Make forget intrinsic safe

Author: tmiasko

core/src/mem/mod.rs

@@ -151,9 +151,14 @@ pub const fn forget<T>(t: T) {
 #[inline]
 #[unstable(feature = "forget_unsized", issue = "none")]
 pub fn forget_unsized<T: ?Sized>(t: T) {
+    #[cfg(bootstrap)]
     // SAFETY: the forget intrinsic could be safe, but there's no point in making it safe since
     // we'll be implementing this function soon via `ManuallyDrop`
-    unsafe { intrinsics::forget(t) }
+    unsafe {
+        intrinsics::forget(t)
+    }
+    #[cfg(not(bootstrap))]
+    intrinsics::forget(t)
 }
 
 /// Returns the size of a type in bytes.