Add convenient GossipSync variant constructors

These constructors fill in the missing types for each variant so that
users don't have to turbofish them manually.

Author: wpaulino

lightning-background-processor/src/lib.rs

@@ -137,6 +137,52 @@ where A::Target: chain::Access, L::Target: Logger {
 	}
 }
 
+impl<P: Deref<Target = P2PGossipSync<G, A, L>>, G: Deref<Target = NetworkGraph<L>>, A: Deref, L: Deref>
+	GossipSync<P, Arc<RapidGossipSync<G, L>>, G, A, L>
+where
+	A::Target: chain::Access,
+	L::Target: Logger,
+{
+	/// Initializes a new [`GossipSync::P2P`] variant.
+	pub fn p2p(gossip_sync: P) -> Self {
+		GossipSync::P2P(gossip_sync)
+	}
+}
+
+impl<R: Deref<Target = RapidGossipSync<G, L>>, G: Deref<Target = NetworkGraph<L>>, L: Deref>
+	GossipSync<
+		Arc<P2PGossipSync<G, Arc<dyn chain::Access + Send + Sync>, L>>,
+		R,
+		G,
+		Arc<dyn chain::Access + Send + Sync>,
+		L,
+	>
+where
+	L::Target: Logger,
+{
+	/// Initializes a new [`GossipSync::Rapid`] variant.
+	pub fn rapid(gossip_sync: R) -> Self {
+		GossipSync::Rapid(gossip_sync)
+	}
+}
+
+impl<L: Deref>
+	GossipSync<
+		Arc<P2PGossipSync<Arc<NetworkGraph<L>>, Arc<dyn chain::Access + Send + Sync>, L>>,
+		Arc<RapidGossipSync<Arc<NetworkGraph<L>>, L>>,
+		Arc<NetworkGraph<L>>,
+		Arc<dyn chain::Access + Send + Sync>,
+		L,
+	>
+where
+	L::Target: Logger,
+{
+	/// Initializes a new [`GossipSync::None`] variant.
+	pub fn none() -> Self {
+		GossipSync::None
+	}
+}
+
 /// Decorates an [`EventHandler`] with common functionality provided by standard [`EventHandler`]s.
 struct DecoratingEventHandler<
 	'a,