Assert that all defined features are in the known features set

Now that the features contexts track the full set of all known
features, rather than the set of supported features, all defined
features should be listed in the context definition macro.

This adds a compile-time assertion to check that all bits for known
features are set in the context known set.

Author: TheBlueMatt

lightning/src/ln/features.rs

@@ -205,6 +205,12 @@ mod sealed {
 				/// [`ODD_BIT`]: #associatedconstant.ODD_BIT
 				const ASSERT_ODD_BIT_PARITY: usize;
 
+				/// Assertion that the bits are set in the context's [`KNOWN_FEATURE_MASK`].
+				///
+				/// [`KNOWN_FEATURE_MASK`]: Context::KNOWN_FEATURE_MASK
+				#[cfg(not(test))] // We violate this constraint with `UnknownFeature`
+				const ASSERT_BITS_IN_MASK: u8;
+
 				/// The byte where the feature is set.
 				const BYTE_OFFSET: usize = Self::EVEN_BIT / 8;
 
@@ -291,6 +297,12 @@ mod sealed {
 
 					// ODD_BIT % 2 == 1
 					const ASSERT_ODD_BIT_PARITY: usize = (<Self as $feature>::ODD_BIT % 2) - 1;
+
+					// (byte & (REQUIRED_MASK | OPTIONAL_MASK)) >> (EVEN_BIT % 8) == 3
+					#[cfg(not(test))] // We violate this constraint with `UnknownFeature`
+					const ASSERT_BITS_IN_MASK: u8 =
+						((<$context>::KNOWN_FEATURE_MASK[<Self as $feature>::BYTE_OFFSET] & (<Self as $feature>::REQUIRED_MASK | <Self as $feature>::OPTIONAL_MASK))
+						 >> (<Self as $feature>::EVEN_BIT % 8)) - 3;
 				}
 			)*
 		};