implement Display instead of to_string()

When implementing Display for a type, the to_string() method can be
called as well. Display is more versatile and is the preferred way.

Signed-off-by: Andreea Florescu <[email protected]>

Author: andreeaflorescu

mmds/src/data_store.rs

@@ -1,6 +1,8 @@
 // Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+use std::fmt;
+
 use serde_json::Value;
 
 /// The Mmds is the Microvm Metadata Service represented as an untyped json.
@@ -16,12 +18,12 @@ pub enum Error {
     UnsupportedValueType,
 }
 
-impl Error {
-    pub fn to_string(&self) -> String {
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
-            Error::NotFound => "The MMDS resource does not exist.".to_string(),
+            Error::NotFound => write!(f, "The MMDS resource does not exist."),
             Error::UnsupportedValueType => {
-                "Cannot add non-strings values to the MMDS data-store.".to_string()
+                write!(f, "Cannot add non-strings values to the MMDS data-store.")
             }
         }
     }

net_util/src/mac.rs

@@ -5,6 +5,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the THIRD-PARTY file.
 
+use std::fmt;
 use std::result::Result;
 
 use serde::de::{Deserialize, Deserializer, Error};
@@ -19,6 +20,17 @@ pub struct MacAddr {
     bytes: [u8; MAC_ADDR_LEN],
 }
 
+impl fmt::Display for MacAddr {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let b = &self.bytes;
+        write!(
+            f,
+            "{:02x}:{:02x}:{:02x}:{:02x}:{:02x}:{:02x}",
+            b[0], b[1], b[2], b[3], b[4], b[5]
+        )
+    }
+}
+
 impl MacAddr {
     /// Try to turn a `&str` into a `MacAddr` object. The method will return the `str` that failed
     /// to be parsed.
@@ -114,15 +126,6 @@ impl MacAddr {
     pub fn get_bytes(&self) -> &[u8] {
         &self.bytes
     }
-
-    /// Obtain the string representation.
-    pub fn to_string(self) -> String {
-        let b = &self.bytes;
-        format!(
-            "{:02x}:{:02x}:{:02x}:{:02x}:{:02x}:{:02x}",
-            b[0], b[1], b[2], b[3], b[4], b[5]
-        )
-    }
 }
 
 impl Serialize for MacAddr {