Add a test for the legacy TLV field logic

Author: TheBlueMatt

lightning/src/util/ser_macros.rs

@@ -1872,4 +1872,33 @@ mod tests {
 		assert_eq!(TuplesOnly::read(&mut none_data_read).unwrap(), None);
 		assert_eq!(none_data_read.position(), unknown_data_variant.len() as u64);
 	}
+
+	#[derive(Debug, PartialEq, Eq)]
+	struct ExpandedField {
+		// Old versions of LDK are presumed to have had something like:
+		// old_field: u8,
+		new_field: (u8, u8),
+	}
+	impl_writeable_tlv_based!(ExpandedField, {
+		(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;
+			}
+		}, |us: &ExpandedField| Some(us.new_field.0))),
+		(1, new_field, required),
+	});
+
+	#[test]
+	fn test_legacy_conversion() {
+		let mut encoded = ExpandedField { new_field: (42, 43) }.encode();
+		assert_eq!(encoded, <Vec<u8>>::from_hex("0700012a01022a2b").unwrap());
+
+		// On read, the post-read action will run, using the old_field value to overwrite the
+		// new_field.
+		encoded[3] = 10;
+		let read = <ExpandedField as Readable>::read(&mut &encoded[..]).unwrap();
+		assert_eq!(read, ExpandedField { new_field: (10, 43) });
+	}
 }