macros: Update offset_of!

- assert! access from core.
- nested unsafe raises an error on older rust compilers on CI.

[ re-split commits as authorized by the author, and update the commit
  summary - Trevor ]

Author: devnexen

src/macros.rs

@@ -418,10 +418,12 @@ macro_rules! offset_of {
         let $Ty { $field: _, .. };
         let data = core::mem::MaybeUninit::<$Ty>::uninit();
         let ptr = data.as_ptr();
+        // nested unsafe, see f!
+        #[allow(unused_unsafe)]
         // SAFETY: computed address is inbounds since we have a stack alloc for T
         let fptr = unsafe { core::ptr::addr_of!((*ptr).$field) };
         let off = (fptr as usize).checked_sub(ptr as usize).unwrap();
-        assert!(off <= core::mem::size_of::<$Ty>());
+        core::assert!(off <= core::mem::size_of::<$Ty>());
         off
     }};
 }