diff --git a/src/btree.rs b/src/btree.rs
index 2267f2f..13a079c 100644
--- a/src/btree.rs
+++ b/src/btree.rs
@@ -56,9 +56,9 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     /// assert_eq!(bimap.len(), 3);
     /// ```
     pub fn len(&self) -> usize {
@@ -75,7 +75,7 @@ where
     ///
     /// let mut bimap = BiBTreeMap::new();
     /// assert!(bimap.is_empty());
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert!(!bimap.is_empty());
     /// bimap.remove_by_right(&1);
     /// assert!(bimap.is_empty());
@@ -92,9 +92,9 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     /// bimap.clear();
     /// assert!(bimap.len() == 0);
     /// ```
@@ -114,9 +114,9 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// for (left, right) in bimap.iter() {
     ///     println!("({}, {})", left, right);
@@ -139,9 +139,9 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// for char_value in bimap.left_values() {
     ///     println!("{}", char_value);
@@ -164,9 +164,9 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// for int_value in bimap.right_values() {
     ///     println!("{}", int_value);
@@ -191,7 +191,7 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert_eq!(bimap.get_by_left(&'a'), Some(&1));
     /// assert_eq!(bimap.get_by_left(&'z'), None);
     /// ```
@@ -216,7 +216,7 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert_eq!(bimap.get_by_right(&1), Some(&'a'));
     /// assert_eq!(bimap.get_by_right(&2), None);
     /// ```
@@ -241,7 +241,7 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert!(bimap.contains_left(&'a'));
     /// assert!(!bimap.contains_left(&'b'));
     /// ```
@@ -266,7 +266,7 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert!(bimap.contains_right(&1));
     /// assert!(!bimap.contains_right(&2));
     /// ```
@@ -293,9 +293,9 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// assert_eq!(bimap.remove_by_left(&'b'), Some(('b', 2)));
     /// assert_eq!(bimap.remove_by_left(&'b'), None);
@@ -331,9 +331,9 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// assert_eq!(bimap.remove_by_right(&2), Some(('b', 2)));
     /// assert_eq!(bimap.remove_by_right(&2), None);
@@ -365,9 +365,9 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// bimap.retain(|&l, _| l != 'b');
     /// assert_eq!(bimap.len(), 2);
@@ -509,10 +509,10 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
-    /// bimap.insert('d', 4);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
+    /// bimap.insert('d', 4).expect_neither();
     ///
     /// for (left, right) in bimap.left_range('b'..'d') {
     ///     println!("({}, {})", left, right);
@@ -546,7 +546,7 @@ where
     /// use bimap::BiBTreeMap;
     ///
     /// let mut bimap = BiBTreeMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// bimap.insert('b', 2);
     /// bimap.insert('c', 3);
     /// bimap.insert('d', 4);
@@ -641,7 +641,7 @@ where
     {
         let mut bimap = BiBTreeMap::new();
         for (left, right) in iter {
-            bimap.insert(left, right);
+            let _ = bimap.insert(left, right);
         }
         bimap
     }
@@ -682,7 +682,7 @@ where
 {
     fn extend<T: IntoIterator<Item = (L, R)>>(&mut self, iter: T) {
         iter.into_iter().for_each(move |(l, r)| {
-            self.insert(l, r);
+            let _ = self.insert(l, r);
         });
     }
 }
@@ -952,8 +952,8 @@ mod tests {
     #[test]
     fn clone() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
         let bimap2 = bimap.clone();
         assert_eq!(bimap, bimap2);
     }
@@ -961,13 +961,13 @@ mod tests {
     #[test]
     fn deep_clone() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
         let mut bimap2 = bimap.clone();
 
         // would panic if clone() didn't deep clone
-        bimap.insert('b', 5);
-        bimap2.insert('a', 12);
+        assert!(bimap.insert('b', 5).did_overwrite());
+        assert!(bimap2.insert('a', 12).did_overwrite());
         bimap2.remove_by_left(&'a');
         bimap.remove_by_right(&2);
     }
@@ -977,10 +977,10 @@ mod tests {
         let mut bimap = BiBTreeMap::new();
         assert_eq!("{}", format!("{:?}", bimap));
 
-        bimap.insert('a', 1);
+        bimap.insert('a', 1).expect_neither();
         assert_eq!("{'a' <> 1}", format!("{:?}", bimap));
 
-        bimap.insert('b', 2);
+        bimap.insert('b', 2).expect_neither();
         assert_eq!("{'a' <> 1, 'b' <> 2}", format!("{:?}", bimap));
     }
 
@@ -993,18 +993,18 @@ mod tests {
     fn eq() {
         let mut bimap = BiBTreeMap::new();
         assert_eq!(bimap, bimap);
-        bimap.insert('a', 1);
+        bimap.insert('a', 1).expect_neither();
         assert_eq!(bimap, bimap);
-        bimap.insert('b', 2);
+        bimap.insert('b', 2).expect_neither();
         assert_eq!(bimap, bimap);
 
         let mut bimap2 = BiBTreeMap::new();
         assert_ne!(bimap, bimap2);
-        bimap2.insert('a', 1);
+        bimap2.insert('a', 1).expect_neither();
         assert_ne!(bimap, bimap2);
-        bimap2.insert('b', 2);
+        bimap2.insert('b', 2).expect_neither();
         assert_eq!(bimap, bimap2);
-        bimap2.insert('c', 3);
+        bimap2.insert('c', 3).expect_neither();
         assert_ne!(bimap, bimap2);
     }
 
@@ -1019,17 +1019,17 @@ mod tests {
             ('b', 3),
         ]);
         let mut bimap2 = BiBTreeMap::new();
-        bimap2.insert('a', 4);
-        bimap2.insert('b', 3);
+        bimap2.insert('a', 4).expect_neither();
+        bimap2.insert('b', 3).expect_neither();
         assert_eq!(bimap, bimap2);
     }
 
     #[test]
     fn into_iter() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
         let pairs = bimap.into_iter().collect::<Vec<_>>();
         assert_eq!(pairs, vec![('a', 3), ('b', 2), ('c', 1)]);
     }
@@ -1037,9 +1037,9 @@ mod tests {
     #[test]
     fn into_iter_ref() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
         let pairs = (&bimap).into_iter().collect::<Vec<_>>();
         assert_eq!(pairs, vec![(&'a', &3), (&'b', &2), (&'c', &1)]);
     }
@@ -1047,13 +1047,13 @@ mod tests {
     #[test]
     fn extend() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
         bimap.extend(vec![('c', 3), ('b', 1), ('a', 4)]);
         let mut bimap2 = BiBTreeMap::new();
-        bimap2.insert('a', 4);
-        bimap2.insert('b', 1);
-        bimap2.insert('c', 3);
+        bimap2.insert('a', 4).expect_neither();
+        bimap2.insert('b', 1).expect_neither();
+        bimap2.insert('c', 3).expect_neither();
         assert_eq!(bimap, bimap2);
     }
 
@@ -1075,9 +1075,9 @@ mod tests {
     #[test]
     fn iter() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
-        bimap.insert('c', 3);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 3).expect_neither();
         let pairs = bimap.iter().map(|(c, i)| (*c, *i)).collect::<Vec<_>>();
         assert_eq!(pairs, vec![('a', 1), ('b', 2), ('c', 3)]);
     }
@@ -1085,9 +1085,9 @@ mod tests {
     #[test]
     fn iter_rev() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
-        bimap.insert('c', 3);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 3).expect_neither();
 
         let mut iter = bimap.iter();
         assert_eq!(iter.next_back(), Some((&'c', &3)));
@@ -1098,9 +1098,9 @@ mod tests {
     #[test]
     fn into_iter_rev() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
-        bimap.insert('c', 3);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 3).expect_neither();
 
         let mut iter = bimap.into_iter();
         assert_eq!(iter.next_back(), Some(('c', 3)));
@@ -1111,9 +1111,9 @@ mod tests {
     #[test]
     fn left_values() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
         let left_values = bimap.left_values().cloned().collect::<Vec<_>>();
         assert_eq!(left_values, vec!['a', 'b', 'c'])
     }
@@ -1121,9 +1121,9 @@ mod tests {
     #[test]
     fn left_values_rev() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
 
         let mut iter = bimap.left_values();
         assert_eq!(iter.next_back(), Some(&'c'));
@@ -1134,9 +1134,9 @@ mod tests {
     #[test]
     fn right_values() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
         let right_values = bimap.right_values().cloned().collect::<Vec<_>>();
         assert_eq!(right_values, vec![1, 2, 3])
     }
@@ -1144,9 +1144,9 @@ mod tests {
     #[test]
     fn right_values_rev() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
 
         let mut iter = bimap.right_values();
         assert_eq!(iter.next_back(), Some(&3));
@@ -1157,10 +1157,10 @@ mod tests {
     #[test]
     fn left_range() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 4);
-        bimap.insert('b', 3);
-        bimap.insert('c', 2);
-        bimap.insert('d', 1);
+        bimap.insert('a', 4).expect_neither();
+        bimap.insert('b', 3).expect_neither();
+        bimap.insert('c', 2).expect_neither();
+        bimap.insert('d', 1).expect_neither();
         let left_range = bimap
             .left_range('b'..'d')
             .map(|(l, r)| (*l, *r))
@@ -1171,10 +1171,10 @@ mod tests {
     #[test]
     fn left_range_rev() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 4);
-        bimap.insert('b', 3);
-        bimap.insert('c', 2);
-        bimap.insert('d', 1);
+        bimap.insert('a', 4).expect_neither();
+        bimap.insert('b', 3).expect_neither();
+        bimap.insert('c', 2).expect_neither();
+        bimap.insert('d', 1).expect_neither();
         let mut left_range = bimap.left_range('b'..'d');
 
         assert_eq!(left_range.next_back(), Some((&'c', &2)));
@@ -1185,10 +1185,10 @@ mod tests {
     #[test]
     fn right_range() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 4);
-        bimap.insert('b', 3);
-        bimap.insert('c', 2);
-        bimap.insert('d', 1);
+        bimap.insert('a', 4).expect_neither();
+        bimap.insert('b', 3).expect_neither();
+        bimap.insert('c', 2).expect_neither();
+        bimap.insert('d', 1).expect_neither();
         let right_range = bimap
             .right_range(2..4)
             .map(|(l, r)| (*l, *r))
@@ -1199,10 +1199,10 @@ mod tests {
     #[test]
     fn right_range_rev() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 4);
-        bimap.insert('b', 3);
-        bimap.insert('c', 2);
-        bimap.insert('d', 1);
+        bimap.insert('a', 4).expect_neither();
+        bimap.insert('b', 3).expect_neither();
+        bimap.insert('c', 2).expect_neither();
+        bimap.insert('d', 1).expect_neither();
         let mut right_range = bimap.right_range(2..4);
 
         assert_eq!(right_range.next_back(), Some((&'b', &3)));
diff --git a/src/hash.rs b/src/hash.rs
index c56f84c..23ca7f7 100644
--- a/src/hash.rs
+++ b/src/hash.rs
@@ -75,9 +75,9 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     /// assert_eq!(bimap.len(), 3);
     /// ```
     pub fn len(&self) -> usize {
@@ -94,7 +94,7 @@ where
     ///
     /// let mut bimap = BiHashMap::new();
     /// assert!(bimap.is_empty());
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert!(!bimap.is_empty());
     /// bimap.remove_by_right(&1);
     /// assert!(bimap.is_empty());
@@ -126,9 +126,9 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     /// bimap.clear();
     /// assert!(bimap.len() == 0);
     /// ```
@@ -148,9 +148,9 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// for (left, right) in bimap.iter() {
     ///     println!("({}, {})", left, right);
@@ -173,9 +173,9 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// for char_value in bimap.left_values() {
     ///     println!("{}", char_value);
@@ -198,9 +198,9 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// for int_value in bimap.right_values() {
     ///     println!("{}", int_value);
@@ -232,7 +232,7 @@ where
     /// let s_left = RandomState::new();
     /// let s_right = RandomState::new();
     /// let mut bimap = BiHashMap::<char, i32>::with_hashers(s_left, s_right);
-    /// bimap.insert('a', 42);
+    /// bimap.insert('a', 42).expect_neither();
     /// ```
     pub fn with_hashers(hash_builder_left: LS, hash_builder_right: RS) -> Self {
         Self {
@@ -299,8 +299,8 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::<char, i32>::with_capacity(100);
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
     /// assert!(bimap.capacity() >= 100);
     /// bimap.shrink_to_fit();
     /// assert!(bimap.capacity() >= 2);
@@ -323,8 +323,8 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::<char, i32>::with_capacity(100);
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
     /// assert!(bimap.capacity() >= 100);
     /// bimap.shrink_to(10);
     /// assert!(bimap.capacity() >= 10);
@@ -348,7 +348,7 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert_eq!(bimap.get_by_left(&'a'), Some(&1));
     /// assert_eq!(bimap.get_by_left(&'z'), None);
     /// ```
@@ -372,7 +372,7 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert_eq!(bimap.get_by_right(&1), Some(&'a'));
     /// assert_eq!(bimap.get_by_right(&2), None);
     /// ```
@@ -396,7 +396,7 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert!(bimap.contains_left(&'a'));
     /// assert!(!bimap.contains_left(&'b'));
     /// ```
@@ -420,7 +420,7 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
+    /// bimap.insert('a', 1).expect_neither();
     /// assert!(bimap.contains_right(&1));
     /// assert!(!bimap.contains_right(&2));
     /// ```
@@ -446,9 +446,9 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// assert_eq!(bimap.remove_by_left(&'b'), Some(('b', 2)));
     /// assert_eq!(bimap.remove_by_left(&'b'), None);
@@ -483,9 +483,9 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     ///
     /// assert_eq!(bimap.remove_by_right(&2), Some(('b', 2)));
     /// assert_eq!(bimap.remove_by_right(&2), None);
@@ -612,9 +612,9 @@ where
     /// use bimap::BiHashMap;
     ///
     /// let mut bimap = BiHashMap::new();
-    /// bimap.insert('a', 1);
-    /// bimap.insert('b', 2);
-    /// bimap.insert('c', 3);
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('b', 2).expect_neither();
+    /// bimap.insert('c', 3).expect_neither();
     /// bimap.retain(|&l, &r| r >= 2);
     /// assert_eq!(bimap.len(), 2);
     /// assert_eq!(bimap.get_by_left(&'b'), Some(&2));
@@ -660,7 +660,7 @@ where
             self.right2left.hasher().clone(),
         );
         for (l, r) in self.iter() {
-            new_bimap.insert(l.clone(), r.clone());
+            new_bimap.insert(l.clone(), r.clone()).expect_neither();
         }
         new_bimap
     }
@@ -742,7 +742,7 @@ where
             }
         };
         for (left, right) in iter {
-            bimap.insert(left, right);
+            let _ = bimap.insert(left, right);
         }
         bimap
     }
@@ -785,7 +785,7 @@ where
 {
     fn extend<T: IntoIterator<Item = (L, R)>>(&mut self, iter: T) {
         iter.into_iter().for_each(move |(l, r)| {
-            self.insert(l, r);
+            let _ = self.insert(l, r);
         });
     }
 }
@@ -932,8 +932,8 @@ mod tests {
     #[test]
     fn clone() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
         let bimap2 = bimap.clone();
         assert_eq!(bimap, bimap2);
     }
@@ -941,13 +941,13 @@ mod tests {
     #[test]
     fn deep_clone() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
         let mut bimap2 = bimap.clone();
 
         // would panic if clone() didn't deep clone
-        bimap.insert('b', 5);
-        bimap2.insert('a', 12);
+        assert!(bimap.insert('b', 5).did_overwrite());
+        assert!(bimap2.insert('a', 12).did_overwrite());
         bimap2.remove_by_left(&'a');
         bimap.remove_by_right(&2);
     }
@@ -957,10 +957,10 @@ mod tests {
         let mut bimap = BiHashMap::new();
         assert_eq!("{}", format!("{:?}", bimap));
 
-        bimap.insert('a', 1);
+        bimap.insert('a', 1).expect_neither();
         assert_eq!("{'a' <> 1}", format!("{:?}", bimap));
 
-        bimap.insert('b', 2);
+        bimap.insert('b', 2).expect_neither();
         let expected1 = "{'a' <> 1, 'b' <> 2}";
         let expected2 = "{'b' <> 2, 'a' <> 1}";
         let formatted = format!("{:?}", bimap);
@@ -976,18 +976,18 @@ mod tests {
     fn eq() {
         let mut bimap = BiHashMap::new();
         assert_eq!(bimap, bimap);
-        bimap.insert('a', 1);
+        bimap.insert('a', 1).expect_neither();
         assert_eq!(bimap, bimap);
-        bimap.insert('b', 2);
+        bimap.insert('b', 2).expect_neither();
         assert_eq!(bimap, bimap);
 
         let mut bimap2 = BiHashMap::new();
         assert_ne!(bimap, bimap2);
-        bimap2.insert('a', 1);
+        bimap2.insert('a', 1).expect_neither();
         assert_ne!(bimap, bimap2);
-        bimap2.insert('b', 2);
+        bimap2.insert('b', 2).expect_neither();
         assert_eq!(bimap, bimap2);
-        bimap2.insert('c', 3);
+        bimap2.insert('c', 3).expect_neither();
         assert_ne!(bimap, bimap2);
     }
 
@@ -1002,17 +1002,17 @@ mod tests {
             ('b', 3),
         ]);
         let mut bimap2 = BiHashMap::new();
-        bimap2.insert('a', 4);
-        bimap2.insert('b', 3);
+        bimap2.insert('a', 4).expect_neither();
+        bimap2.insert('b', 3).expect_neither();
         assert_eq!(bimap, bimap2);
     }
 
     #[test]
     fn into_iter() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
         let mut pairs = bimap.into_iter().collect::<Vec<_>>();
         pairs.sort();
         assert_eq!(pairs, vec![('a', 3), ('b', 2), ('c', 1)]);
@@ -1021,9 +1021,9 @@ mod tests {
     #[test]
     fn into_iter_ref() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
         let mut pairs = (&bimap).into_iter().collect::<Vec<_>>();
         pairs.sort();
         assert_eq!(pairs, vec![(&'a', &3), (&'b', &2), (&'c', &1)]);
@@ -1032,22 +1032,22 @@ mod tests {
     #[test]
     fn extend() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
         bimap.extend(vec![('c', 3), ('b', 1), ('a', 4)]);
         let mut bimap2 = BiHashMap::new();
-        bimap2.insert('a', 4);
-        bimap2.insert('b', 1);
-        bimap2.insert('c', 3);
+        bimap2.insert('a', 4).expect_neither();
+        bimap2.insert('b', 1).expect_neither();
+        bimap2.insert('c', 3).expect_neither();
         assert_eq!(bimap, bimap2);
     }
 
     #[test]
     fn iter() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
-        bimap.insert('c', 3);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 3).expect_neither();
         let mut pairs = bimap.iter().map(|(c, i)| (*c, *i)).collect::<Vec<_>>();
         pairs.sort();
         assert_eq!(pairs, vec![('a', 1), ('b', 2), ('c', 3)]);
@@ -1056,9 +1056,9 @@ mod tests {
     #[test]
     fn left_values() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
         let mut left_values = bimap.left_values().cloned().collect::<Vec<_>>();
         left_values.sort();
         assert_eq!(left_values, vec!['a', 'b', 'c'])
@@ -1067,9 +1067,9 @@ mod tests {
     #[test]
     fn right_values() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 3);
-        bimap.insert('b', 2);
-        bimap.insert('c', 1);
+        bimap.insert('a', 3).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 1).expect_neither();
         let mut right_values = bimap.right_values().cloned().collect::<Vec<_>>();
         right_values.sort();
         assert_eq!(right_values, vec![1, 2, 3])
@@ -1086,7 +1086,7 @@ mod tests {
         let s_left = hash_map::RandomState::new();
         let s_right = hash_map::RandomState::new();
         let mut bimap = BiHashMap::<char, i32>::with_hashers(s_left, s_right);
-        bimap.insert('a', 42);
+        bimap.insert('a', 42).expect_neither();
         assert_eq!(Some(&'a'), bimap.get_by_right(&42));
         assert_eq!(Some(&42), bimap.get_by_left(&'a'));
     }
@@ -1111,8 +1111,8 @@ mod tests {
         assert_eq!(bimap.len(), 0);
         assert!(bimap.capacity() >= 100);
 
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
         assert!(!bimap.is_empty());
         assert_eq!(bimap.len(), 2);
         assert!(bimap.capacity() >= 100);
@@ -1130,8 +1130,8 @@ mod tests {
         assert_eq!(bimap.len(), 0);
         assert!(bimap.capacity() >= 100);
 
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
         assert!(!bimap.is_empty());
         assert_eq!(bimap.len(), 2);
         assert!(bimap.capacity() >= 100);
@@ -1201,9 +1201,9 @@ mod tests {
     #[test]
     fn retain_calls_f_once() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
-        bimap.insert('c', 3);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 3).expect_neither();
 
         // retain one element
         let mut i = 0;
diff --git a/src/lib.rs b/src/lib.rs
index f814032..209c221 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -62,8 +62,8 @@
 //! use bimap::BiMap;
 //!
 //! let mut bimap = BiMap::new();
-//! bimap.insert('a', 1);
-//! bimap.insert('b', 1); // what to do here?
+//! bimap.insert('a', 1).expect_neither();
+//! let _ = bimap.insert('b', 1); // what to do here?
 //! ```
 //!
 //! In order to maintain the bijection, the bimap cannot have both left-right
@@ -135,7 +135,7 @@
 //!
 //! // insert both Foos into a bimap
 //! let mut bimap = BiMap::new();
-//! bimap.insert(foo1, 99);
+//! bimap.insert(foo1, 99).expect_neither();
 //! let overwritten = bimap.insert(foo2, 100);
 //!
 //! // foo1 is overwritten and returned
@@ -198,11 +198,14 @@
 #[allow(unused_imports)]
 #[macro_use]
 extern crate alloc;
+extern crate core;
 
 mod mem;
 
 pub mod btree;
+
 pub use btree::BiBTreeMap;
+use core::fmt::Debug;
 
 #[cfg(feature = "std")]
 pub mod hash;
@@ -225,6 +228,7 @@ pub mod serde;
 /// The previous left-right pairs, if any, that were overwritten by a call to
 /// the [`insert`](BiHashMap::insert) method of a bimap.
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[must_use = "This `Overwritten` may not be a `Neither` variant, which should be handled"]
 pub enum Overwritten<L, R> {
     /// Neither the left nor the right value previously existed in the bimap.
     Neither,
@@ -266,6 +270,34 @@ impl<L, R> Overwritten<L, R> {
     pub fn did_overwrite(&self) -> bool {
         !matches!(self, Overwritten::Neither)
     }
+
+    /// Panics if the `Overwritten` variant is not `Neither`.
+    /// This method is useful for consume Overwritten like `unwrap`.
+    ///
+    /// # Examples
+    ///
+    /// ```rust,should_panic
+    /// use bimap::{BiMap, Overwritten};
+    ///
+    /// let mut bimap = BiMap::new();
+    /// bimap.insert('a', 1).expect_neither();
+    /// bimap.insert('a', 2).expect_neither();
+    /// ```
+    pub fn expect_neither(&self) {
+        match self {
+            Overwritten::Neither => (),
+            _ => panic!(
+                "Expected Overwritten::Neither, got {}",
+                match self {
+                    Overwritten::Left(_, _) => "Overwritten::Left",
+                    Overwritten::Right(_, _) => "Overwritten::Right",
+                    Overwritten::Pair(_, _) => "Overwritten::Pair",
+                    Overwritten::Both(_, _) => "Overwritten::Both",
+                    _ => unreachable!(),
+                }
+            ),
+        }
+    }
 }
 
 #[cfg(test)]
diff --git a/src/serde.rs b/src/serde.rs
index 83fd9cc..be470b7 100644
--- a/src/serde.rs
+++ b/src/serde.rs
@@ -123,9 +123,9 @@
 //! let mut bimap = BiHashMap::new();
 //!
 //! // insert some pairs
-//! bimap.insert('A', 1);
-//! bimap.insert('B', 2);
-//! bimap.insert('C', 3);
+//! bimap.insert('A', 1).expect_neither();
+//! bimap.insert('B', 2).expect_neither();
+//! bimap.insert('C', 3).expect_neither();
 //!
 //! // serialize the bimap
 //! let json = serde_json::to_string(&bimap).unwrap();
@@ -196,7 +196,7 @@ where
             None => BiHashMap::<L, R, LS, RS>::with_hashers(LS::default(), RS::default()),
         };
         while let Some((l, r)) = entries.next_entry()? {
-            map.insert(l, r);
+            let _ = map.insert(l, r);
         }
         Ok(map)
     }
@@ -246,7 +246,7 @@ where
     fn visit_map<A: MapAccess<'de>>(self, mut entries: A) -> Result<Self::Value, A::Error> {
         let mut map = BiBTreeMap::new();
         while let Some((l, r)) = entries.next_entry()? {
-            map.insert(l, r);
+            map.insert(l, r).expect_neither();
         }
         Ok(map)
     }
@@ -274,9 +274,9 @@ mod tests {
     #[test]
     fn serde_hash() {
         let mut bimap = BiHashMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
-        bimap.insert('c', 3);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 3).expect_neither();
 
         let json = serde_json::to_string(&bimap).unwrap();
         let bimap2 = serde_json::from_str(&json).unwrap();
@@ -295,9 +295,9 @@ mod tests {
         >::with_capacity_and_hashers(
             4, hasher_builder.clone(), hasher_builder.clone()
         );
-        bimap.insert('f', 1);
-        bimap.insert('g', 2);
-        bimap.insert('h', 3);
+        bimap.insert('f', 1).expect_neither();
+        bimap.insert('g', 2).expect_neither();
+        bimap.insert('h', 3).expect_neither();
 
         let json = serde_json::to_string(&bimap).unwrap();
         let bimap2 = serde_json::from_str(&json).unwrap();
@@ -316,9 +316,9 @@ mod tests {
         >::with_capacity_and_hashers(
             4, hasher_builder.clone(), hasher_builder.clone()
         );
-        bimap.insert('x', 1);
-        bimap.insert('y', 2);
-        bimap.insert('z', 3);
+        bimap.insert('x', 1).expect_neither();
+        bimap.insert('y', 2).expect_neither();
+        bimap.insert('z', 3).expect_neither();
 
         let json = serde_json::to_string(&bimap).unwrap();
         let bimap2 = serde_json::from_str(&json).unwrap();
@@ -329,9 +329,9 @@ mod tests {
     #[test]
     fn serde_btree() {
         let mut bimap = BiBTreeMap::new();
-        bimap.insert('a', 1);
-        bimap.insert('b', 2);
-        bimap.insert('c', 3);
+        bimap.insert('a', 1).expect_neither();
+        bimap.insert('b', 2).expect_neither();
+        bimap.insert('c', 3).expect_neither();
 
         let json = serde_json::to_string(&bimap).unwrap();
         let bimap2 = serde_json::from_str(&json).unwrap();
diff --git a/tests/hello_world.rs b/tests/hello_world.rs
index 658268a..e0842ba 100644
--- a/tests/hello_world.rs
+++ b/tests/hello_world.rs
@@ -5,9 +5,9 @@ fn main() {
     // A bijective map between letters of the English alphabet and their positions.
     let mut alphabet = bimap::BiMap::<char, u8>::new();
 
-    alphabet.insert('A', 1);
+    alphabet.insert('A', 1).expect_neither();
     // some letters omitted for brevity
-    alphabet.insert('Z', 26);
+    alphabet.insert('Z', 26).expect_neither();
 
     println!("A is at position {}", alphabet.get_by_left(&'A').unwrap());
     println!("{} is at position 26", alphabet.get_by_right(&26).unwrap());