use mem::conjure_zst directly

library/proc_macro/src/bridge/selfless_reify.rs

@@ -55,7 +55,7 @@ pub(super) const fn reify_to_extern_c_fn_hrt_bridge<
         let f = unsafe {
             // SAFETY: `F` satisfies all criteria for "out of thin air"
             // reconstructability (see module-level doc comment).
-            mem::MaybeUninit::<F>::uninit().assume_init()
+            mem::conjure_zst::<F>()
         };
         f(bridge)
     }

library/proc_macro/src/lib.rs

@@ -27,6 +27,7 @@
 #![feature(restricted_std)]
 #![feature(rustc_attrs)]
 #![feature(extend_one)]
+#![feature(mem_conjure_zst)]
 #![recursion_limit = "256"]
 #![allow(internal_features)]
 #![deny(ffi_unwind_calls)]