diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 2e4a8a15d2088..f168458bff1d2 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -32,7 +32,7 @@
 #![feature(slice_partition_dedup)]
 #![feature(copy_within)]
 #![feature(int_error_matching)]
-#![deny(rust_2018_idioms)]
+#![warn(rust_2018_idioms)]
 
 extern crate test;