Make error a type member of FromPayload trait rather than a type variable.

Fix demo test compilation errors.

zcash_extensions/src/transparent/demo.rs

@@ -18,14 +18,15 @@
 //! - `tx_b`: `[ TzeIn(tx_a, preimage_1) -> TzeOut(value, hash_2) ]`
 //! - `tx_c`: `[ TzeIn(tx_b, preimage_2) -> [any output types...] ]`
 
-use blake2b_simd::Params;
 use std::convert::TryFrom;
 use std::fmt;
 
-use zcash_primitives::extensions::transparent::{
-    Extension, ExtensionTxBuilder, FromPayload, ToPayload,
+use blake2b_simd::Params;
+
+use zcash_primitives::{
+    extensions::transparent::{Extension, ExtensionTxBuilder, FromPayload, ToPayload},
+    transaction::components::{amount::Amount, OutPoint, TzeOut},
 };
-use zcash_primitives::transaction::components::{amount::Amount, OutPoint, TzeOut};
 
 mod open {
     pub const MODE: usize = 0;
@@ -100,8 +101,10 @@ impl TryFrom<(usize, Precondition)> for Precondition {
     }
 }
 
-impl FromPayload<Error> for Precondition {
-    fn from_payload(mode: usize, payload: &[u8]) -> Result<Self, Error> {
+impl FromPayload for Precondition {
+    type Error = Error;
+
+    fn from_payload(mode: usize, payload: &[u8]) -> Result<Self, Self::Error> {
         match mode {
             open::MODE => {
                 if payload.len() == 32 {
@@ -163,8 +166,10 @@ impl TryFrom<(usize, Witness)> for Witness {
     }
 }
 
-impl FromPayload<Error> for Witness {
-    fn from_payload(mode: usize, payload: &[u8]) -> Result<Self, Error> {
+impl FromPayload for Witness {
+    type Error = Error;
+
+    fn from_payload(mode: usize, payload: &[u8]) -> Result<Self, Self::Error> {
         match mode {
             open::MODE => {
                 if payload.len() == 32 {
@@ -361,12 +366,14 @@ impl<'a, B: ExtensionTxBuilder<'a>> DemoBuilder<'a, B> {
 #[cfg(test)]
 mod tests {
     use blake2b_simd::Params;
-    use zcash_extensions_api::transparent::{self as tze, Extension, FromPayload, ToPayload};
 
     use super::{close, open, Context, Precondition, Program, Witness};
-    use crate::transaction::{
-        components::{Amount, OutPoint, TzeIn, TzeOut},
-        Transaction, TransactionData,
+    use zcash_primitives::{
+        extensions::transparent::{self as tze, Extension, FromPayload, ToPayload},
+        transaction::{
+            components::{Amount, OutPoint, TzeIn, TzeOut},
+            Transaction, TransactionData,
+        },
     };
 
     #[test]
@@ -476,7 +483,7 @@ mod tests {
             precondition: tze::Precondition::from(0, &Precondition::open(hash_1)),
         };
 
-        println!("{:x?}", precondition.payload);
+        // println!("{:x?}", precondition.payload);
 
         let mut mtx_a = TransactionData::nu4();
         mtx_a.tze_outputs.push(out_a);

zcash_primitives/src/extensions/transparent.rs

@@ -3,9 +3,11 @@
 use crate::transaction::components::{Amount, OutPoint, TzeOut};
 use std::fmt;
 
-pub trait FromPayload<E>: Sized {
+pub trait FromPayload: Sized {
+    type Error;
+
     /// Parses an extension type from a mode and payload.
-    fn from_payload(mode: usize, payload: &[u8]) -> Result<Self, E>;
+    fn from_payload(mode: usize, payload: &[u8]) -> Result<Self, Self::Error>;
 }
 
 pub trait ToPayload {
@@ -30,6 +32,10 @@ impl Precondition {
             payload,
         }
     }
+
+    pub fn try_to<P: FromPayload>(&self) -> Result<P, P::Error> {
+        P::from_payload(self.mode, &self.payload)
+    }
 }
 
 /// Data that satisfies the precondition for prior encumbered funds, enabling them to be
@@ -96,8 +102,8 @@ pub trait Extension<C> {
         context: &C,
     ) -> Result<(), Self::Error>
     where
-        Self::P: FromPayload<Self::Error>,
-        Self::W: FromPayload<Self::Error>,
+        Self::P: FromPayload<Error = Self::Error>,
+        Self::W: FromPayload<Error = Self::Error>,
     {
         self.verify_inner(
             &Self::P::from_payload(precondition.mode, &precondition.payload)?,

zcash_primitives/src/transaction/builder.rs

@@ -275,6 +275,10 @@ struct TzeInputs<'a, BuildCtx> {
 }
 
 impl<'a, BuildCtx> TzeInputs<'a, BuildCtx> {
+    fn default() -> Self {
+        TzeInputs { builders: vec![] }
+    }
+
     fn push<WBuilder, W: ToPayload>(
         &mut self,
         extension_id: usize,
@@ -392,7 +396,7 @@ impl<'a, P: consensus::Parameters, R: RngCore + CryptoRng> Builder<'a, P, R> {
             spends: vec![],
             outputs: vec![],
             transparent_inputs: TransparentInputs::default(),
-            tze_inputs: TzeInputs { builders: vec![] },
+            tze_inputs: TzeInputs::default(),
             change_address: None,
             phantom: PhantomData,
         }
@@ -815,7 +819,6 @@ mod tests {
     use rand_core::OsRng;
     use std::marker::PhantomData;
 
-    use super::{Builder, Error};
     use crate::{
         consensus,
         consensus::TestNetwork,
@@ -828,6 +831,8 @@ mod tests {
         zip32::{ExtendedFullViewingKey, ExtendedSpendingKey},
     };
 
+    use super::{Builder, Error, TzeInputs};
+
     #[test]
     fn fails_on_negative_output() {
         let extsk = ExtendedSpendingKey::master(&[]);
@@ -854,7 +859,7 @@ mod tests {
             TestNetwork::activation_height(NetworkUpgrade::Sapling).unwrap();
 
         // Create a builder with 0 fee, so we can construct t outputs
-        let mut builder = builder::Builder::<TestNetwork, OsRng> {
+        let mut builder = builder::Builder::<'_, TestNetwork, OsRng> {
             rng: OsRng,
             height: sapling_activation_height,
             mtx: TransactionData::new(),
@@ -863,6 +868,7 @@ mod tests {
             spends: vec![],
             outputs: vec![],
             transparent_inputs: TransparentInputs::default(),
+            tze_inputs: TzeInputs::default(),
             change_address: None,
             phantom: PhantomData,
         };