Use iterators directly in process_onion_failure_inner

Now that `create_onion_keys_generic` returns an iterator rather
than relying on a callback, making `process_onion_failure_inner`
chain the non-trampoline and trampoline iterators is trivial.

We do so here, avoiding the extra vec allocation.

Author: TheBlueMatt

lightning/src/ln/onion_utils.rs

@@ -1041,39 +1041,34 @@ where
 		}
 	}
 
-	let (num_blinded_hops, num_trampoline_hops) =
-		path.blinded_tail.as_ref().map_or((0, 0), |bt| (bt.hops.len(), bt.trampoline_hops.len()));
-
-	// We are first collecting all the unblinded `RouteHop`s inside `onion_keys`. Then, if applicable,
-	// we will add all the `TrampolineHop`s, and finally, the blinded hops.
-	let mut onion_keys =
-		Vec::with_capacity(path.hops.len() + num_trampoline_hops + num_blinded_hops);
+	let num_blinded_hops = path.blinded_tail.as_ref().map_or(0, |bt| bt.hops.len());
 
 	// if we have Trampoline hops, the blinded hops are part of the inner Trampoline onion
 	let nontrampoline_bp =
 		if path.has_trampoline_hops() { None } else { path.blinded_tail.as_ref() };
 	let nontrampoline_hops =
-		construct_onion_keys_generic(secp_ctx, &path.hops, nontrampoline_bp, outer_session_priv);
-	for (shared_secret, _, _, route_hop_option, _) in nontrampoline_hops {
-		onion_keys.push((route_hop_option.map(|rh| ErrorHop::RouteHop(rh)), shared_secret));
-	}
+		construct_onion_keys_generic(secp_ctx, &path.hops, nontrampoline_bp, outer_session_priv)
+			.map(|(shared_secret, _, _, route_hop_option, _)| {
+				(route_hop_option.map(|rh| ErrorHop::RouteHop(rh)), shared_secret)
+			});
 
-	if path.has_trampoline_hops() {
+	let trampoline_hops = if path.has_trampoline_hops() {
 		// Trampoline hops are part of the blinded tail, so this can never panic
 		let blinded_tail = path.blinded_tail.as_ref();
 		let hops = &blinded_tail.unwrap().trampoline_hops;
 		let inner_session_priv =
 			inner_session_priv.expect("Trampoline hops always have an inner session priv");
-		let trampoline_hops =
-			construct_onion_keys_generic(secp_ctx, hops, blinded_tail, inner_session_priv);
-		for (shared_secret, _, _, trampoline_hop_option, _) in trampoline_hops {
-			onion_keys
-				.push((trampoline_hop_option.map(|th| ErrorHop::TrampolineHop(th)), shared_secret));
-		}
-	}
+		Some(construct_onion_keys_generic(secp_ctx, hops, blinded_tail, inner_session_priv).map(
+			|(shared_secret, _, _, route_hop_option, _)| {
+				(route_hop_option.map(|th| ErrorHop::TrampolineHop(th)), shared_secret)
+			},
+		))
+	} else {
+		None
+	};
 
 	// Handle packed channel/node updates for passing back for the route handler
-	let mut iterator = onion_keys.into_iter().peekable();
+	let mut iterator = nontrampoline_hops.chain(trampoline_hops.into_iter().flatten()).peekable();
 	while let Some((route_hop_option, shared_secret)) = iterator.next() {
 		let route_hop = match route_hop_option.as_ref() {
 			Some(hop) => hop,