Do not require ptr validity in rc::data_offset

Author: CAD97

src/liballoc/lib.rs

@@ -100,6 +100,7 @@
 #![feature(fundamental)]
 #![feature(internal_uninit_const)]
 #![feature(lang_items)]
+#![feature(layout_for_ptr)]
 #![feature(libc)]
 #![feature(negative_impls)]
 #![feature(new_uninit)]

src/liballoc/rc.rs

@@ -245,7 +245,7 @@ use core::hash::{Hash, Hasher};
 use core::intrinsics::abort;
 use core::iter;
 use core::marker::{self, PhantomData, Unpin, Unsize};
-use core::mem::{self, align_of, align_of_val, forget, size_of_val};
+use core::mem::{self, align_of, align_of_val_raw, forget, size_of_val};
 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
@@ -2114,7 +2114,7 @@ unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
     // Because it is ?Sized, it will always be the last field in memory.
     // Note: This is a detail of the current implementation of the compiler,
     // and is not a guaranteed language detail. Do not rely on it outside of std.
-    unsafe { data_offset_align(align_of_val(&*ptr)) }
+    unsafe { data_offset_align(align_of_val_raw(ptr)) }
 }
 
 /// Computes the offset of the data field within `RcBox`.