New GroupingMap::fold_in

Author: Philippe-Cholet

src/grouping_map.rs

@@ -188,7 +188,7 @@ where
         R: Clone,
         FO: FnMut(R, &K, V) -> R,
     {
-        self.fold_with(|_, _| init.clone(), operation)
+        self.fold_in(init, operation, HashMap::new())
     }
 
     /// Groups elements from the `GroupingMap` source by key and applies `operation` to the elements
@@ -643,4 +643,15 @@ where
             map,
         )
     }
+
+    /// Apply [`fold`](Self::fold) with a provided empty map
+    /// (`BTreeMap` or `HashMap` with any hasher).
+    pub fn fold_in<FO, R, M>(self, init: R, operation: FO, map: M) -> M
+    where
+        R: Clone,
+        FO: FnMut(R, &K, V) -> R,
+        M: Map<Key = K, Value = R>,
+    {
+        self.fold_with_in(|_, _| init.clone(), operation, map)
+    }
 }