a

crates/core/src/main.rs

@@ -14,6 +14,7 @@ use iced_layershell::settings::{LayerShellSettings, Settings, StartMode};
 use iced_layershell::to_layer_message;
 
 use common::{BarModule, ModuleEffect, ModuleMsg, Service as ModuleService, ServicePayloadKind};
+use services::IntoSubscription;
 
 mod popup;
 
@@ -96,8 +97,9 @@ fn gather_subscriptions(bar: &Bar) -> Subscription<Message> {
         bar.routes
             .values()
             .flat_map(|wants| wants.iter())
-            .map(|service| {
-                services::subscription(service)
+            .map(|&service| {
+                service
+                    .into_subscription()
                     .map(|event| Message::Event(Msg::Subscription(event)))
             })
             .collect::<Vec<_>>(),

crates/services/src/lib.rs

@@ -1,14 +1,24 @@
 //! Service registry: turns a route key into the subscription that backs it.
 //! Adding a service is a new crate under `crates/services/` plus one arm in
-//! `subscription` — core never changes.
+//! the impl below — core never changes.
 
 use iced::Subscription;
 
 use common::{Service, ServicePayloadKind};
 
-/// The subscription backing a route key.
-pub fn subscription(service: &Service) -> Subscription<ServicePayloadKind> {
-    match service {
+/// Conversion from a route key to the subscription backing it.
+///
+/// An extension trait (not `impl From<Service> for Subscription<...>`) because
+/// both `Service` and `Subscription` are foreign to this crate — the orphan
+/// rule blocks a foreign trait (`From`) on a foreign type (`Subscription`).
+pub trait IntoSubscription {
+    fn into_subscription(self) -> Subscription<ServicePayloadKind>;
+}
+
+impl IntoSubscription for Service {
+    fn into_subscription(self) -> Subscription<ServicePayloadKind> {
+        match self {
             Service::Clock(_) => service_datetime::ClockTicker::run(),
         }
     }
+}