Put Default behind cfg_attr(test)

Rationale
---------

We avoid implementing `Default` on consensus-critical types because it's
easy to miss an incorrect use in a review. It's easy to hide a
`default()` in a call like `unwrap_or_default()` or even more subtle
methods.

zebra-chain/src/block/hash.rs

@@ -20,8 +20,8 @@ use proptest_derive::Arbitrary;
 ///
 /// Note: Zebra displays transaction and block hashes in big-endian byte-order,
 /// following the u256 convention set by Bitcoin and zcashd.
-#[derive(Copy, Clone, Eq, PartialEq, Hash, Serialize, Deserialize, Default)]
-#[cfg_attr(any(test, feature = "proptest-impl"), derive(Arbitrary))]
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Serialize, Deserialize)]
+#[cfg_attr(any(test, feature = "proptest-impl"), derive(Arbitrary, Default))]
 pub struct Hash(pub [u8; 32]);
 
 impl Hash {

zebra-chain/src/block/header.rs

@@ -22,7 +22,7 @@ use proptest_derive::Arbitrary;
 /// backwards reference (previous header hash) present in the block
 /// header. Each block points backwards to its parent, all the way
 /// back to the genesis block (the first block in the blockchain).
-#[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize, Default)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
 pub struct Header {
     /// The block's version field. This is supposed to be `4`:
     ///

zebra-chain/src/block/merkle.rs

@@ -69,8 +69,8 @@ use proptest_derive::Arbitrary;
 /// aggressive anti-DoS mechanism.
 ///
 /// [ZIP-244]: https://zips.z.cash/zip-0244
-#[derive(Clone, Copy, Eq, PartialEq, Serialize, Deserialize, Default)]
-#[cfg_attr(any(test, feature = "proptest-impl"), derive(Arbitrary))]
+#[derive(Clone, Copy, Eq, PartialEq, Serialize, Deserialize)]
+#[cfg_attr(any(test, feature = "proptest-impl"), derive(Arbitrary, Default))]
 pub struct Root(pub [u8; 32]);
 
 impl fmt::Debug for Root {

zebra-chain/src/sapling/commitment.rs

@@ -157,7 +157,8 @@ impl NoteCommitment {
 /// [`NotSmallOrderValueCommitment`].
 ///
 /// <https://zips.z.cash/protocol/protocol.pdf#concretehomomorphiccommit>
-#[derive(Clone, Copy, Deserialize, PartialEq, Eq, Serialize, Default)]
+#[derive(Clone, Copy, Deserialize, PartialEq, Eq, Serialize)]
+#[cfg_attr(any(test, feature = "proptest-impl"), derive(Default))]
 pub struct ValueCommitment(#[serde(with = "serde_helpers::AffinePoint")] jubjub::AffinePoint);
 
 impl<'a> std::ops::Add<&'a ValueCommitment> for ValueCommitment {
@@ -301,7 +302,8 @@ lazy_static! {
 ///
 /// <https://zips.z.cash/protocol/protocol.pdf#spenddesc>
 /// <https://zips.z.cash/protocol/protocol.pdf#outputdesc>
-#[derive(Debug, Clone, Copy, Deserialize, PartialEq, Eq, Serialize, Default)]
+#[derive(Debug, Clone, Copy, Deserialize, PartialEq, Eq, Serialize)]
+#[cfg_attr(any(test, feature = "proptest-impl"), derive(Default))]
 pub struct NotSmallOrderValueCommitment(ValueCommitment);
 
 impl TryFrom<ValueCommitment> for NotSmallOrderValueCommitment {

zebra-chain/src/work/difficulty.rs

@@ -62,7 +62,8 @@ mod tests;
 /// > (see equations in the Zcash Specification [section 7.7.4])
 ///
 /// [section 7.7.4]: https://zips.z.cash/protocol/protocol.pdf#nbits
-#[derive(Clone, Copy, Eq, PartialEq, Serialize, Deserialize, Default)]
+#[derive(Clone, Copy, Eq, PartialEq, Serialize, Deserialize)]
+#[cfg_attr(any(test, feature = "proptest-impl"), derive(Default))]
 pub struct CompactDifficulty(pub(crate) u32);
 
 /// An invalid CompactDifficulty value, for testing.

zebra-chain/src/work/equihash.rs

@@ -93,6 +93,7 @@ impl Clone for Solution {
 
 impl Eq for Solution {}
 
+#[cfg(any(test, feature = "proptest-impl"))]
 impl Default for Solution {
     fn default() -> Self {
         Self([0; SOLUTION_SIZE])