Improve Debug implementations of OccupiedError.

library/alloc/src/collections/btree/map/entry.rs

@@ -86,8 +86,9 @@ pub struct OccupiedError<'a, K: 'a, V: 'a> {
 impl<K: Debug + Ord, V: Debug> Debug for OccupiedError<'_, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("OccupiedError")
-            .field("entry", &self.entry)
+            .field("key", self.entry.key())
-            .field("value", &self.value)
+            .field("old_value", self.entry.get())
+            .field("new_value", &self.value)
             .finish()
     }
 }

library/std/src/collections/hash/map.rs

@@ -1889,7 +1889,6 @@ impl<K: Debug, V> Debug for VacantEntry<'_, K, V> {
 ///
 /// Contains the occupied entry, and the value that was not inserted.
 #[unstable(feature = "map_try_insert", issue = "none")]
-#[derive(Debug)]
 pub struct OccupiedError<'a, K: 'a, V: 'a> {
     /// The entry in the map that was already occupied.
     pub entry: OccupiedEntry<'a, K, V>,
@@ -1897,6 +1896,17 @@ pub struct OccupiedError<'a, K: 'a, V: 'a> {
     pub value: V,
 }
 
+#[unstable(feature = "map_try_insert", issue = "none")]
+impl<K: Debug, V: Debug> Debug for OccupiedError<'_, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("OccupiedError")
+            .field("key", self.entry.key())
+            .field("old_value", self.entry.get())
+            .field("new_value", &self.value)
+            .finish()
+    }
+}
+
 #[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);