f Assertion checks for bit parity and proximity

Author: jkczyz

lightning/src/ln/features.rs

@@ -38,6 +38,13 @@ mod sealed { // You should just use the type aliases instead.
 				/// The bit used to signify that the feature is optional.
 				const ODD_BIT: usize = $odd_bit;
 
+				/// Assertion that [`ODD_BIT`] is actually odd.
+				const ASSERT_BIT_PARITY: usize;
+
+				/// Assertion that [`ODD_BIT`] is at most `EVEN_BIT + 1`. Allows for special cases
+				/// where a feature only has one bit and thus `EVEN_BIT == ODD_BIT`.
+				const ASSERT_BIT_PROXIMITY: usize;
+
 				/// The byte where the feature is set.
 				const BYTE_OFFSET: usize = Self::EVEN_BIT / 8;
 
@@ -78,7 +85,14 @@ mod sealed { // You should just use the type aliases instead.
 			}
 
 			$(
-				impl $feature for $context {}
+				impl $feature for $context {
+					// ODD_BIT % 2 == 1
+					const ASSERT_BIT_PARITY: usize = (<Self as $feature>::ODD_BIT % 2) - 1;
+
+					// ODD_BIT == EVEN_BIT + 1 || ODD_BIT == EVEN_BIT
+					const ASSERT_BIT_PROXIMITY: usize =
+						1 - (<Self as $feature>::ODD_BIT - <Self as $feature>::EVEN_BIT);
+				}
 			)*
 		}
 	}