Remove Default impl from MemoBytes

Memo fields have two ways to encode an empty memo:

- 0xF6 followed by all-zeroes, encoding "there is no memo".
- All-zeroes, encoding the empty UTF-8 string.

In almost all cases you want the former, but users thinking about byte
slices may expect MemoBytes::default() to result in the latter. To
ensure clarity, we now require calling either MemoBytes::default() or
MemoBytes::from_bytes(&[]) to be explicit.

No such confusion exists for the Memo enum, because the two types are
visibly separated as different enum cases, and Memo::Empty makes sense
as the default.

zcash_client_backend/src/welding_rig.rs

@@ -345,7 +345,7 @@ mod tests {
             Some(extfvk.fvk.ovk),
             note.clone(),
             to,
-            MemoBytes::default(),
+            MemoBytes::empty(),
             &mut rng,
         );
         let cmu = note.cmu().to_repr().as_ref().to_owned();

zcash_client_sqlite/src/lib.rs

@@ -603,7 +603,7 @@ mod tests {
             Some(extfvk.fvk.ovk),
             note.clone(),
             to,
-            MemoBytes::default(),
+            MemoBytes::empty(),
             &mut rng,
         );
         let cmu = note.cmu().to_repr().as_ref().to_vec();
@@ -663,7 +663,7 @@ mod tests {
                 Some(extfvk.fvk.ovk),
                 note.clone(),
                 to,
-                MemoBytes::default(),
+                MemoBytes::empty(),
                 &mut rng,
             );
             let cmu = note.cmu().to_repr().as_ref().to_vec();
@@ -691,7 +691,7 @@ mod tests {
                 Some(extfvk.fvk.ovk),
                 note.clone(),
                 change_addr,
-                MemoBytes::default(),
+                MemoBytes::empty(),
                 &mut rng,
             );
             let cmu = note.cmu().to_repr().as_ref().to_vec();

zcash_primitives/benches/note_decryption.rs

@@ -36,7 +36,7 @@ fn bench_note_decryption(c: &mut Criterion) {
         let note = pa.create_note(value, rseed).unwrap();
         let cmu = note.cmu();
 
-        let mut ne = SaplingNoteEncryption::new(None, note, pa, MemoBytes::default(), &mut rng);
+        let mut ne = SaplingNoteEncryption::new(None, note, pa, MemoBytes::empty(), &mut rng);
         let ephemeral_key = ne.epk().clone().into();
         let enc_ciphertext = ne.encrypt_note_plaintext();
         let out_ciphertext = ne.encrypt_outgoing_plaintext(&cv, &cmu);

zcash_primitives/src/memo.rs

@@ -58,14 +58,6 @@ impl fmt::Debug for MemoBytes {
     }
 }
 
-impl Default for MemoBytes {
-    fn default() -> Self {
-        let mut bytes = [0u8; 512];
-        bytes[0] = 0xF6;
-        MemoBytes(Box::new(bytes))
-    }
-}
-
 impl PartialEq for MemoBytes {
     fn eq(&self, rhs: &MemoBytes) -> bool {
         self.0[..] == rhs.0[..]
@@ -87,10 +79,22 @@ impl Ord for MemoBytes {
 }
 
 impl MemoBytes {
-    /// Creates a `MemoBytes` from a slice.
+    /// Creates a `MemoBytes` indicating that no memo is present.
+    pub fn empty() -> Self {
+        let mut bytes = [0u8; 512];
+        bytes[0] = 0xF6;
+        MemoBytes(Box::new(bytes))
+    }
+
+    /// Creates a `MemoBytes` from a slice, exactly as provided.
     ///
     /// Returns an error if the provided slice is longer than 512 bytes. Slices shorter
     /// than 512 bytes are padded with null bytes.
+    ///
+    /// Note that passing an empty slice to this API (or an all-zeroes slice) will result
+    /// in a memo representing an empty string. What you almost certainly want in this
+    /// case is [`MemoBytes::empty`], which uses a specific encoding to indicate that no
+    /// memo is present.
     pub fn from_bytes(bytes: &[u8]) -> Result<Self, Error> {
         if bytes.len() > 512 {
             return Err(Error::TooLong(bytes.len()));
@@ -220,7 +224,7 @@ impl From<&Memo> for MemoBytes {
     /// Serializes the `Memo` per ZIP 302.
     fn from(memo: &Memo) -> Self {
         match memo {
-            Memo::Empty => MemoBytes::default(),
+            Memo::Empty => MemoBytes::empty(),
             Memo::Text(s) => {
                 let mut bytes = [0u8; 512];
                 let s_bytes = s.0.as_bytes();

zcash_primitives/src/note_encryption.rs

@@ -135,7 +135,7 @@ pub fn prf_ock(
 /// let note = to.create_note(value, Rseed::BeforeZip212(rcm)).unwrap();
 /// let cmu = note.cmu();
 ///
-/// let mut enc = SaplingNoteEncryption::new(ovk, note, to, MemoBytes::default(), &mut rng);
+/// let mut enc = SaplingNoteEncryption::new(ovk, note, to, MemoBytes::empty(), &mut rng);
 /// let encCiphertext = enc.encrypt_note_plaintext();
 /// let outCiphertext = enc.encrypt_outgoing_plaintext(&cv.commitment().into(), &cmu);
 /// ```
@@ -681,8 +681,7 @@ mod tests {
         let cmu = note.cmu();
 
         let ovk = OutgoingViewingKey([0; 32]);
-        let mut ne =
-            SaplingNoteEncryption::new(Some(ovk), note, pa, MemoBytes::default(), &mut rng);
+        let mut ne = SaplingNoteEncryption::new(Some(ovk), note, pa, MemoBytes::empty(), &mut rng);
         let epk = ne.epk().clone().into();
         let enc_ciphertext = ne.encrypt_note_plaintext();
         let out_ciphertext = ne.encrypt_outgoing_plaintext(&cv, &cmu);

zcash_primitives/src/transaction/builder.rs

@@ -142,7 +142,7 @@ impl SaplingOutput {
             ovk,
             to,
             note,
-            memo: memo.unwrap_or_default(),
+            memo: memo.unwrap_or_else(MemoBytes::empty),
         })
     }