Implement entry_and_modify

src/liballoc/btree/map.rs

@@ -2102,6 +2102,40 @@ impl<'a, K: Ord, V> Entry<'a, K, V> {
             Vacant(ref entry) => entry.key(),
         }
     }
+
+    /// Provides in-place mutable access to an occupied entry before any
+    /// potential inserts into the map.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(entry_and_modify)]
+    /// use std::collections::BTreeMap;
+    ///
+    /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
+    ///
+    /// map.entry("poneyland")
+    ///    .and_modify(|e| { *e += 1 })
+    ///    .or_insert(42);
+    /// assert_eq!(map["poneyland"], 42);
+    ///
+    /// map.entry("poneyland")
+    ///    .and_modify(|e| { *e += 1 })
+    ///    .or_insert(42);
+    /// assert_eq!(map["poneyland"], 43);
+    /// ```
+    #[unstable(feature = "entry_and_modify", issue = "44733")]
+    pub fn and_modify<F>(self, mut f: F) -> Self
+        where F: FnMut(&mut V)
+    {
+        match self {
+            Occupied(mut entry) => {
+                f(entry.get_mut());
+                Occupied(entry)
+            },
+            Vacant(entry) => Vacant(entry),
+        }
+    }
 }
 
 impl<'a, K: Ord, V: Default> Entry<'a, K, V> {