4 files changed, 45 insertions, 0 deletions
diff --git a/library/alloc/src/collections/btree/map/entry.rs b/library/alloc/src/collections/btree/map/entry.rs
index cc508f30a61..a5ece31d67b 100644
--- a/library/alloc/src/collections/btree/map/entry.rs
+++ b/library/alloc/src/collections/btree/map/entry.rs
@@ -93,6 +93,19 @@ impl<K: Debug + Ord, V: Debug> Debug for OccupiedError<'_, K, V> {
     }
 }
 
+#[unstable(feature = "map_try_insert", issue = "none")]
+impl<'a, K: Debug + Ord, V: Debug> fmt::Display for OccupiedError<'a, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "failed to insert {:?}, key {:?} already exists with value {:?}",
+            self.value,
+            self.entry.key(),
+            self.entry.get(),
+        )
+    }
+}
+
 impl<'a, K: Ord, V> Entry<'a, K, V> {
     /// Ensures a value is in the entry by inserting the default if empty, and returns
     /// a mutable reference to the value in the entry.
diff --git a/library/std/src/collections/hash/map.rs b/library/std/src/collections/hash/map.rs
index 7a50b49007c..a4a7b256654 100644
--- a/library/std/src/collections/hash/map.rs
+++ b/library/std/src/collections/hash/map.rs
@@ -1907,6 +1907,19 @@ impl<K: Debug, V: Debug> Debug for OccupiedError<'_, K, V> {
     }
 }
 
+#[unstable(feature = "map_try_insert", issue = "none")]
+impl<'a, K: Debug, V: Debug> fmt::Display for OccupiedError<'a, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "failed to insert {:?}, key {:?} already exists with value {:?}",
+            self.value,
+            self.entry.key(),
+            self.entry.get(),
+        )
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S> {
     type Item = (&'a K, &'a V);
diff --git a/library/std/src/error.rs b/library/std/src/error.rs
index 94338c7b04d..a7f744ce515 100644
--- a/library/std/src/error.rs
+++ b/library/std/src/error.rs
@@ -470,6 +470,24 @@ impl Error for char::DecodeUtf16Error {
     }
 }
 
+#[unstable(feature = "map_try_insert", issue = "none")]
+impl<'a, K: Debug + Ord, V: Debug> Error
+    for crate::collections::btree_map::OccupiedError<'a, K, V>
+{
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "key already exists"
+    }
+}
+
+#[unstable(feature = "map_try_insert", issue = "none")]
+impl<'a, K: Debug, V: Debug> Error for crate::collections::hash_map::OccupiedError<'a, K, V> {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "key already exists"
+    }
+}
+
 #[stable(feature = "box_error", since = "1.8.0")]
 impl<T: Error> Error for Box<T> {
     #[allow(deprecated, deprecated_in_future)]
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index ba49dee38e6..7e86311bbb0 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -281,6 +281,7 @@
 #![feature(linkage)]
 #![feature(llvm_asm)]
 #![feature(log_syntax)]
+#![feature(map_try_insert)]
 #![feature(maybe_uninit_extra)]
 #![feature(maybe_uninit_ref)]
 #![feature(maybe_uninit_slice)]