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

@@ -23,22 +23,12 @@ impl Coin {
     }
 }
 
-impl TryFrom<proto::cosmos::base::v1beta1::Coin> for Coin {
-    type Error = ErrorReport;
-
-    fn try_from(proto: proto::cosmos::base::v1beta1::Coin) -> Result<Coin> {
-        Coin::try_from(&proto)
-    }
-}
-
-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()?,
-        })
+impl Default for Coin {
+    fn default() -> Coin {
+        Coin {
+            denom: Denom::default(),
+            amount: 0
+        }
     }
 }
 
@@ -57,6 +47,32 @@ impl From<&Coin> for proto::cosmos::base::v1beta1::Coin {
     }
 }
 
+impl TryFrom<proto::cosmos::base::v1beta1::Coin> for Coin {
+    type Error = ErrorReport;
+
+    fn try_from(proto: proto::cosmos::base::v1beta1::Coin) -> Result<Coin> {
+        Coin::try_from(&proto)
+    }
+}
+
+impl TryFrom<&proto::cosmos::base::v1beta1::Coin> for Coin {
+    type Error = ErrorReport;
+
+    fn try_from(proto: &proto::cosmos::base::v1beta1::Coin) -> Result<Coin> {
+        // 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()?,
+            })
+        }
+    }
+}
+
+
+
 impl fmt::Display for Coin {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // See: https://github.com/cosmos/cosmos-sdk/blob/v0.42.4/types/coin.go#L643-L645
@@ -66,10 +82,21 @@ impl fmt::Display for Coin {
 
 #[cfg(test)]
 mod tests {
+    use crate::proto;
     use super::Coin;
 
     #[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 {