From 5b2e8b4c66c56ef50a853de5a6ccb7f20a0d7921 Mon Sep 17 00:00:00 2001
From: Wolfgang Bumiller <w.bumiller@proxmox.com>
Date: Wed, 17 Nov 2021 10:04:03 +0100
Subject: [PATCH] Revert "lang: get offsetof const fn ready"

This reverts commit 8f89f9ad60948d4853c05801414cfd152ddef16a.

generates broken code in release builds on current rustc
1.55
---
 proxmox-lang/src/lib.rs | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/proxmox-lang/src/lib.rs b/proxmox-lang/src/lib.rs
index 06e0b319..f5c6ebec 100644
--- a/proxmox-lang/src/lib.rs
+++ b/proxmox-lang/src/lib.rs
@@ -72,21 +72,22 @@ macro_rules! static_assert_size {
 /// #[repr(C)]
 /// struct Stuff {
 ///     first: u32,
-///     second: u16,
-///     third: u16,
-///     fourth: u32,
+///     second: u32,
 /// }
 ///
-/// assert_eq!(offsetof!(Stuff, first), 0);
 /// assert_eq!(offsetof!(Stuff, second), 4);
-/// assert_eq!(offsetof!(Stuff, third), 6);
-/// assert_eq!(offsetof!(Stuff, fourth), 8);
 ///
 /// ```
+// FIXME: With 1.56 we get `const transmute` which may help making this usable `const fn` as we can
+// avoid dereferencing the raw pointer by transmuting `0usize` into a proper reference instead.
+//
+// So with 1.56, do this instead:
+//
+//     unsafe { &(std::mem::transmute::<_, &$ty>(0usize).$field) as *const _ as usize }
 #[macro_export]
 macro_rules! offsetof {
     ($ty:ty, $field:ident) => {
-        unsafe { &(std::mem::transmute::<_, &$ty>(0usize).$field) as *const _ as usize }
+        unsafe { &(*(std::ptr::null::<$ty>())).$field as *const _ as usize }
     };
 }