f - add clarifying comments

Author: jkczyz

lightning/src/offers/offer.rs

@@ -384,9 +384,13 @@ macro_rules! offer_builder_methods { (
 	}
 
 	fn build_without_checks($($self_mut)* $self: $self_type) -> Offer {
-		// Create the metadata for stateless verification of an InvoiceRequest.
 		if let Some(mut metadata) = $self.offer.metadata.take() {
+
+			// Create the metadata for stateless verification of an InvoiceRequest.
 			if metadata.has_derivation_material() {
+
+				// Don't derive keys if no blinded paths were given since this means the signing
+				// pubkey must be the node id of an announced node.
 				if $self.offer.paths.is_none() {
 					metadata = metadata.without_keys();
 				}
@@ -398,6 +402,9 @@ macro_rules! offer_builder_methods { (
 					tlv_stream.node_id = None;
 				}
 
+				// Either replace the signing pubkey with the derived pubkey or include the metadata
+				// for verification. In the former case, the blinded paths must include the metadata
+				// instead.
 				let (derived_metadata, keys) = metadata.derive_from(tlv_stream, $self.secp_ctx);
 				match keys {
 					Some(keys) => $self.offer.signing_pubkey = Some(keys.public_key()),