Allow unused feature in std_detect

crates/std_detect/src/lib.rs

@@ -22,7 +22,7 @@
 #![allow(internal_features)]
 // Temporary hack: needed to build against toolchains from before the mass feature renaming.
 // Remove this as soon as the stdarch submodule is updated on nightly.
-#![allow(stable_features)]
+#![allow(stable_features, unused_features)]
 #![feature(stdsimd)]
 #![cfg_attr(
     all(