The return type of ChangeValue::new is now optional; it returns None

if a memo is given for the transparent pool. Use `ChangeValue::shielded`
to avoid this error case when creating a `ChangeValue` known to be for a
shielded pool.

Co-authored-by: Jack Grigg <jack@electriccoin.co>
Signed-off-by: Daira-Emma Hopwood <daira@jacaranda.org>

zcash_client_backend/CHANGELOG.md

@@ -10,7 +10,7 @@ and this library adheres to Rust's notion of
 ### Added
 - `zcash_client_backend::data_api`:
   - `chain::BlockCache` trait, behind the `sync` feature flag.
-- `zcash_client_backend::fees::ChangeValue::transparent`
+- `zcash_client_backend::fees::ChangeValue::{transparent, shielded}`
 - `zcash_client_backend::scanning`:
   - `testing` module
 - `zcash_client_backend::sync` module, behind the `sync` feature flag.
@@ -31,6 +31,10 @@ and this library adheres to Rust's notion of
   - The return type of `ChangeValue::output_pool`, and the type of the
     `output_pool` argument to `ChangeValue::new`, have changed from
     `ShieldedProtocol` to `zcash_protocol::PoolType`.
+  - The return type of `ChangeValue::new` is now optional; it returns `None`
+    if a memo is given for the transparent pool. Use `ChangeValue::shielded`
+    to avoid this error case when creating a `ChangeValue` known to be for a
+    shielded pool.
 - `zcash_client_backend::wallet::Recipient` variants have changed. Instead of
   wrapping protocol-address types, the `Recipient` type now wraps a
   `zcash_address::ZcashAddress`. This simplifies the process of tracking the

zcash_client_backend/src/fees.rs

@@ -31,12 +31,16 @@ pub struct ChangeValue {
 
 impl ChangeValue {
     /// Constructs a new change value from its constituent parts.
-    pub fn new(output_pool: PoolType, value: NonNegativeAmount, memo: Option<MemoBytes>) -> Self {
-        Self {
+    pub fn new(
+        output_pool: PoolType,
+        value: NonNegativeAmount,
+        memo: Option<MemoBytes>,
+    ) -> Option<Self> {
+        (matches!(output_pool, PoolType::Shielded(_)) || memo.is_none()).then_some(Self {
             output_pool,
             value,
             memo,
-        }
+        })
     }
 
     /// Constructs a new change value that will be created as a transparent output.
@@ -48,23 +52,28 @@ impl ChangeValue {
         }
     }
 
-    /// Constructs a new change value that will be created as a Sapling output.
-    pub fn sapling(value: NonNegativeAmount, memo: Option<MemoBytes>) -> Self {
+    /// Constructs a new change value that will be created as a shielded output.
+    pub fn shielded(
+        protocol: ShieldedProtocol,
+        value: NonNegativeAmount,
+        memo: Option<MemoBytes>,
+    ) -> Self {
         Self {
-            output_pool: PoolType::Shielded(ShieldedProtocol::Sapling),
+            output_pool: PoolType::Shielded(protocol),
             value,
             memo,
         }
     }
 
+    /// Constructs a new change value that will be created as a Sapling output.
+    pub fn sapling(value: NonNegativeAmount, memo: Option<MemoBytes>) -> Self {
+        Self::shielded(ShieldedProtocol::Sapling, value, memo)
+    }
+
     /// Constructs a new change value that will be created as an Orchard output.
     #[cfg(feature = "orchard")]
     pub fn orchard(value: NonNegativeAmount, memo: Option<MemoBytes>) -> Self {
-        Self {
-            output_pool: PoolType::Shielded(ShieldedProtocol::Orchard),
-            value,
-            memo,
-        }
+        Self::shielded(ShieldedProtocol::Orchard, value, memo)
     }
 
     /// Returns the pool to which the change output should be sent.

zcash_client_backend/src/fees/common.rs

@@ -6,7 +6,7 @@ use zcash_primitives::{
         fees::{transparent, FeeRule},
     },
 };
-use zcash_protocol::{PoolType, ShieldedProtocol};
+use zcash_protocol::ShieldedProtocol;
 
 use super::{
     sapling as sapling_fees, ChangeError, ChangeValue, DustAction, DustOutputPolicy,
@@ -219,8 +219,8 @@ where
                     })
                 }
                 DustAction::AllowDustChange => TransactionBalance::new(
-                    vec![ChangeValue::new(
-                        PoolType::Shielded(change_pool),
+                    vec![ChangeValue::shielded(
+                        change_pool,
                         proposed_change,
                         change_memo,
                     )],
@@ -235,8 +235,8 @@ where
             }
         } else {
             TransactionBalance::new(
-                vec![ChangeValue::new(
-                    PoolType::Shielded(change_pool),
+                vec![ChangeValue::shielded(
+                    change_pool,
                     proposed_change,
                     change_memo,
                 )],