Add an explicit_type TLV syntax for avoiding certain cases of type inference

This new syntax is used to fix "dependency on fallback of ! -> ()".
This avoids cases where code compiles with a fallback of the
never type leading to the unit type. The behaviour in Rust edition 2024
would make this a compile error.

See: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/builtin/static.DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK.html#

Author: dunxen

lightning/src/ln/chan_utils.rs

@@ -1412,7 +1412,7 @@ impl Readable for CommitmentTransaction {
 			(8, keys, required),
 			(10, built, required),
 			(12, htlcs, required_vec),
-			(14, _legacy_deserialization_prevention_marker, option),
+			(14, _legacy_deserialization_prevention_marker, (option, explicit_type: ())),
 			(15, channel_type_features, option),
 		});
 

lightning/src/util/ser_macros.rs

@@ -281,6 +281,12 @@ macro_rules! _check_decoded_tlv_order {
 	($last_seen_type: expr, $typ: expr, $type: expr, $field: ident, option) => {{
 		// no-op
 	}};
+	($last_seen_type: expr, $typ: expr, $type: expr, $field: ident, (option, explicit_type: $fieldty: ty)) => {{
+		// no-op
+	}};
+	($last_seen_type: expr, $typ: expr, $type: expr, $field: ident, (required, explicit_type: $fieldty: ty)) => {{
+		_check_decoded_tlv_order!($last_seen_type, $typ, $type, $field, required);
+	}};
 	($last_seen_type: expr, $typ: expr, $type: expr, $field: ident, required_vec) => {{
 		$crate::_check_decoded_tlv_order!($last_seen_type, $typ, $type, $field, required);
 	}};
@@ -332,6 +338,12 @@ macro_rules! _check_missing_tlv {
 	($last_seen_type: expr, $type: expr, $field: ident, option) => {{
 		// no-op
 	}};
+	($last_seen_type: expr, $type: expr, $field: ident, (option, explicit_type: $fieldty: ty)) => {{
+		// no-op
+	}};
+	($last_seen_type: expr, $type: expr, $field: ident, (required, explicit_type: $fieldty: ty)) => {{
+		_check_missing_tlv!($last_seen_type, $type, $field, required);
+	}};
 	($last_seen_type: expr, $type: expr, $field: ident, optional_vec) => {{
 		// no-op
 	}};
@@ -372,6 +384,14 @@ macro_rules! _decode_tlv {
 	($outer_reader: expr, $reader: expr, $field: ident, option) => {{
 		$field = Some($crate::util::ser::Readable::read(&mut $reader)?);
 	}};
+	($outer_reader: expr, $reader: expr, $field: ident, (option, explicit_type: $fieldty: ty)) => {{
+		$field = Some($crate::util::ser::Readable::read(&mut $reader)?);
+		let field: $fieldty = _decode_tlv!($outer_reader, $reader, $field, option); $field = Some(field);
+	}};
+	($outer_reader: expr, $reader: expr, $field: ident, (required, explicit_type: $fieldty: ty)) => {{
+		$field = Some($crate::util::ser::Readable::read(&mut $reader)?);
+		let field: $fieldty = _decode_tlv!($outer_reader, $reader, $field, required); $field = field;
+	}};
 	($outer_reader: expr, $reader: expr, $field: ident, optional_vec) => {{
 		let f: $crate::util::ser::WithoutLength<Vec<_>> = $crate::util::ser::Readable::read(&mut $reader)?;
 		$field = Some(f.0);
@@ -795,6 +815,12 @@ macro_rules! _init_tlv_field_var {
 	($field: ident, optional_vec) => {
 		let mut $field = Some(Vec::new());
 	};
+	($field: ident, (option, explicit_type: $fieldty: ty)) => {
+		let mut $field: Option<$fieldty> = None;
+	};
+	($field: ident, (required, explicit_type: $fieldty: ty)) => {
+		let mut $field = $crate::util::ser::RequiredWrapper::<$fieldty>(None);
+	};
 	($field: ident, (option, encoding: ($fieldty: ty, $encoding: ident))) => {
 		$crate::_init_tlv_field_var!($field, option);
 	};