Re: [PATCH] rust: retain pointer mut-ness in `container_of!`

[Benno Lossin <benno.lossin@proton.me>]

On Fri Apr 11, 2025 at 2:25 PM CEST, Tamir Duberstein wrote:
> On Fri, Apr 11, 2025 at 5:02 AM Alice Ryhl <aliceryhl@google.com> wrote:
>> On Wed, Apr 09, 2025 at 10:43:16AM -0400, Tamir Duberstein wrote:
>> > @@ -199,9 +199,8 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
>> >  #[macro_export]
>> >  macro_rules! container_of {
>> >      ($ptr:expr, $type:ty, $($f:tt)*) => {{
>> > -        let ptr = $ptr as *const _ as *const u8;
>> >          let offset: usize = ::core::mem::offset_of!($type, $($f)*);
>> > -        ptr.sub(offset) as *const $type
>> > +        $ptr.byte_sub(offset).cast::<$type>()
>> >      }}
>> >  }
>>
>> This implementation does not check the type of `ptr`. Would we not want
>> it to have the type of the field?
>
> I might be missing it but ISTM that the current implementation doesn't
> check that either.
>
> It's not obvious to me how you'd implement such a check; given `$ptr`
> and `$f`, how do you get your hands on the type of `$ptr->$($f)*`?

I don't think it's possible with current rust, but maybe with field
projection (:

    ($ptr:expr, $type:ty, $($f:tt)*) => {{
        // do not run this code, only use it for type-checking:
        let _ = || {
            let mut ptr = $ptr;
            ptr = $ptr.cast::<<field_of!($t, $($f)*) as Field>::Type>();
        };
        let offset: usize = ::core::mem::offset_of!($type, $($f)*);
        $ptr.byte_sub(offset).cast::<$type>()
    }}

---
Cheers,
Benno