Adds some debug logging (#837)

polkadot/network/src/legacy/collator_pool.rs

@@ -65,6 +65,7 @@ impl CollationSlot {
 	}
 }
 
+#[derive(Debug)]
 enum SlotEntries {
 	Blank,
 	// not queried yet
@@ -75,7 +76,7 @@ enum SlotEntries {
 
 impl SlotEntries {
 	fn received_collation(&mut self, collation: Collation) {
-		*self = match ::std::mem::replace(self, SlotEntries::Blank) {
+		*self = match std::mem::replace(self, SlotEntries::Blank) {
 			SlotEntries::Blank => SlotEntries::Pending(vec![collation]),
 			SlotEntries::Pending(mut cs) => {
 				cs.push(collation);
@@ -185,6 +186,12 @@ impl CollatorPool {
 	/// The collator should be registered for the parachain of the collation as a precondition of this function.
 	/// The collation should have been checked for integrity of signature before passing to this function.
 	pub fn on_collation(&mut self, collator_id: CollatorId, relay_parent: Hash, collation: Collation) {
+		log::debug!(
+			target: "collator-pool", "On collation from collator {} for relay parent {}",
+			collator_id,
+			relay_parent,
+		);
+
 		if let Some((para_id, _)) = self.collators.get(&collator_id) {
 			debug_assert_eq!(para_id, &collation.info.parachain_index);