Comment in the implementation that the assert isn't guaranteed.

This could be made guaranteed in future because it's UB to fail it right now, but the other way would introduce unsoundness, so this is the best way to add it for now.

Author: scottmcm

library/core/src/mem/mod.rs

@@ -720,6 +720,8 @@ pub unsafe fn uninitialized<T>() -> T {
 #[unstable(feature = "mem_conjure_zst", issue = "95383")]
 #[track_caller]
 pub const unsafe fn conjure_zst<T>() -> T {
+    // This is not a guarantee exposed to clients, but it'll easily optimize out
+    // in the sound cases, so we might as well check because we can.
     assert!(size_of::<T>() == 0); // FIXME: Use assert_eq! once that's allowed in const
 
     // SAFETY: because the caller must guarantee that it's inhabited and zero-sized,