diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index 31b71732635c..5cdc692708a6 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/compiler/rustc_feature/src/builtin_attrs.rs
@@ -535,11 +535,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     gated!(
-        ffi_pure, Normal, template!(Word), WarnFollowing,
+        unsafe ffi_pure, Normal, template!(Word), WarnFollowing,
         EncodeCrossCrate::No, experimental!(ffi_pure)
     ),
     gated!(
-        ffi_const, Normal, template!(Word), WarnFollowing,
+        unsafe ffi_const, Normal, template!(Word), WarnFollowing,
         EncodeCrossCrate::No, experimental!(ffi_const)
     ),
     gated!(