Mark unreachable match clause as unreachable! (paritytech#620)

* Mark unreachable match clause as unreachable!

* Fix indentation

validation/src/lib.rs

@@ -835,7 +835,13 @@ impl<C, TxApi> futures03::Future for CreateProposal<C, TxApi> where
 
 				data
 			},
-			CreateProposalState::Switching => return Poll::Pending,
+			CreateProposalState::Switching =>
+				unreachable!(
+					"State Switching are only created on call, \
+					and immediately swapped out; \
+					the data being read is from state; \
+					thus Switching will never be reachable here; qed"
+				),
 			CreateProposalState::Fired(mut future) => {
 				let ret = Pin::new(&mut future).poll(cx);
 				self.state = CreateProposalState::Fired(future);
@@ -844,17 +850,14 @@ impl<C, TxApi> futures03::Future for CreateProposal<C, TxApi> where
 		};
 
 		// 2. propose
-		let future = tokio_executor::blocking::run(move || {
+		let mut future = tokio_executor::blocking::run(move || {
 			let proposed_candidates = data.table.proposed_set();
 			data.propose_with(proposed_candidates)
 		});
+		let polled = Pin::new(&mut future).poll(cx);
 		self.state = CreateProposalState::Fired(future);
 
-		match &mut self.state {
-			CreateProposalState::Fired(future) => Pin::new(future).poll(cx),
-			CreateProposalState::Switching | CreateProposalState::Pending(_) =>
-				Poll::Pending,
-		}
+		polled
 	}
 }