diff --git a/src/librustc_data_structures/obligation_forest/mod.rs b/src/librustc_data_structures/obligation_forest/mod.rs
index 4490e5f86d2..557e5e2186f 100644
--- a/src/librustc_data_structures/obligation_forest/mod.rs
+++ b/src/librustc_data_structures/obligation_forest/mod.rs
@@ -263,7 +263,7 @@ impl<O: ForestObligation> ObligationForest<O> {
             done_cache: Default::default(),
             waiting_cache: Default::default(),
             scratch: Some(vec![]),
-            obligation_tree_id_generator: (0..).map(|i| ObligationTreeId(i)),
+            obligation_tree_id_generator: (0..).map(ObligationTreeId),
             error_cache: Default::default(),
         }
     }