use take and remove Copy

src/stream/stream/min_by_key.rs

@@ -54,7 +54,7 @@ where
                 }
                 Poll::Pending
             }
-            None => Poll::Ready(*this.min),
+            None => Poll::Ready(this.min.take()),
         }
     }
 }

src/stream/stream/mod.rs

@@ -633,6 +633,7 @@ extension_trait! {
         ) -> impl Future<Output = Option<Self::Item>> [MinByKeyFuture<Self, Self::Item, K>]
         where
             Self: Sized,
+            Self::Item: Ord,
             K: FnMut(&Self::Item) -> Self::Item,
         {
             MinByKeyFuture::new(self, key_by)