diff --git a/src/librustc/middle/subst.rs b/src/librustc/middle/subst.rs
index ddc817ffc023e..f8c6d3d934123 100644
--- a/src/librustc/middle/subst.rs
+++ b/src/librustc/middle/subst.rs
@@ -555,6 +555,11 @@ impl<'a,T> Iterator for EnumeratedItems<'a,T> {
             None
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.vec.as_slice().len();
+        (size, Some(size))
+    }
 }
 
 impl<T> IntoIterator for VecPerParamSpace<T> {