instead of failing eval

deploy.swarm-ui.enable and swarm.ui.enable are declared again as
nullOr bool, default null, and do nothing. Setting either adds an eval
warning that swarm-ui now runs wherever swarm-controller runs.

docs/swarm/ui.md

@@ -337,7 +337,8 @@ Where each credential lives and who reads it:
 services.hyperhive.deploy.swarm-controller.enable = true;   # the UI comes with it
 ```
 
-The UI has no switch of its own, and doesn't follow `allSwarmServices`:
+The UI has no switch of its own (setting `deploy.swarm-ui.enable` or
+`swarm.ui.enable` has no effect, and warns at evaluation), and doesn't follow `allSwarmServices`:
 the UI is a view onto the controller's state and reaches it over that
 daemon's socket, so the host that runs the controller is the host that
 can serve the UI. A hive that merely _uses_ a swarm has nothing to serve.

nix/host-modules/deploy.nix

@@ -28,13 +28,23 @@
 # spread across the service modules, so all of them have a single home
 # and a single file to delete when the deprecation window closes — the
 # shape ./swarm-peers-removed.nix already uses.
-{ lib, ... }:
+{ config, lib, ... }:
 let
-  # Shared by the two removed spellings of the swarm UI's own switch below.
+  # The swarm UI's own switch, in both spellings. Accepted and ignored: a
-  swarmUiSwitchRemoved = ''
+  # definition only produces a warning, so a host that still sets one keeps
-    swarm-ui now runs wherever swarm-controller runs. Remove this
+  # evaluating. `null` is "not set".
-    definition; services.hyperhive.deploy.swarm-controller.enable decides it.
+  swarmUiSwitchIgnored = lib.mkOption {
+    type = lib.types.nullOr lib.types.bool;
+    default = null;
+    visible = false;
+    description = ''
+      No effect. swarm-ui runs wherever swarm-controller runs.
     '';
+  };
+  swarmUiSwitchWarning =
+    path: value:
+    lib.optional (value != null)
+      "services.hyperhive.${path} no longer has any effect: swarm-ui now runs wherever swarm-controller runs. Remove this setting; services.hyperhive.deploy.swarm-controller.enable decides it.";
 
   # Shared by the two removed spellings of the status-publishing secret below.
   statusSecretRemoved = ''
@@ -98,20 +108,6 @@ in
         be deleted.
       ''
     )
-    (lib.mkRemovedOptionModule [
-      "services"
-      "hyperhive"
-      "swarm"
-      "ui"
-      "enable"
-    ] swarmUiSwitchRemoved)
-    (lib.mkRemovedOptionModule [
-      "services"
-      "hyperhive"
-      "deploy"
-      "swarm-ui"
-      "enable"
-    ] swarmUiSwitchRemoved)
     (lib.mkRenamedOptionModule
       [ "services" "hyperhive" "swarm" "authelia" "enable" ]
       [ "services" "hyperhive" "deploy" "authelia" "enable" ]
@@ -451,7 +447,7 @@ in
       [ "services" "hyperhive" "deploy" "victoriametrics" "package" ]
     )
     # With this one `services.hyperhive.swarm.ui` holds nothing at all: its
-    # `enable` is removed above and its `package` is renamed here.
+    # `enable` is an ignored switch (declared below) and its `package` is renamed here.
     (lib.mkRenamedOptionModule
       [ "services" "hyperhive" "swarm" "ui" "package" ]
       [ "services" "hyperhive" "deploy" "swarm-ui" "package" ]
@@ -490,7 +486,14 @@ in
   # ⚠️ Only `deploy.forgejo.enable` is declared here. The rest of
   # `deploy.forgejo` is in ./hive-forge/default.nix: the knobs live with the
   # module that reads them; this file stays the registry of toggles.
+  options.services.hyperhive.swarm.ui.enable = swarmUiSwitchIgnored;
+
+  config.warnings =
+    swarmUiSwitchWarning "deploy.swarm-ui.enable" config.services.hyperhive.deploy.swarm-ui.enable
+    ++ swarmUiSwitchWarning "swarm.ui.enable" config.services.hyperhive.swarm.ui.enable;
+
   options.services.hyperhive.deploy = {
+    swarm-ui.enable = swarmUiSwitchIgnored;
     forgejo.enable = lib.mkOption {
       type = lib.types.bool;
       default = false;

nix/module-eval/swarm-services-switch.nix

@@ -75,8 +75,8 @@ let
   # saying "it is not there" would hold just as well if it were never there.
   # Package stubbed per this file's header: the vhost roots at it.
   #
-  # Also sets both removed UI switches to `false`: the vhost must render
+  # Also sets both ignored UI switches to `false`: the vhost must render
-  # regardless, and each must fail evaluation through its assertion. Neither
+  # regardless, and each must evaluate and warn. Neither
   # is read anywhere else, so they change nothing the other arms see.
   swarmUiHere = hive {
     deploy.swarm-controller.enable = true;
@@ -245,24 +245,23 @@ let
       ok = swarmUiHere.services.nginx.virtualHosts ? "t.local";
     }
     {
-      # Control: `swarmServicesHere` sets neither removed switch.
+      # Control: `swarmServicesHere` sets neither ignored switch.
-      name = "a definition of a removed swarm UI switch fails evaluation, naming the option and the controller";
+      name = "a definition of an ignored swarm UI switch evaluates and warns, naming the option and the controller";
       ok =
         let
-          refused =
+          warned =
             m: path:
             lib.any (
-              a:
+              w:
-              !a.assertion
+              lib.hasInfix "services.hyperhive.${path} no longer has any effect" w
-              && lib.hasInfix "services.hyperhive.${path}" a.message
+              && lib.hasInfix "swarm-ui now runs wherever swarm-controller runs" w
-              && lib.hasInfix "swarm-ui now runs wherever swarm-controller runs" a.message
+            ) m.warnings;
-            ) m.assertions;
           paths = [
             "deploy.swarm-ui.enable"
             "swarm.ui.enable"
           ];
         in
-        lib.all (refused swarmUiHere) paths && !(lib.any (refused swarmServicesHere) paths);
+        lib.all (warned swarmUiHere) paths && !(lib.any (warned swarmServicesHere) paths);
     }
     {
       # The swarm-services toggle enables them explicitly, from its own