f more representative test

Author: TheBlueMatt

lightning/src/util/ser_macros.rs

@@ -1862,22 +1862,32 @@ mod tests {
 		(0, old_field, (legacy, u8, {
 			// Sadly the type-checker needs some help
 			let _: &(u8, u8) = &new_field;
-			if let Some(old_field) = old_field {
-				new_field.0 = old_field;
+			// If new_field.0 is 0, we assume we hit the default_value case below and overwrite the
+			// value with the old data.
+			if new_field == (0, 0) {
+				if let Some(old_field) = old_field {
+					new_field.0 = old_field;
+				}
 			}
 		}, |us: &ExpandedField| Some(us.new_field.0))),
-		(1, new_field, required),
+		(1, new_field, (default_value, (0, 0))),
 	});
 
 	#[test]
 	fn test_legacy_conversion() {
-		let mut encoded = ExpandedField { new_field: (42, 43) }.encode();
-		assert_eq!(encoded, <Vec<u8>>::from_hex("0700012a01022a2b").unwrap());
+		let mut encoded = ExpandedField { new_field: (43, 42) }.encode();
+		assert_eq!(encoded, <Vec<u8>>::from_hex("0700012b01022b2a").unwrap());
 
-		// On read, the post-read action will run, using the old_field value to overwrite the
-		// new_field.
+		// On read, we'll read the `new_field` and have a value other than `(0, 0)`, causing us to
+		// ignore the old field value (in byte 3).
 		encoded[3] = 10;
 		let read = <ExpandedField as Readable>::read(&mut &encoded[..]).unwrap();
-		assert_eq!(read, ExpandedField { new_field: (10, 43) });
+		assert_eq!(read, ExpandedField { new_field: (43, 42) });
+
+		// On read, if we read an old `ExpandedField` that just has a type-0 `old_field` entry,
+		// we'll copy that into the first position of `new_field`.
+		let encoded = <Vec<u8>>::from_hex("0300012a").unwrap();
+		let read = <ExpandedField as Readable>::read(&mut &encoded[..]).unwrap();
+		assert_eq!(read, ExpandedField { new_field: (42, 0) });
 	}
 }