use value receiver

cid.go

@@ -410,7 +410,7 @@ func (c *Cid) UnmarshalJSON(b []byte) error {
 //
 // Note that this formatting comes from the IPLD specification
 // (https://github.com/ipld/specs/tree/master/ipld)
-func (c *Cid) MarshalJSON() ([]byte, error) {
+func (c Cid) MarshalJSON() ([]byte, error) {
 	return []byte(fmt.Sprintf("{\"/\":\"%s\"}", c.String())), nil
 }
 

cid_test.go

@@ -383,3 +383,32 @@ func TestFromJson(t *testing.T) {
 		t.Fatal("json parsing failed")
 	}
 }
+
+func TestJsonRoundTrip(t *testing.T) {
+	exp, err := Decode("zb2rhhFAEMepUBbGyP1k8tGfz7BSciKXP6GHuUeUsJBaK6cqG")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify it works for a *Cid.
+	enc, err := json.Marshal(exp)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var actual Cid
+	err = json.Unmarshal(enc, &actual)
+	if !exp.Equals(&actual) {
+		t.Fatal("cids not equal for *Cid")
+	}
+
+	// Verify it works for a Cid.
+	enc, err = json.Marshal(*exp)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var actual2 Cid
+	err = json.Unmarshal(enc, &actual2)
+	if !(*exp).Equals(&actual2) {
+		t.Fatal("cids not equal for *Cid")
+	}
+}