Define each feature only in terms of an odd bit

Feature flags are generally assigned in pairs -- an even bit and an odd
bit. Remove the need to specify both when defining a feature. Instead,
define it in terms of an odd bit and infer the even bit.

Remove the special case where initial_routing_sync has no even bit. Now,
it (bit 2) is considered known by the implementation.

Author: jkczyz

lightning/src/ln/features.rs

@@ -115,19 +115,29 @@ mod sealed {
 	///
 	/// [`Context`]: trait.Context.html
 	macro_rules! define_feature {
-		($even_bit: expr, $odd_bit: expr, $feature: ident, [$($context: ty),+], $doc: expr) => {
+		($odd_bit: expr, $feature: ident, [$($context: ty),+], $doc: expr) => {
 			#[doc = $doc]
 			///
 			/// See [BOLT #9] for details.
 			///
 			/// [BOLT #9]: https://github.com/lightningnetwork/lightning-rfc/blob/master/09-features.md
 			pub trait $feature: Context {
 				/// The bit used to signify that the feature is required.
-				const EVEN_BIT: usize = $even_bit;
+				const EVEN_BIT: usize = $odd_bit - 1;
 
 				/// The bit used to signify that the feature is optional.
 				const ODD_BIT: usize = $odd_bit;
 
+				/// Assertion that [`EVEN_BIT`] is actually even.
+				///
+				/// [`EVEN_BIT`]: #associatedconstant.EVEN_BIT
+				const ASSERT_EVEN_BIT_PARITY: usize;
+
+				/// Assertion that [`ODD_BIT`] is actually odd.
+				///
+				/// [`ODD_BIT`]: #associatedconstant.ODD_BIT
+				const ASSERT_ODD_BIT_PARITY: usize;
+
 				/// The byte where the feature is set.
 				const BYTE_OFFSET: usize = Self::EVEN_BIT / 8;
 
@@ -170,23 +180,29 @@ mod sealed {
 			}
 
 			$(
-				impl $feature for $context {}
+				impl $feature for $context {
+					// EVEN_BIT % 2 == 0
+					const ASSERT_EVEN_BIT_PARITY: usize = 0 - (<Self as $feature>::EVEN_BIT % 2);
+
+					// ODD_BIT % 2 == 1
+					const ASSERT_ODD_BIT_PARITY: usize = (<Self as $feature>::ODD_BIT % 2) - 1;
+				}
 			)*
 		}
 	}
 
-	define_feature!(0, 1, DataLossProtect, [InitContext, NodeContext],
+	define_feature!(1, DataLossProtect, [InitContext, NodeContext],
 		"Feature flags for `option_data_loss_protect`.");
 	// NOTE: Per Bolt #9, initial_routing_sync has no even bit.
-	define_feature!(3, 3, InitialRoutingSync, [InitContext],
+	define_feature!(3, InitialRoutingSync, [InitContext],
 		"Feature flags for `initial_routing_sync`.");
-	define_feature!(4, 5, UpfrontShutdownScript, [InitContext, NodeContext],
+	define_feature!(5, UpfrontShutdownScript, [InitContext, NodeContext],
 		"Feature flags for `option_upfront_shutdown_script`.");
-	define_feature!(8, 9, VariableLengthOnion, [InitContext, NodeContext],
+	define_feature!(9, VariableLengthOnion, [InitContext, NodeContext],
 		"Feature flags for `var_onion_optin`.");
-	define_feature!(14, 15, PaymentSecret, [InitContext, NodeContext],
+	define_feature!(15, PaymentSecret, [InitContext, NodeContext],
 		"Feature flags for `payment_secret`.");
-	define_feature!(16, 17, BasicMPP, [InitContext, NodeContext],
+	define_feature!(17, BasicMPP, [InitContext, NodeContext],
 		"Feature flags for `basic_mpp`.");
 }