Add basic_mpp support to InvoiceBuilder

Since InvoiceFeatures are an implementation detail of InvoiceBuilder, an
explicit call is needed to support the basic_mpp feature. Since it is
dependent on the payment_secret feature, conditionally define the
builder's method only when payment_secret has been set.

Author: jkczyz

lightning-invoice/src/lib.rs

@@ -607,6 +607,20 @@ impl<D: tb::Bool, H: tb::Bool, T: tb::Bool, C: tb::Bool> InvoiceBuilder<D, H, T,
 	}
 }
 
+impl<D: tb::Bool, H: tb::Bool, T: tb::Bool, C: tb::Bool> InvoiceBuilder<D, H, T, C, tb::True> {
+	/// Sets the `basic_mpp` feature as optional.
+	pub fn basic_mpp(mut self) -> Self {
+		self.tagged_fields = self.tagged_fields
+			.drain(..)
+			.map(|field| match field {
+				TaggedField::Features(f) => TaggedField::Features(f.set_basic_mpp_optional()),
+				_ => field,
+			})
+			.collect();
+		self
+	}
+}
+
 impl<S: tb::Bool> InvoiceBuilder<tb::True, tb::True, tb::True, tb::True, S> {
 	/// Builds and signs an invoice using the supplied `sign_function`. This function MAY NOT fail
 	/// and MUST produce a recoverable signature valid for the given hash and if applicable also for

lightning-invoice/src/utils.rs

@@ -64,6 +64,7 @@ where
 		.payee_pub_key(our_node_pubkey)
 		.payment_hash(Hash::from_slice(&payment_hash.0).unwrap())
 		.payment_secret(payment_secret)
+		.basic_mpp()
 		.min_final_cltv_expiry(MIN_FINAL_CLTV_EXPIRY.into());
 	if let Some(amt) = amt_msat {
 		invoice = invoice.amount_pico_btc(amt * 10);