Fix field_offset! macro to avoid ptr -> int casts

These have been removed in recent nightlies,
since pointers can't nessicarrily have a meaningfull integer value.
I beleive this is because of issues with pointer 'provenance'.
Here is a good blog post explaining the issue in depth:
https://www.ralfj.de/blog/2020/12/14/provenance.html

Author: Techcable

lib/derive/tests/funcs.rs

@@ -1,4 +1,3 @@
-#![feature(const_raw_ptr_to_usize_cast)]
 use std::os::raw::c_void;
 
 use static_reflect_derive::reflect_func;

src/lib.rs

@@ -13,18 +13,12 @@
 #![feature(
     const_panic, const_option, // We use Option::unwrap
     const_fn_fn_ptr_basics, // We use PhantomData<fn() -> T>
+    const_fn_trait_bound,
     // Used for field_offset macro
     const_raw_ptr_deref,
     const_maybe_uninit_as_ptr,
     const_ptr_offset_from,
 )]
-/*
- * This is required on recent nightly
- *
- * However it breaks on the version that docs.rs is using (as of this writing).
- * Therefore, we have to turn it off there.
- */
-#![cfg_attr(not(feature="docs-rs"), feature(const_fn_trait_bound))]
 #![cfg_attr(feature = "never", feature(never_type))]
 
 mod macros;

src/macros.rs

@@ -20,7 +20,7 @@ macro_rules! field_offset {
         unsafe {
             let uninit = core::mem::MaybeUninit::<$target>::uninit();
             let base = uninit.as_ptr();
-            (core::ptr::addr_of!((*base)$(.$field)*).cast::<u8>().offset_from(base as *const u8))
+            (core::ptr::addr_of!((*base)$(.$field)*).cast::<u8>().offset_from(base as *const u8) as usize)
         }
     }
 }