Add test find_alias_in_file_email_already_exists

When the email of the alias we're looking is already in the file, expect
an `AliasSearchError::EmailExists` error.

Oh man, this was a tough one. After much searching, finally figured out
how to implement the `PartialEq` trait for my error type so that we
could actually test it.

Many thanks to @peterbudai for an example in the 'redux' project of how
to do this
(https://github.com/peterbudai/redux/blob/ef5d47a0a64cef9fa9e1e9c6f21badc46fa283fc/src/lib.rs):

    #[cfg(test)]
    impl PartialEq<Error> for Error {
        fn eq(&self, other: &Error) -> bool {
           match *self {
               Error::Eof => match *other { Error::Eof => true, _ => false },
               Error::InvalidInput => match *other { Error::InvalidInput => true, _ => false },
               Error::IoError(_)  => match *other { Error::IoError(_) => true, _ => false },
           }
        }
    }

With that example, I was able to correctly build an equality function to
get past my compiler errors which complained that

    an implementation of `std::cmp::PartialEq` might be missing for `std::io::error::Error`

when I tried to `#[derive(PartialEq)]` on my `AliasSearchError` type.

2 files changed, 32 insertions, 9 deletions

diff --git a/src/main.rs b/src/main.rs
index aebabef..4c0cf26 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -97,6 +97,26 @@ impl From<io::Error> for AliasSearchError {
     }
 }
 
+#[cfg(test)]
+impl PartialEq<AliasSearchError> for AliasSearchError {
+    fn eq(&self, other: &AliasSearchError) -> bool {
+        match *self {
+            AliasSearchError::NotFound => match *other {
+                AliasSearchError::NotFound => true,
+                _ => false,
+            },
+            AliasSearchError::EmailExists => match *other {
+                AliasSearchError::EmailExists => true,
+                _ => false,
+            },
+            AliasSearchError::Io(_) => match *other {
+                AliasSearchError::Io(_) => true,
+                _ => false,
+            },
+        }
+    }
+}
+
 fn find_alias_in_file(alias: &Alias, file: &str) -> Result<Vec<String>, AliasSearchError> {
     let mut matches = Vec::new();
     let f = try!(File::open(file));
diff --git a/src/tests.rs b/src/tests.rs
index ccacd29..a6f2777 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -1,4 +1,4 @@
-use super::{Alias, find_alias_in_file};
+use super::{Alias, AliasSearchError, find_alias_in_file};
 
 #[test]
 fn new_alias_with_only_email() {
@@ -42,13 +42,16 @@ fn new_alias_with_special_characters() {
 
 
 #[test]
-fn find_alias_in_file_finds_a_match() {
-    find_alias_in_file(
-        &Alias {
-            alias: "farnsworth-hubert".to_owned(),
-            name: "Hubert Farnsworth".to_owned(),
-            email: "<professor@planetexpress.com>".to_owned()
-        },
-        "./testdata/aliases"
+fn find_alias_in_file_email_already_exists() {
+    assert_eq!(
+        Err(AliasSearchError::EmailExists),
+        find_alias_in_file(
+            &Alias {
+                alias: "farnsworth-hubert".to_owned(),
+                name: "Hubert Farnsworth".to_owned(),
+                email: "<professor@planetexpress.com>".to_owned()
+            },
+            "./testdata/aliases"
+        )
     );
 }