Disallow holders from selecting 0-reserve in legacy channels

diff --git a/fuzz/src/chanmon_consistency.rs b/fuzz/src/chanmon_consistency.rs
index 4a182c3..4ff0e4a 100644
--- a/fuzz/src/chanmon_consistency.rs
+++ b/fuzz/src/chanmon_consistency.rs
@@ -628,7 +628,7 @@ fn assert_action_timeout_awaiting_response(action: &msgs::ErrorAction) {
 	);
 }
 
-#[derive(Copy, Clone)]
+#[derive(Clone, Copy, PartialEq)]
 enum ChanType {
 	Legacy,
 	KeyedAnchors,
@@ -2082,19 +2082,20 @@ impl<'a, Out: Output + MaybeSend + MaybeSync> Harness<'a, Out> {
 		connect_peers(&nodes[0], &nodes[1]);
 		connect_peers(&nodes[1], &nodes[2]);
 
+		let set_0reserve = chan_type != ChanType::Legacy;
 		// Create 3 channels between A-B and 3 channels between B-C (6 total).
 		//
 		// Use distinct version numbers for each funding transaction so each test
 		// channel gets its own txid and funding outpoint.
 		// A-B: channel 2 A and B have 0-reserve (trusted open + trusted accept),
-		//      channel 3 A has 0-reserve (trusted accept).
+		//      channel 3 A has 0-reserve (trusted accept), if channels are non-legacy.
 		make_channel(&nodes[0], &nodes[1], 1, false, false, &mut chain_state);
-		make_channel(&nodes[0], &nodes[1], 2, true, true, &mut chain_state);
-		make_channel(&nodes[0], &nodes[1], 3, false, true, &mut chain_state);
+		make_channel(&nodes[0], &nodes[1], 2, set_0reserve, set_0reserve, &mut chain_state);
+		make_channel(&nodes[0], &nodes[1], 3, false, set_0reserve, &mut chain_state);
 		// B-C: channel 4 B has 0-reserve (via trusted accept),
-		//      channel 5 C has 0-reserve (via trusted open).
-		make_channel(&nodes[1], &nodes[2], 4, false, true, &mut chain_state);
-		make_channel(&nodes[1], &nodes[2], 5, true, false, &mut chain_state);
+		//      channel 5 C has 0-reserve (via trusted open), if channels are non-legacy.
+		make_channel(&nodes[1], &nodes[2], 4, false, set_0reserve, &mut chain_state);
+		make_channel(&nodes[1], &nodes[2], 5, set_0reserve, false, &mut chain_state);
 		make_channel(&nodes[1], &nodes[2], 6, false, false, &mut chain_state);
 
 		// Wipe the transactions-broadcasted set to make sure we don't broadcast
diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index 863256f..063b530 100644
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -3832,6 +3832,14 @@ impl<SP: SignerProvider> ChannelContext<SP> {
 				"Funding must be smaller than the total bitcoin supply. It was {channel_value_satoshis}"
 			)));
 		}
+		if !channel_type.supports_anchors_zero_fee_htlc_tx()
+			&& !channel_type.supports_anchor_zero_fee_commitments()
+			&& holder_selected_channel_reserve_satoshis == 0
+		{
+			return Err(ChannelError::close(
+				"0-reserve is not allowed on legacy channels".to_owned(),
+			));
+		}
 		if msg_channel_reserve_satoshis > channel_value_satoshis {
 			return Err(ChannelError::close(format!(
 				"Bogus channel_reserve_satoshis ({msg_channel_reserve_satoshis}). Must be no greater than channel_value_satoshis: {channel_value_satoshis}"
@@ -4323,6 +4331,14 @@ impl<SP: SignerProvider> ChannelContext<SP> {
 		}
 
 		let channel_type = get_initial_channel_type(&config, their_features);
+		if !channel_type.supports_anchors_zero_fee_htlc_tx()
+			&& !channel_type.supports_anchor_zero_fee_commitments()
+			&& holder_selected_channel_reserve_satoshis == 0
+		{
+			return Err(APIError::APIMisuseError {
+				err: "0-reserve is not allowed on legacy channels".to_owned(),
+			});
+		}
 		debug_assert!(!channel_type.supports_any_optional_bits());
 		debug_assert!(!channel_type
 			.requires_unknown_bits_from(&channelmanager::provided_channel_type_features(&config)));
@@ -4869,6 +4885,14 @@ impl<SP: SignerProvider> ChannelContext<SP> {
 		}
 
 		let channel_type = funding.get_channel_type();
+		if !channel_type.supports_anchors_zero_fee_htlc_tx()
+			&& !channel_type.supports_anchor_zero_fee_commitments()
+			&& funding.holder_selected_channel_reserve_satoshis == 0
+		{
+			return Err(ChannelError::close(
+				"0-reserve is not allowed on legacy channels".to_owned(),
+			));
+		}
 		if common_fields.max_accepted_htlcs > max_htlcs(channel_type) {
 			return Err(ChannelError::close(format!(
 				"max_accepted_htlcs was {}. It must not be larger than {}",
@@ -6576,6 +6600,13 @@ impl<SP: SignerProvider> ChannelContext<SP> {
 		}
 
 		let next_channel_type = get_initial_channel_type(user_config, &eligible_features);
+		if !next_channel_type.supports_anchors_zero_fee_htlc_tx()
+			&& !next_channel_type.supports_anchor_zero_fee_commitments()
+			&& funding.holder_selected_channel_reserve_satoshis == 0
+		{
+			// 0-reserve is not allowed on legacy channels
+			return Err(());
+		}
 
 		self.feerate_per_kw =
 			selected_commitment_sat_per_1000_weight(&fee_estimator, &next_channel_type);
diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index 19fd2f9..55fe83b 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -3617,6 +3617,8 @@ pub enum TrustedChannelFeatures {
 	/// with a revoked commitment transaction *for free*.
 	///
 	/// Note that there is no guarantee that the counterparty accepts such a channel themselves.
+	///
+	/// The zero-reserve feature is not allowed on legacy / anchorless channels.
 	ZeroReserve,
 	/// Sets the combination of [`TrustedChannelFeatures::ZeroConf`] and [`TrustedChannelFeatures::ZeroReserve`]
 	ZeroConfZeroReserve,
@@ -3873,6 +3875,8 @@ impl<
 	/// transaction *for free*.
 	///
 	/// Note that there is no guarantee that the counterparty accepts such a channel.
+	///
+	/// The zero-reserve feature is not allowed on legacy / anchorless channels.
 	pub fn create_channel_to_trusted_peer_0reserve(
 		&self, their_network_key: PublicKey, channel_value_satoshis: u64, push_msat: u64,
 		user_channel_id: u128, temporary_channel_id: Option<ChannelId>,