Merge rust-bitcoin/rust-bitcoin#3386: transaction: Remove `Default` implementations

d0a30096b4 transaction: Remove Default implementations (Tobin C. Harding)

Pull request description:

  Currently we provide `Default` implementations for a couple of types in the `transaction` module, the values returned are meaningless and it seems these impls were added to make writing test code easier. In hindsight this was the wrong thing to do.

  Break the API and remove the `Default` implementations for `OutPoint` and `TxIn`.

  Add an associated const `TxIn::EMPTY_COINBASE` that is, as the name suggests, an empty transaction input with the prevout set to all zeros as for the coinbase transaction.

ACKs for top commit:
  apoelstra:
    ACK d0a30096b4 successfully ran local tests

Tree-SHA512: f76c2984d5f57fb6cb0792b4feb555434dd4ad3439926d30fe1be42432ace990888ad61d724fe701cb30b6f201f66b4670b7c59cc8de82d7a61ee71883a7fbd7

bitcoin/src/blockdata/transaction.rs

@@ -127,10 +127,6 @@ impl OutPoint {
     pub fn is_null(&self) -> bool { *self == OutPoint::null() }
 }
 
-impl Default for OutPoint {
-    fn default() -> Self { OutPoint::null() }
-}
-
 impl fmt::Display for OutPoint {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "{}:{}", self.txid, self.vout)
@@ -249,6 +245,14 @@ pub struct TxIn {
 }
 
 impl TxIn {
+    /// An empty transaction input with the previous output as for a coinbase transaction.
+    pub const EMPTY_COINBASE: TxIn = TxIn {
+        previous_output: OutPoint::COINBASE_PREVOUT,
+        script_sig: ScriptBuf::new(),
+        sequence: Sequence::MAX,
+        witness: Witness::new(),
+    };
+
     /// Returns the input base weight.
     ///
     /// Base weight excludes the witness and script.
@@ -313,17 +317,6 @@ impl TxIn {
     pub fn total_size(&self) -> usize { self.base_size() + self.witness.size() }
 }
 
-impl Default for TxIn {
-    fn default() -> TxIn {
-        TxIn {
-            previous_output: OutPoint::default(),
-            script_sig: ScriptBuf::new(),
-            sequence: Sequence::MAX,
-            witness: Witness::default(),
-        }
-    }
-}
-
 /// Bitcoin transaction output.
 ///
 /// Defines new coins to be created as a result of the transaction,
@@ -1540,16 +1533,6 @@ mod tests {
         assert!(txin.is_ok());
     }
 
-    #[test]
-    fn txin_default() {
-        let txin = TxIn::default();
-        assert_eq!(txin.previous_output, OutPoint::default());
-        assert_eq!(txin.script_sig, ScriptBuf::new());
-        assert_eq!(txin.sequence, Sequence::from_consensus(0xFFFFFFFF));
-        assert_eq!(txin.previous_output, OutPoint::default());
-        assert_eq!(txin.witness.len(), 0);
-    }
-
     #[test]
     fn is_coinbase() {
         use crate::constants;
@@ -2251,7 +2234,7 @@ mod tests {
 
     #[test]
     fn outpoint_format() {
-        let outpoint = OutPoint::default();
+        let outpoint = OutPoint::COINBASE_PREVOUT;
 
         let debug = "OutPoint { txid: 0000000000000000000000000000000000000000000000000000000000000000, vout: 4294967295 }";
         assert_eq!(debug, format!("{:?}", &outpoint));

bitcoin/src/crypto/sighash.rs

@@ -1487,7 +1487,7 @@ mod tests {
         let tx = Transaction {
             version: transaction::Version::ONE,
             lock_time: absolute::LockTime::ZERO,
-            input: vec![TxIn::default(), TxIn::default()],
+            input: vec![TxIn::EMPTY_COINBASE, TxIn::EMPTY_COINBASE],
             output: vec![TxOut::NULL],
         };
         let script = ScriptBuf::new();
@@ -1676,7 +1676,7 @@ mod tests {
         let dumb_tx = Transaction {
             version: transaction::Version::TWO,
             lock_time: absolute::LockTime::ZERO,
-            input: vec![TxIn::default()],
+            input: vec![TxIn::EMPTY_COINBASE],
             output: vec![],
         };
         let mut c = SighashCache::new(&dumb_tx);

bitcoin/src/psbt/mod.rs

@@ -2155,7 +2155,7 @@ mod tests {
                             vout: 0,
                         },
                         sequence: Sequence::ENABLE_LOCKTIME_NO_RBF,
-                        ..Default::default()
+                        ..TxIn::EMPTY_COINBASE
                     }
                 ],
                 output: vec![
@@ -2186,7 +2186,7 @@ mod tests {
                                     vout: 1,
                                 },
                                 sequence: Sequence::MAX,
-                                ..Default::default()
+                                ..TxIn::EMPTY_COINBASE
                             },
                             TxIn {
                                 previous_output: OutPoint {
@@ -2194,7 +2194,7 @@ mod tests {
                                     vout: 1,
                                 },
                                 sequence: Sequence::MAX,
-                                ..Default::default()
+                                ..TxIn::EMPTY_COINBASE
                             }
                         ],
                         output: vec![
@@ -2258,7 +2258,7 @@ mod tests {
         let unsigned_tx = Transaction {
             version: transaction::Version::TWO,
             lock_time: absolute::LockTime::ZERO,
-            input: vec![TxIn::default(), TxIn::default()],
+            input: vec![TxIn::EMPTY_COINBASE, TxIn::EMPTY_COINBASE],
             output: vec![TxOut::NULL],
         };
         let mut psbt = Psbt::from_unsigned_tx(unsigned_tx).unwrap();