deny -> warn

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;