Generate TLV write length impls

Add a writeable TLV helper macro that emits both write and
serialized_length from the same field list. Reuse the shared TLV length
helper from impl_ser_tlv_based so the generated read/write path and the
new custom-read path stay aligned.

Use the new helper for the hot channel funding and commitment
transaction TLV writers while leaving their custom read implementations
unchanged.

Author: joostjager

lightning/src/ln/chan_utils.rs

@@ -1169,24 +1169,21 @@ impl_ser_tlv_based!(CounterpartyChannelTransactionParameters, {
 	(2, selected_contest_delay, required),
 });
 
-impl Writeable for ChannelTransactionParameters {
-	#[rustfmt::skip]
-	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
-		let legacy_deserialization_prevention_marker = legacy_deserialization_prevention_marker_for_channel_type_features(&self.channel_type_features);
-		write_tlv_fields!(writer, {
-			(0, self.holder_pubkeys, required),
-			(2, self.holder_selected_contest_delay, required),
-			(4, self.is_outbound_from_holder, required),
-			(6, self.counterparty_parameters, option),
-			(8, self.funding_outpoint, option),
-			(10, legacy_deserialization_prevention_marker, option),
-			(11, self.channel_type_features, required),
-			(12, self.splice_parent_funding_txid, option),
-			(13, self.channel_value_satoshis, required),
-		});
-		Ok(())
-	}
-}
+impl_writeable_tlv_based!(ChannelTransactionParameters, self, {
+	(0, self.holder_pubkeys, required),
+	(2, self.holder_selected_contest_delay, required),
+	(4, self.is_outbound_from_holder, required),
+	(6, self.counterparty_parameters, option),
+	(8, self.funding_outpoint, option),
+	(
+		10,
+		legacy_deserialization_prevention_marker_for_channel_type_features(&self.channel_type_features),
+		option
+	),
+	(11, self.channel_type_features, required),
+	(12, self.splice_parent_funding_txid, option),
+	(13, self.channel_value_satoshis, required),
+});
 
 impl ReadableArgs<Option<u64>> for ChannelTransactionParameters {
 	#[rustfmt::skip]
@@ -1634,25 +1631,22 @@ impl PartialEq for CommitmentTransaction {
 	}
 }
 
-impl Writeable for CommitmentTransaction {
-	#[rustfmt::skip]
-	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
-		let legacy_deserialization_prevention_marker = legacy_deserialization_prevention_marker_for_channel_type_features(&self.channel_type_features);
-		write_tlv_fields!(writer, {
-			(0, self.commitment_number, required),
-			(1, self.to_broadcaster_delay, option),
-			(2, self.to_broadcaster_value_sat, required),
-			(4, self.to_countersignatory_value_sat, required),
-			(6, self.feerate_per_kw, required),
-			(8, self.keys, required),
-			(10, self.built, required),
-			(12, self.nondust_htlcs, required_vec),
-			(14, legacy_deserialization_prevention_marker, option),
-			(15, self.channel_type_features, required),
-		});
-		Ok(())
-	}
-}
+impl_writeable_tlv_based!(CommitmentTransaction, self, {
+	(0, self.commitment_number, required),
+	(1, self.to_broadcaster_delay, option),
+	(2, self.to_broadcaster_value_sat, required),
+	(4, self.to_countersignatory_value_sat, required),
+	(6, self.feerate_per_kw, required),
+	(8, self.keys, required),
+	(10, self.built, required),
+	(12, self.nondust_htlcs, required_vec),
+	(
+		14,
+		legacy_deserialization_prevention_marker_for_channel_type_features(&self.channel_type_features),
+		option
+	),
+	(15, self.channel_type_features, required),
+});
 
 impl Readable for CommitmentTransaction {
 	#[rustfmt::skip]

lightning/src/ln/channel.rs

@@ -2596,22 +2596,17 @@ pub(super) struct FundingScope {
 	minimum_depth_override: Option<u32>,
 }
 
-impl Writeable for FundingScope {
-	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
-		write_tlv_fields!(writer, {
-			(1, self.value_to_self_msat, required),
-			(3, self.counterparty_selected_channel_reserve_satoshis, option),
-			(5, self.holder_selected_channel_reserve_satoshis, required),
-			(7, self.channel_transaction_parameters, (required: ReadableArgs, None)),
-			(9, self.funding_transaction, option),
-			(11, self.funding_tx_confirmed_in, option),
-			(13, self.funding_tx_confirmation_height, required),
-			(15, self.short_channel_id, option),
-			(17, self.minimum_depth_override, option),
-		});
-		Ok(())
-	}
-}
+impl_writeable_tlv_based!(FundingScope, self, {
+	(1, self.value_to_self_msat, required),
+	(3, self.counterparty_selected_channel_reserve_satoshis, option),
+	(5, self.holder_selected_channel_reserve_satoshis, required),
+	(7, self.channel_transaction_parameters, (required: ReadableArgs, None)),
+	(9, self.funding_transaction, option),
+	(11, self.funding_tx_confirmed_in, option),
+	(13, self.funding_tx_confirmation_height, required),
+	(15, self.short_channel_id, option),
+	(17, self.minimum_depth_override, option),
+});
 
 impl Readable for FundingScope {
 	#[rustfmt::skip]

lightning/src/util/ser_macros.rs

@@ -839,6 +839,58 @@ macro_rules! write_tlv_fields {
 	}
 }
 
+#[doc(hidden)]
+#[macro_export]
+macro_rules! _tlv_fields_serialized_length {
+	({$(($type: expr, $field: expr, $fieldty: tt $(, $self: ident)?)),* $(,)*}) => { {
+		use $crate::util::ser::BigSize;
+		let len = {
+			#[allow(unused_mut)]
+			let mut len = $crate::util::ser::LengthCalculatingWriter(0);
+			$(
+				$crate::_get_varint_length_prefixed_tlv_length!(len, $type, &$field, $fieldty $(, $self)?);
+			)*
+			len.0
+		};
+		let mut len_calc = $crate::util::ser::LengthCalculatingWriter(0);
+		BigSize(len as u64).write(&mut len_calc).expect("No in-memory data may fail to serialize");
+		len + len_calc.0
+	} }
+}
+
+/// Implements [`Writeable`] for a type serialized as a length-prefixed TLV stream.
+///
+/// This is useful for types that share the TLV-writing format used by
+/// [`impl_ser_tlv_based`] but need a custom read implementation. The field list uses the
+/// same entries accepted by [`write_tlv_fields`], and the macro derives both `write` and
+/// `serialized_length` from that list so the two paths stay aligned.
+///
+/// The `$self` argument names the generated `self` binding, allowing field expressions to refer
+/// to it explicitly.
+///
+/// [`Writeable`]: crate::util::ser::Writeable
+/// [`impl_ser_tlv_based`]: crate::impl_ser_tlv_based
+/// [`write_tlv_fields`]: crate::write_tlv_fields
+macro_rules! impl_writeable_tlv_based {
+	($st: ty, $self: ident, {$(($type: expr, $field: expr, $fieldty: tt)),* $(,)*}) => {
+		impl $crate::util::ser::Writeable for $st {
+			fn write<W: $crate::util::ser::Writer>(&$self, writer: &mut W) -> Result<(), $crate::io::Error> {
+				write_tlv_fields!(writer, {
+					$(($type, $field, $fieldty)),*
+				});
+				Ok(())
+			}
+
+			#[inline]
+			fn serialized_length(&$self) -> usize {
+				$crate::_tlv_fields_serialized_length!({
+					$(($type, $field, $fieldty)),*
+				})
+			}
+		}
+	}
+}
+
 /// Reads a prefix added by [`write_ver_prefix`], above. Takes the current version of the
 /// serialization logic for this object. This is compared against the
 /// `$min_version_that_can_read_this` added by [`write_ver_prefix`].
@@ -1095,18 +1147,9 @@ macro_rules! impl_ser_tlv_based {
 
 			#[inline]
 			fn serialized_length(&self) -> usize {
-				use $crate::util::ser::BigSize;
-				let len = {
-					#[allow(unused_mut)]
-					let mut len = $crate::util::ser::LengthCalculatingWriter(0);
-					$(
-						$crate::_get_varint_length_prefixed_tlv_length!(len, $type, &self.$field, $fieldty, self);
-					)*
-					len.0
-				};
-				let mut len_calc = $crate::util::ser::LengthCalculatingWriter(0);
-				BigSize(len as u64).write(&mut len_calc).expect("No in-memory data may fail to serialize");
-				len + len_calc.0
+				$crate::_tlv_fields_serialized_length!({
+					$(($type, self.$field, $fieldty, self)),*
+				})
 			}
 		}