a

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 {
-        Service::Clock(_) => service_datetime::ClockTicker::run(),
+/// 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(),
+        }
     }
 }