Add first_entry and last_entry similar to BTreeMap

src/map.rs

@@ -1098,6 +1098,13 @@ impl<K, V, S> IndexMap<K, V, S> {
         self.as_entries_mut().first_mut().map(Bucket::ref_mut)
     }
 
+    /// Get the first entry in the map for in-place manipulation.
+    ///
+    /// Computes in **O(1)** time.
+    pub fn first_entry(&mut self) -> Option<IndexedEntry<'_, K, V>> {
+        self.get_index_entry(0)
+    }
+
     /// Get the last key-value pair
     ///
     /// Computes in **O(1)** time.
@@ -1112,6 +1119,13 @@ impl<K, V, S> IndexMap<K, V, S> {
         self.as_entries_mut().last_mut().map(Bucket::ref_mut)
     }
 
+    /// Get the last entry in the map for in-place manipulation.
+    ///
+    /// Computes in **O(1)** time.
+    pub fn last_entry(&mut self) -> Option<IndexedEntry<'_, K, V>> {
+        self.get_index_entry(self.len().checked_sub(1)?)
+    }
+
     /// Remove the key-value pair by index
     ///
     /// Valid indices are *0 <= index < self.len()*

src/map/tests.rs

@@ -391,6 +391,8 @@ fn get_index_entry() {
     let mut map = IndexMap::new();
 
     assert!(map.get_index_entry(0).is_none());
+    assert!(map.first_entry().is_none());
+    assert!(map.last_entry().is_none());
 
     map.insert(0, "0");
     map.insert(1, "1");
@@ -414,6 +416,18 @@ fn get_index_entry() {
     }
 
     assert_eq!(*map.get(&3).unwrap(), "4");
+
+    {
+        let e = map.first_entry().unwrap();
+        assert_eq!(*e.key(), 0);
+        assert_eq!(*e.get(), "0");
+    }
+
+    {
+        let e = map.last_entry().unwrap();
+        assert_eq!(*e.key(), 2);
+        assert_eq!(*e.get(), "2");
+    }
 }
 
 #[test]