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lang: get offsetof const fn ready

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Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
This commit is contained in:
Wolfgang Bumiller 2021-10-12 14:10:19 +02:00
parent af33a97547
commit 8f89f9ad60
1 changed files with 7 additions and 8 deletions
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15
proxmox-lang/src/lib.rs
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@ -72,22 +72,21 @@ macro_rules! static_assert_size {
/// #[repr(C)]
/// struct Stuff {
/// first: u32,
/// second: u32,
/// second: u16,
/// third: u16,
/// fourth: 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, $field:ident) => {
unsafe { &(*(std::ptr::null::<$ty>())).$field as *const _ as usize }
unsafe { &(std::mem::transmute::<_, &$ty>(0usize).$field) as *const _ as usize }
};
}