New GroupingMap::collect_in

Author: Philippe-Cholet

src/grouping_map.rs

@@ -256,16 +256,7 @@ where
     where
         C: Default + Extend<V>,
     {
-        let mut destination_map = HashMap::new();
-
-        self.iter.for_each(|(key, val)| {
-            destination_map
-                .entry(key)
-                .or_insert_with(C::default)
-                .extend(Some(val));
-        });
-
-        destination_map
+        self.collect_in(HashMap::new())
     }
 
     /// Groups elements from the `GroupingMap` source by key and finds the maximum of each group.
@@ -667,4 +658,20 @@ where
             map,
         )
     }
+
+    /// Apply [`collect`](Self::collect) with a provided empty map
+    /// (`BTreeMap` or `HashMap` with any hasher).
+    pub fn collect_in<C, M>(self, mut map: M) -> M
+    where
+        C: Default + Extend<V>,
+        M: Map<Key = K, Value = C>,
+    {
+        debug_assert!(map.is_empty());
+
+        self.iter.for_each(|(key, val)| {
+            map.entry_or_default(key).extend(Some(val));
+        });
+
+        map
+    }
 }