add Option::reduce

Author: Qelxiros

library/core/src/option.rs

@@ -1961,6 +1961,42 @@ impl<T> Option<T> {
             _ => None,
         }
     }
+
+    /// Reduces two options into one, using the provided function if both are `Some`.
+    ///
+    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
+    /// Otherwise, if only one of `self` and `other` is `Some`, that one is returned.
+    /// If both `self` and `other` are `None`, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_reduce)]
+    ///
+    /// let s12 = Some(12);
+    /// let s17 = Some(17);
+    /// let n = None;
+    /// let f = |a, b| a + b;
+    ///
+    /// assert_eq!(s12.reduce(s17, f), Some(29));
+    /// assert_eq!(s12.reduce(n, f), Some(12));
+    /// assert_eq!(n.reduce(s17, f), Some(17));
+    /// assert_eq!(n.reduce(n, f), None);
+    /// ```
+    #[unstable(feature = "option_reduce", issue = "144273")]
+    pub fn reduce<U, R, F>(self, other: Option<U>, f: F) -> Option<R>
+    where
+        T: Into<R>,
+        U: Into<R>,
+        F: FnOnce(T, U) -> R,
+    {
+        match (self, other) {
+            (Some(a), Some(b)) => Some(f(a, b)),
+            (Some(a), _) => Some(a.into()),
+            (_, Some(b)) => Some(b.into()),
+            _ => None,
+        }
+    }
 }
 
 impl<T, U> Option<(T, U)> {

library/coretests/tests/lib.rs

@@ -77,6 +77,7 @@
 #![feature(never_type)]
 #![feature(next_index)]
 #![feature(numfmt)]
+#![feature(option_reduce)]
 #![feature(pattern)]
 #![feature(pointer_is_aligned_to)]
 #![feature(portable_simd)]