2 files changed, 54 insertions, 0 deletions
diff --git a/src/liballoc/collections/btree/map.rs b/src/liballoc/collections/btree/map.rs
index a1e59b2e6af..650582850ab 100644
--- a/src/liballoc/collections/btree/map.rs
+++ b/src/liballoc/collections/btree/map.rs
@@ -2378,6 +2378,33 @@ impl<'a, K: Ord, V> Entry<'a, K, V> {
         }
     }
 
+    #[unstable(feature = "or_insert_with_key", issue = "70996")]
+    /// Ensures a value is in the entry by inserting, if empty, the result of the default function,
+    /// which takes the key as its argument, and returns a mutable reference to the value in the
+    /// entry.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::BTreeMap;
+    ///
+    /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();
+    ///
+    /// map.entry("poneyland").or_insert_with_key(|key| key.chars().count());
+    ///
+    /// assert_eq!(map["poneyland"], 9);
+    /// ```
+    #[inline]
+    pub fn or_insert_with_key<F: FnOnce(&K) -> V>(self, default: F) -> &'a mut V {
+        match self {
+            Occupied(entry) => entry.into_mut(),
+            Vacant(entry) => {
+                let value = default(entry.key());
+                entry.insert(value)
+            }
+        }
+    }
+
     /// Returns a reference to this entry's key.
     ///
     /// # Examples
diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index d1cb8e92d56..277b37dd310 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -1943,6 +1943,33 @@ impl<'a, K, V> Entry<'a, K, V> {
         }
     }
 
+    #[unstable(feature = "or_insert_with_key", issue = "70996")]
+    /// Ensures a value is in the entry by inserting, if empty, the result of the default function,
+    /// which takes the key as its argument, and returns a mutable reference to the value in the
+    /// entry.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let mut map: HashMap<&str, usize> = HashMap::new();
+    ///
+    /// map.entry("poneyland").or_insert_with_key(|key| key.chars().count());
+    ///
+    /// assert_eq!(map["poneyland"], 9);
+    /// ```
+    #[inline]
+    pub fn or_insert_with_key<F: FnOnce(&K) -> V>(self, default: F) -> &'a mut V {
+        match self {
+            Occupied(entry) => entry.into_mut(),
+            Vacant(entry) => {
+                let value = default(entry.key());
+                entry.insert(value)
+            }
+        }
+    }
+
     /// Returns a reference to this entry's key.
     ///
     /// # Examples