Merge PR #4454: Implement MarshalJSON for Coins Type

.pending/improvements/sdk/4259-Coins-that-are-

@@ -0,0 +1,2 @@
+#4259 `Coins` that are `nil` are now JSON encoded as an empty array `[]`.
+Decoding remains unchanged and behavior is left intact.

types/coin.go

@@ -1,6 +1,7 @@
 package types
 
 import (
+	"encoding/json"
 	"fmt"
 	"regexp"
 	"sort"
@@ -151,6 +152,18 @@ func NewCoins(coins ...Coin) Coins {
 	return newCoins
 }
 
+type coinsJSON Coins
+
+// MarshalJSON implements a custom JSON marshaller for the Coins type to allow
+// nil Coins to be encoded as an empty array.
+func (coins Coins) MarshalJSON() ([]byte, error) {
+	if coins == nil {
+		return json.Marshal(coinsJSON(Coins{}))
+	}
+
+	return json.Marshal(coinsJSON(coins))
+}
+
 func (coins Coins) String() string {
 	if len(coins) == 0 {
 		return ""

types/coin_test.go

@@ -4,6 +4,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/cosmos/cosmos-sdk/codec"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -621,3 +622,35 @@ func TestFindDup(t *testing.T) {
 		})
 	}
 }
+
+func TestMarshalJSONCoins(t *testing.T) {
+	cdc := codec.New()
+	RegisterCodec(cdc)
+
+	testCases := []struct {
+		name      string
+		input     Coins
+		strOutput string
+	}{
+		{"nil coins", nil, `[]`},
+		{"empty coins", Coins{}, `[]`},
+		{"non-empty coins", NewCoins(NewInt64Coin("foo", 50)), `[{"denom":"foo","amount":"50"}]`},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			bz, err := cdc.MarshalJSON(tc.input)
+			require.NoError(t, err)
+			require.Equal(t, tc.strOutput, string(bz))
+
+			var newCoins Coins
+			require.NoError(t, cdc.UnmarshalJSON(bz, &newCoins))
+
+			if tc.input.Empty() {
+				require.Nil(t, newCoins)
+			} else {
+				require.Equal(t, tc.input, newCoins)
+			}
+		})
+	}
+}