Remove the network-specific 0xf6 convention for empty memos

CHANGELOG.md

@@ -95,8 +95,7 @@ The entries below are relative to the `zcash_primitives::sapling` module as of
   - `SaplingBuilder::add_spend` now takes `extsk` by reference. Also, it no
     longer takes a `diversifier` argument as the diversifier may be obtained
     from the note.
-  - `SaplingBuilder::add_output` now takes an `Option<[u8; 512]>` memo instead
-    of a `MemoBytes`.
+  - `SaplingBuilder::add_output` now takes `[u8; 512]` instead of `MemoBytes`.
   - `SaplingBuilder::build` no longer takes a prover, proving context, progress
     notifier, or target height. Instead, it has `SpendProver, OutputProver`
     generic parameters and returns `(UnauthorizedBundle, SaplingMetadata)`. The

src/builder.rs

@@ -142,7 +142,7 @@ pub struct SaplingOutputInfo {
     /// `None` represents the `ovk = ⊥` case.
     ovk: Option<OutgoingViewingKey>,
     note: Note,
-    memo: Option<[u8; 512]>,
+    memo: [u8; 512],
     rcv: ValueCommitTrapdoor,
 }
 
@@ -165,7 +165,7 @@ impl SaplingOutputInfo {
             None,
             dummy_to,
             NoteValue::from_raw(0),
-            None,
+            [0u8; 512],
             zip212_enforcement,
         )
     }
@@ -175,7 +175,7 @@ impl SaplingOutputInfo {
         ovk: Option<OutgoingViewingKey>,
         to: PaymentAddress,
         value: NoteValue,
-        memo: Option<[u8; 512]>,
+        memo: [u8; 512],
         zip212_enforcement: Zip212Enforcement,
     ) -> Self {
         let rseed = generate_random_rseed_internal(zip212_enforcement, rng);
@@ -194,16 +194,7 @@ impl SaplingOutputInfo {
         self,
         rng: &mut R,
     ) -> OutputDescription<circuit::Output> {
-        let encryptor = sapling_note_encryption::<R>(
-            self.ovk,
-            self.note.clone(),
-            self.memo.unwrap_or_else(|| {
-                let mut memo = [0; 512];
-                memo[0] = 0xf6;
-                memo
-            }),
-            rng,
-        );
+        let encryptor = sapling_note_encryption::<R>(self.ovk, self.note.clone(), self.memo, rng);
 
         // Construct the value commitment.
         let cv = ValueCommitment::derive(self.note.value(), self.rcv.clone());
@@ -381,7 +372,7 @@ impl SaplingBuilder {
         ovk: Option<OutgoingViewingKey>,
         to: PaymentAddress,
         value: NoteValue,
-        memo: Option<[u8; 512]>,
+        memo: [u8; 512],
     ) -> Result<(), Error> {
         let output = SaplingOutputInfo::new_internal(
             &mut rng,