Auto merge of #37054 - rednum:master, r=alexcrichton

Add or and or_else for ordering.

Fixes https://github.com/rust-lang/rust/issues/37053 (see discussion in rust-lang/rfcs#1677).

1 files changed, 74 insertions, 0 deletions

diff --git a/src/libcore/cmp.rs b/src/libcore/cmp.rs
index e0f976e4161..0daf658a0f4 100644
--- a/src/libcore/cmp.rs
+++ b/src/libcore/cmp.rs
@@ -248,6 +248,80 @@ impl Ordering {
             Greater => Less,
         }
     }
+
+    /// Chains two orderings.
+    ///
+    /// Returns `self` when it's not `Equal`. Otherwise returns `other`.
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ordering_chaining)]
+    ///
+    /// use std::cmp::Ordering;
+    ///
+    /// let result = Ordering::Equal.then(Ordering::Less);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Less.then(Ordering::Equal);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Less.then(Ordering::Greater);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Equal.then(Ordering::Equal);
+    /// assert_eq!(result, Ordering::Equal);
+    ///
+    /// let x: (i64, i64, i64) = (1, 2, 7);
+    /// let y: (i64, i64, i64) = (1, 5, 3);
+    /// let result = x.0.cmp(&y.0).then(x.1.cmp(&y.1)).then(x.2.cmp(&y.2));
+    ///
+    /// assert_eq!(result, Ordering::Less);
+    /// ```
+    #[unstable(feature = "ordering_chaining", issue = "37053")]
+    pub fn then(self, other: Ordering) -> Ordering {
+        match self {
+            Equal => other,
+            _ => self,
+        }
+    }
+
+    /// Chains the ordering with the given function.
+    ///
+    /// Returns `self` when it's not `Equal`. Otherwise calls `f` and returns
+    /// the result.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ordering_chaining)]
+    ///
+    /// use std::cmp::Ordering;
+    ///
+    /// let result = Ordering::Equal.then_with(|| Ordering::Less);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Less.then_with(|| Ordering::Equal);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Less.then_with(|| Ordering::Greater);
+    /// assert_eq!(result, Ordering::Less);
+    ///
+    /// let result = Ordering::Equal.then_with(|| Ordering::Equal);
+    /// assert_eq!(result, Ordering::Equal);
+    ///
+    /// let x: (i64, i64, i64) = (1, 2, 7);
+    /// let y: (i64, i64, i64)  = (1, 5, 3);
+    /// let result = x.0.cmp(&y.0).then_with(|| x.1.cmp(&y.1)).then_with(|| x.2.cmp(&y.2));
+    ///
+    /// assert_eq!(result, Ordering::Less);
+    /// ```
+    #[unstable(feature = "ordering_chaining", issue = "37053")]
+    pub fn then_with<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).