Improve return type of NonEmpty::split

Fixes #60[^issue-60].

As pointed out in #60, splitting a `NonEmpty` that contains a single
element can be ambiguous to a `NonEmpty` equivalent to `(x, [x])`. A
better representation for this return type is `(&T, &[T],
Option<&T>)`.

[^issue-60]: #60
Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

Author: FintanH

src/lib.rs

@@ -516,17 +516,17 @@ impl<T> NonEmpty<T> {
     ///
     /// // Guaranteed to have the last element and the elements
     /// // preceding it.
-    /// assert_eq!(non_empty.split(), (&1, &[2, 3, 4][..], &5));
+    /// assert_eq!(non_empty.split(), (&1, &[2, 3, 4][..], Some(&5)));
     ///
     /// let non_empty = NonEmpty::new(1);
     ///
     /// // Guaranteed to have the last element.
-    /// assert_eq!(non_empty.split(), (&1, &[][..], &1));
+    /// assert_eq!(non_empty.split(), (&1, &[][..], None));
     /// ```
-    pub fn split(&self) -> (&T, &[T], &T) {
+    pub fn split(&self) -> (&T, &[T], Option<&T>) {
         match self.tail.split_last() {
-            None => (&self.head, &[], &self.head),
-            Some((last, middle)) => (&self.head, middle, last),
+            None => (&self.head, &[], None),
+            Some((last, middle)) => (&self.head, middle, Some(last)),
         }
     }