cosmrs: support coins with amount 0, empty denom

These can be used when the gas fee is zero, for example

Fixes #477

cosmrs/src/base/coin.rs

@@ -4,7 +4,7 @@ use serde::{Deserialize, Serialize};
 use std::fmt;
 
 /// Coin defines a token with a denomination and an amount.
-#[derive(Serialize, Deserialize, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
+#[derive(Clone, Debug, Default, Eq, PartialEq, PartialOrd, Ord, Serialize, Deserialize)]
 pub struct Coin {
     /// Denomination
     pub denom: Denom,
@@ -35,10 +35,15 @@ impl TryFrom<&proto::cosmos::base::v1beta1::Coin> for Coin {
     type Error = ErrorReport;
 
     fn try_from(proto: &proto::cosmos::base::v1beta1::Coin) -> Result<Coin> {
-        Ok(Coin {
-            denom: proto.denom.parse()?,
-            amount: proto.amount.parse()?,
-        })
+        // Support an empty denom when the amount is `0`. See cosmos/cosmos-rust#477
+        if proto.denom.is_empty() && proto.amount == "0" {
+            Ok(Coin::default())
+        } else {
+            Ok(Coin {
+                denom: proto.denom.parse()?,
+                amount: proto.amount.parse()?,
+            })
+        }
     }
 }
 
@@ -67,9 +72,20 @@ impl fmt::Display for Coin {
 #[cfg(test)]
 mod tests {
     use super::Coin;
+    use crate::proto;
 
     #[test]
     fn new() {
         Coin::new(1000, "uatom").unwrap();
     }
+
+    #[test]
+    fn zero_value_coin_with_empty_denom() {
+        let zero_proto = proto::cosmos::base::v1beta1::Coin::from(Coin::default());
+        assert_eq!(&zero_proto.denom, "");
+        assert_eq!(&zero_proto.amount, "0");
+
+        let zero_coin = Coin::try_from(zero_proto).unwrap();
+        assert_eq!(zero_coin, Coin::default())
+    }
 }

cosmrs/src/base/denom.rs

@@ -3,7 +3,7 @@ use serde::{de, de::Error as _, ser, Deserialize, Serialize};
 use std::{fmt, str::FromStr};
 
 /// Denomination.
-#[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
+#[derive(Clone, Debug, Default, Eq, PartialEq, PartialOrd, Ord)]
 pub struct Denom(String);
 
 impl Denom {