Fix the 'field_offset!' macro on latest nightly

I believe the old version was actually undefined behavior :(

Author: Techcable

src/lib.rs

@@ -15,7 +15,8 @@
     const_fn_fn_ptr_basics, // We use PhantomData<fn() -> T>
     // Used for field_offset macro
     const_raw_ptr_deref,
-    const_raw_ptr_to_usize_cast,
+    const_maybe_uninit_as_ptr,
+    const_ptr_offset_from,
 )]
 /*
  * This is required on recent nightly

src/macros.rs

@@ -18,12 +18,9 @@ macro_rules! define_extern_type {
 macro_rules! field_offset {
     ($target:path, $($field:ident),+) => {
         unsafe {
-            /*
-             * I'm going to assume the dereference is safe,
-             * because of the presense of '(*uninit.as_mut_ptr()).field'
-             * in the documentation for std::ptr::addr_of_mut
-             */
-            (std::ptr::addr_of!((*(1 as *mut $target))$(.$field)*) as usize) - 1
+            let uninit = core::mem::MaybeUninit::<$target>::uninit();
+            let base = uninit.as_ptr();
+            (core::ptr::addr_of!((*base)$(.$field)*).cast::<u8>().offset_from(base as *const u8))
         }
     }
 }