mac: Handle improperly-formatted addresses

This commit introduces a new variant to the `ParseError` enumeration,
`ParseError::Format`. This error is returned by
`MacAddress::from_string()` when the input string is not correctly
formatted.

src/mac.rs

@@ -6,6 +6,7 @@ use std::num;
 #[derive(Debug)]
 pub enum ParseError {
     Value(num::ParseIntError),
+    Format,
 }
 
 
@@ -20,6 +21,7 @@ impl fmt::Display for ParseError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
             ParseError::Value(ref e) => e.fmt(f),
+            ParseError::Format => write!(f, "Invalid MAC address format"),
         }
     }
 }
@@ -29,6 +31,7 @@ impl error::Error for ParseError {
     fn cause(&self) -> Option<&error::Error> {
         match *self {
             ParseError::Value(ref e) => Some(e),
+            ParseError::Format => None,
         }
     }
 }
@@ -41,15 +44,21 @@ pub struct MacAddress {
 
 impl MacAddress {
     pub fn from_string(s: &str) -> Result<Self, ParseError> {
-        s.split(":")
+        let split: Result<Vec<_>, _> = s.split(":")
             .map(|b| u8::from_str_radix(b, 16))
-            .collect::<Result<Vec<u8>, _>>()
-            .and_then(|parts| {
+            .collect();
+        match split {
+            Ok(parts) => {
                 let mut addr = [0u8; 6];
+                if parts.len() == 6 {
                     addr.copy_from_slice(&parts[..6]);
                     Ok(MacAddress { addr })
-            })
-            .map_err(ParseError::from)
+                } else {
+                    Err(ParseError::Format)
+                }
+            },
+            Err(e) => Err(ParseError::from(e)),
+        }
     }
 
     pub fn as_bytes(&self) -> &[u8; 6] {