diff --git a/src/lib.rs b/src/lib.rs
index dfa5463bc9..c26d48fc17 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -85,6 +85,7 @@
 #![cfg_attr(feature = "const-fn", feature(const_fn))]
 #![cfg_attr(feature = "const-fn", feature(const_manually_drop_new))]
 #![cfg_attr(feature = "const-fn", feature(untagged_unions))]
+#![cfg_attr(feature = "const-fn", feature(const_fn_union))]
 #![cfg_attr(feature = "smaller-atomics", feature(core_intrinsics))]
 #![no_std]