Add then and then_with for ordering.

src/libcore/cmp.rs

@@ -245,6 +245,76 @@ impl Ordering {
             Greater => Less,
         }
     }
+
+    /// Chain two orderings.
+    ///
+    /// Returns `self` when it's not `Equal`. Otherwise returns `other`.
+    /// # Examples
+    ///
+    /// ```
+    /// use std::cmp::Ordering;
+    ///
+    /// let result = Ordering::Equal.or(Ordering::Less);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Less.or(Ordering::Equal);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Less.or(Ordering::Greater);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Equal.or(Ordering::Equal);
+    /// assert_eq!(result, Ordering::Equal);
+    ///
+    /// let x = (1, 2, 7);
+    /// let y = (1, 5, 3);
+    /// let result = x.0.cmp(y.0).or(x.1.cmp(y.1)).or(x.2.cmp(y.2));
+    ///
+    /// assert_eq!(result, Ordering::Less);
+    /// ```
+    #[unstable(feature = "ordering_chaining", issue = "37053")]
+    pub fn or(self, other: Ordering) -> Ordering {
+        match self {
+            Equal => other,
+            _ => self,
+        }
+    }
+
+    /// Chain the ordering with given function.
+    ///
+    /// Returns `self` when it's not `Equal`. Otherwise calls `f` and returns
+    /// the result.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::cmp::Ordering;
+    ///
+    /// let result = Ordering::Equal.or_else(|| Ordering::Less);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Less.or_else(|| Ordering::Equal);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Less.or_else(|| Ordering::Greater);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Equal.or_else(|| Ordering::Equal);
+    /// assert_eq!(result, Ordering::Equal);
+    ///
+    /// let x = (1, 2, 7);
+    /// let y = (1, 5, 3);
+    /// let result = x.0.cmp(&y.0).or_else(|| x.1.cmp(&y.1)).or_else(|| x.2.cmp(&y.2));
+    ///
+    /// assert_eq!(result, Ordering::Less);
+    /// ```
+    #[unstable(feature = "ordering_chaining", issue = "37053")]
+    pub fn or_else<F: FnOnce() -> Ordering>(self, f: F) -> Ordering {
+        match self {
+            Equal => f(),
+            _ => self,
+        }
+    }
 }
 
 /// Trait for types that form a [total order](https://en.wikipedia.org/wiki/Total_order).

src/libcoretest/cmp.rs

@@ -41,6 +41,32 @@ fn test_ordering_order() {
     assert_eq!(Greater.cmp(&Less), Greater);
 }
 
+#[test]
+fn test_ordering_or() {
+    assert_eq!(Equal.or(Less), Less);
+    assert_eq!(Equal.or(Equal), Equal);
+    assert_eq!(Equal.or(Greater), Greater);
+    assert_eq!(Less.or(Less), Less);
+    assert_eq!(Less.or(Equal), Less);
+    assert_eq!(Less.or(Greater), Less);
+    assert_eq!(Greater.or(Less), Greater);
+    assert_eq!(Greater.or(Equal), Greater);
+    assert_eq!(Greater.or(Greater), Greater);
+}
+
+#[test]
+fn test_ordering_or_else() {
+    assert_eq!(Equal.or_else(|| Less), Less);
+    assert_eq!(Equal.or_else(|| Equal), Equal);
+    assert_eq!(Equal.or_else(|| Greater), Greater);
+    assert_eq!(Less.or_else(|| Less), Less);
+    assert_eq!(Less.or_else(|| Equal), Less);
+    assert_eq!(Less.or_else(|| Greater), Less);
+    assert_eq!(Greater.or_else(|| Less), Greater);
+    assert_eq!(Greater.or_else(|| Equal), Greater);
+    assert_eq!(Greater.or_else(|| Greater), Greater);
+}
+
 #[test]
 fn test_user_defined_eq() {
     // Our type.

src/libcoretest/lib.rs

@@ -34,6 +34,7 @@
 #![feature(unique)]
 #![feature(iter_max_by)]
 #![feature(iter_min_by)]
+#![feature(ordering_chaining)]
 
 extern crate core;
 extern crate test;