New GroupingMap::product_in

src/grouping_map.rs

@@ -559,7 +559,7 @@ where
     where
         V: Mul<V, Output = V>,
     {
-        self.reduce(|acc, _, val| acc * val)
+        self.product_in(HashMap::new())
     }
 }
 
@@ -786,4 +786,14 @@ where
     {
         self.reduce_in(|acc, _, val| acc + val, map)
     }
+
+    /// Apply [`product`](Self::product) with a provided empty map
+    /// (`BTreeMap` or `HashMap` with any hasher).
+    pub fn product_in<M>(self, map: M) -> M
+    where
+        V: Mul<V, Output = V>,
+        M: Map<Key = K, Value = V>,
+    {
+        self.reduce_in(|acc, _, val| acc * val, map)
+    }
 }