Implement Deserialize trait for ServerAddress

See: #717

Impement the serde deserializer for the ServerAddress(Url) struct
based on the existing implementation of `from_str`.

nats/src/connector.rs

@@ -13,8 +13,11 @@
 
 use lazy_static::__Deref;
 use parking_lot::{Mutex, MutexGuard};
+use serde::de::{self, Visitor};
+use serde::{Deserialize, Deserializer};
 use std::collections::HashMap;
 use std::convert::TryFrom;
+use std::fmt::{self, Formatter};
 use std::io::prelude::*;
 use std::io::{self, BufReader, Error, ErrorKind};
 use std::net::{Shutdown, SocketAddr, TcpStream, ToSocketAddrs};
@@ -748,6 +751,26 @@ impl IntoServerList for io::Result<Vec<ServerAddress>> {
     }
 }
 
+struct ServerAddressVisitor;
+
+impl<'de> Visitor<'de> for ServerAddressVisitor {
+    type Value = ServerAddress;
+
+    fn expecting<'a>(&self, formatter: &mut Formatter<'a>) -> fmt::Result {
+        formatter.write_str("a valid URL")
+    }
+
+    fn visit_str<E: de::Error>(self, value: &str) -> Result<Self::Value, E> {
+        ServerAddress::from_str(value).map_err(|err| E::custom(format!("{}", err)))
+    }
+}
+
+impl<'de> Deserialize<'de> for ServerAddress {
+    fn deserialize<D: Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
+        deserializer.deserialize_str(ServerAddressVisitor)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -789,4 +812,24 @@ mod tests {
             matches!(address.auth(), AuthStyle::UserPass(username, password) if &username == "myuser" && &password == "mypass")
         );
     }
+
+    #[test]
+    fn deserialize_valid_server_address() {
+        let serialized = "\"nats://myuser:mypass@localhost:4222\"";
+        let address = ServerAddress::from_str("nats://myuser:mypass@localhost:4222").unwrap();
+
+        assert_eq!(
+            serde_json::from_str::<ServerAddress>(serialized).unwrap(),
+            address
+        );
+    }
+
+    #[test]
+    fn deserialize_invalid_server_address() {
+        let serialized = "\"this is not the address\"";
+        let result = serde_json::from_str::<ServerAddress>(serialized);
+
+        assert!(result.is_err());
+        assert!(format!("{}", result.unwrap_err()).contains("NATS server URL is invalid"));
+    }
 }