Add zcash_serialized_size() to ZcashSerialize trait

zebra-chain/src/serialization/tests/prop.rs

@@ -4,7 +4,7 @@ use proptest::prelude::*;
 
 use std::io::Cursor;
 
-use crate::serialization::{ReadZcashExt, WriteZcashExt};
+use crate::serialization::{ReadZcashExt, WriteZcashExt, ZcashSerialize};
 
 proptest! {
     #[test]
@@ -35,4 +35,12 @@ proptest! {
             prop_assert_eq!(bytes, expect_bytes);
         }
     }
+
+    #[test]
+    fn transaction_serialized_size(transaction in any::<crate::transaction::UnminedTx>()) {
+        zebra_test::init();
+
+        // just make sure all transactions are bigger than zero
+        assert!(transaction.transaction.zcash_serialized_size().unwrap() > 0);
+    }
 }

zebra-chain/src/serialization/zcash_serialize.rs

@@ -27,6 +27,12 @@ pub trait ZcashSerialize: Sized {
         self.zcash_serialize(&mut data)?;
         Ok(data)
     }
+
+    /// Get the size of `self` by using the `zcash_serialize_to_vec()` helper
+    /// to get a vector of bytes, then just apply `len()` on it.
+    fn zcash_serialized_size(&self) -> Result<usize, io::Error> {
+        Ok(self.zcash_serialize_to_vec()?.len())
+    }
 }
 
 /// Serialize a `Vec` as a compactsize number of items, then the items. This is