Make `TransactionBuilder::AddOrchardOutput` memo optional

This replaces the previous use of the `NO_MEMO` constant, instead
passing the optionality through to Rust for handling.

src/gtest/test_transaction_builder.cpp

@@ -278,7 +278,7 @@ TEST(TransactionBuilder, TransparentToOrchard)
     // 0.0005 t-ZEC in, 0.0004 z-ZEC out, default fee
     auto builder = TransactionBuilder(consensusParams, 1, orchardAnchor, &keystore);
     builder.AddTransparentInput(COutPoint(uint256S("1234"), 0), scriptPubKey, 50000);
-    builder.AddOrchardOutput(std::nullopt, recipient, 40000);
+    builder.AddOrchardOutput(std::nullopt, recipient, 40000, std::nullopt);
     auto maybeTx = builder.Build();
     EXPECT_TRUE(maybeTx.IsTx());
     if (maybeTx.IsError()) {

src/rust/include/rust/builder.h

@@ -35,6 +35,8 @@ void orchard_builder_free(OrchardBuilderPtr* ptr);
 ///
 /// `ovk` is a pointer to the outgoing viewing key to make this recipient recoverable by,
 /// or `null` to make the recipient unrecoverable by the sender.
+///
+/// `memo` is a pointer to the 512-byte memo field encoding, or `null` for "no memo".
 bool orchard_builder_add_recipient(
     OrchardBuilderPtr* ptr,
     const unsigned char* ovk,

src/rust/src/builder_ffi.rs

@@ -46,9 +46,9 @@ pub extern "C" fn orchard_builder_add_recipient(
         .map(OutgoingViewingKey::from);
     let recipient = unsafe { recipient.as_ref() }.expect("Recipient may not be null.");
     let value = NoteValue::from_raw(value);
-    let memo = unsafe { memo.as_ref() }.expect("Memo may not be null.");
+    let memo = unsafe { memo.as_ref() }.copied();
 
-    match builder.add_recipient(ovk, *recipient, value, Some(*memo)) {
+    match builder.add_recipient(ovk, *recipient, value, memo) {
         Ok(()) => true,
         Err(e) => {
             error!("Failed to add Orchard recipient: {}", e);

src/transaction_builder.cpp

@@ -45,7 +45,7 @@ void Builder::AddOutput(
     const std::optional<uint256>& ovk,
     const libzcash::OrchardRawAddress& to,
     CAmount value,
-    std::array<unsigned char, ZC_MEMO_SIZE> memo)
+    const std::optional<std::array<unsigned char, ZC_MEMO_SIZE>>& memo)
 {
     if (!inner) {
         throw std::logic_error("orchard::Builder has already been used");
@@ -56,7 +56,7 @@ void Builder::AddOutput(
         ovk.has_value() ? ovk->begin() : nullptr,
         to.inner.get(),
         value,
-        memo.data());
+        memo.has_value() ? memo->data() : nullptr);
 }
 
 std::optional<UnauthorizedBundle> Builder::Build() {
@@ -264,7 +264,7 @@ void TransactionBuilder::AddOrchardOutput(
     const std::optional<uint256>& ovk,
     const libzcash::OrchardRawAddress& to,
     CAmount value,
-    std::array<unsigned char, ZC_MEMO_SIZE> memo)
+    const std::optional<std::array<unsigned char, ZC_MEMO_SIZE>>& memo)
 {
     if (!orchardBuilder.has_value()) {
         throw std::runtime_error("TransactionBuilder cannot add Orchard output without Orchard anchor");

src/transaction_builder.h

@@ -64,7 +64,7 @@ public:
         const std::optional<uint256>& ovk,
         const libzcash::OrchardRawAddress& to,
         CAmount value,
-        std::array<unsigned char, ZC_MEMO_SIZE> memo = NO_MEMO);
+        const std::optional<std::array<unsigned char, ZC_MEMO_SIZE>>& memo);
 
     /// Builds a bundle containing the given spent notes and recipients.
     ///
@@ -289,7 +289,7 @@ public:
         const std::optional<uint256>& ovk,
         const libzcash::OrchardRawAddress& to,
         CAmount value,
-        std::array<unsigned char, ZC_MEMO_SIZE> memo = NO_MEMO);
+        const std::optional<std::array<unsigned char, ZC_MEMO_SIZE>>& memo);
 
     // Throws if the anchor does not match the anchor used by
     // previously-added Sapling spends.