Manually implement PartialOrd for ChannelState

Relying on the order in which `ChannelState` variants are defined is
error-prone.

Author: wpaulino

lightning/src/ln/channel.rs

@@ -414,7 +414,7 @@ define_state_flags!(
 	]
 );
 
-#[derive(Copy, Clone, Debug, PartialEq, PartialOrd, Eq)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 enum ChannelState {
 	/// We are negotiating the parameters required for the channel prior to funding it.
 	NegotiatingFunding(NegotiatingFundingFlags),
@@ -433,6 +433,48 @@ enum ChannelState {
 	ShutdownComplete,
 }
 
+impl PartialOrd for ChannelState {
+    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
+		match self {
+			ChannelState::NegotiatingFunding(flags) => match other {
+				ChannelState::NegotiatingFunding(other_flags) => flags.partial_cmp(other_flags),
+				ChannelState::FundingNegotiated => Some(cmp::Ordering::Less),
+				ChannelState::AwaitingChannelReady(_) => Some(cmp::Ordering::Less),
+				ChannelState::ChannelReady(_) => Some(cmp::Ordering::Less),
+				ChannelState::ShutdownComplete => Some(cmp::Ordering::Less),
+			},
+			ChannelState::FundingNegotiated => match other {
+				ChannelState::NegotiatingFunding(_) => Some(cmp::Ordering::Greater),
+				ChannelState::FundingNegotiated => Some(cmp::Ordering::Equal),
+				ChannelState::AwaitingChannelReady(_) => Some(cmp::Ordering::Less),
+				ChannelState::ChannelReady(_) => Some(cmp::Ordering::Less),
+				ChannelState::ShutdownComplete => Some(cmp::Ordering::Less),
+			},
+			ChannelState::AwaitingChannelReady(flags) => match other {
+				ChannelState::NegotiatingFunding(_) => Some(cmp::Ordering::Greater),
+				ChannelState::FundingNegotiated => Some(cmp::Ordering::Greater),
+				ChannelState::AwaitingChannelReady(other_flags) => flags.partial_cmp(other_flags),
+				ChannelState::ChannelReady(_) => Some(cmp::Ordering::Less),
+				ChannelState::ShutdownComplete => Some(cmp::Ordering::Less),
+			},
+			ChannelState::ChannelReady(flags) => match other {
+				ChannelState::NegotiatingFunding(_) => Some(cmp::Ordering::Greater),
+				ChannelState::FundingNegotiated => Some(cmp::Ordering::Greater),
+				ChannelState::AwaitingChannelReady(_) => Some(cmp::Ordering::Greater),
+				ChannelState::ChannelReady(other_flags) => flags.partial_cmp(other_flags),
+				ChannelState::ShutdownComplete => Some(cmp::Ordering::Less),
+			},
+			ChannelState::ShutdownComplete => match other {
+				ChannelState::NegotiatingFunding(_) => Some(cmp::Ordering::Greater),
+				ChannelState::FundingNegotiated => Some(cmp::Ordering::Greater),
+				ChannelState::AwaitingChannelReady(_) => Some(cmp::Ordering::Greater),
+				ChannelState::ChannelReady(_) => Some(cmp::Ordering::Greater),
+				ChannelState::ShutdownComplete => Some(cmp::Ordering::Equal),
+			},
+		}
+    }
+}
+
 macro_rules! impl_state_flag {
 	($get: ident, $set: ident, $clear: ident, $state_flag: expr, [$($state: ident),+]) => {
 		#[allow(unused)]