Expose the key of Entry variants for HashMap and BTreeMap.

src/libcollections/btree/map.rs

@@ -1465,6 +1465,13 @@ impl<'a, K: Ord, V> Entry<'a, K, V> {
 }
 
 impl<'a, K: Ord, V> VacantEntry<'a, K, V> {
+    /// Gets a reference to the key that would be used when inserting a value
+    /// through the VacantEntry.
+    #[unstable(feature = "map_entry_keys", issue = "32281")]
+    pub fn key(&self) -> &K {
+        &self.key
+    }
+
     /// Sets the value of the entry with the VacantEntry's key,
     /// and returns a mutable reference to it.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1509,6 +1516,12 @@ impl<'a, K: Ord, V> VacantEntry<'a, K, V> {
 }
 
 impl<'a, K: Ord, V> OccupiedEntry<'a, K, V> {
+    /// Gets a reference to the key in the entry.
+    #[unstable(feature = "map_entry_keys", issue = "32281")]
+    pub fn key(&self) -> &K {
+        self.handle.reborrow().into_kv().0
+    }
+
     /// Gets a reference to the value in the entry.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get(&self) -> &V {

src/libcollectionstest/btree/map.rs

@@ -395,6 +395,42 @@ fn test_variance() {
     fn vals<'a, 'new>(v: Values<'a, (), &'static str>) -> Values<'a, (), &'new str> { v }
 }
 
+#[test]
+fn test_occupied_entry_key() {
+    let mut a = BTreeMap::new();
+    let key = "hello there";
+    let value = "value goes here";
+    assert!(a.is_empty());
+    a.insert(key.clone(), value.clone());
+    assert_eq!(a.len(), 1);
+    assert_eq!(a[key], value);
+
+    match a.entry(key.clone()) {
+        Vacant(_) => panic!(),
+        Occupied(e) => assert_eq!(key, *e.key()),
+    }
+    assert_eq!(a.len(), 1);
+    assert_eq!(a[key], value);
+}
+
+#[test]
+fn test_vacant_entry_key() {
+    let mut a = BTreeMap::new();
+    let key = "hello there";
+    let value = "value goes here";
+
+    assert!(a.is_empty());
+    match a.entry(key.clone()) {
+        Occupied(_) => panic!(),
+        Vacant(e) => {
+            assert_eq!(key, *e.key());
+            e.insert(value.clone());
+        },
+    }
+    assert_eq!(a.len(), 1);
+    assert_eq!(a[key], value);
+}
+
 mod bench {
     use std::collections::BTreeMap;
     use std::__rand::{Rng, thread_rng};

src/libcollectionstest/lib.rs

@@ -21,6 +21,7 @@
 #![feature(fn_traits)]
 #![feature(enumset)]
 #![feature(iter_arith)]
+#![feature(map_entry_keys)]
 #![feature(pattern)]
 #![feature(rand)]
 #![feature(set_recovery)]

src/libstd/collections/hash/map.rs

@@ -1554,6 +1554,12 @@ impl<'a, K, V> Entry<'a, K, V> {
 }
 
 impl<'a, K, V> OccupiedEntry<'a, K, V> {
+    /// Gets a reference to the key in the entry.
+    #[unstable(feature = "map_entry_keys", issue = "32281")]
+    pub fn key(&self) -> &K {
+        self.elem.read().0
+    }
+
     /// Gets a reference to the value in the entry.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get(&self) -> &V {
@@ -1589,6 +1595,13 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
 }
 
 impl<'a, K: 'a, V: 'a> VacantEntry<'a, K, V> {
+    /// Gets a reference to the key that would be used when inserting a value
+    /// through the VacantEntry.
+    #[unstable(feature = "map_entry_keys", issue = "32281")]
+    pub fn key(&self) -> &K {
+        &self.key
+    }
+
     /// Sets the value of the entry with the VacantEntry's key,
     /// and returns a mutable reference to it
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -2434,4 +2447,40 @@ mod test_map {
         a.insert(item, 0);
         assert!(a.capacity() > a.len());
     }
+
+    #[test]
+    fn test_occupied_entry_key() {
+        let mut a = HashMap::new();
+        let key = "hello there";
+        let value = "value goes here";
+        assert!(a.is_empty());
+        a.insert(key.clone(), value.clone());
+        assert_eq!(a.len(), 1);
+        assert_eq!(a[key], value);
+
+        match a.entry(key.clone()) {
+            Vacant(_) => panic!(),
+            Occupied(e) => assert_eq!(key, *e.key()),
+        }
+        assert_eq!(a.len(), 1);
+        assert_eq!(a[key], value);
+    }
+
+    #[test]
+    fn test_vacant_entry_key() {
+        let mut a = HashMap::new();
+        let key = "hello there";
+        let value = "value goes here";
+
+        assert!(a.is_empty());
+        match a.entry(key.clone()) {
+            Occupied(_) => panic!(),
+            Vacant(e) => {
+                assert_eq!(key, *e.key());
+                e.insert(value.clone());
+            },
+        }
+        assert_eq!(a.len(), 1);
+        assert_eq!(a[key], value);
+    }
 }