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

[Alice Ryhl]

On Fri, Apr 11, 2025 at 3:09 PM Tamir Duberstein <tamird@xxxxxxxxx> wrote:
>
> On Fri, Apr 11, 2025 at 8:38 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
> >
> > On Fri, Apr 11, 2025 at 2:35 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> > >
> > > 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@xxxxxxxxxx> 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>()
> > >     }}
> >
> > You can definitely implement the check with current Rust. You use
> > addr_of! to create a raw pointer with the type of the field, and
> > trigger a type error if `ptr` doesn't have the same type as that other
> > pointer. Something along these lines would do it:
> >
> > let mut ptr = $ptr;
> > let offset: usize = ::core::mem::offset_of!($type, $($f)*);
> > let container = ptr.byte_sub(offset).cast::<$type>();
> > if false {
> >     ptr = ::core::ptr::addr_of!((*container).$($f)*).cast_mut();
> > }
> > container
>
> It's a nice idea. Wouldn't it require `$ptr` to be `*mut _` and not
> work with `*const _`?

As far as I can tell, it should work with const pointers too. The rhs
of the last assignment gets downgraded back to a const pointer in that
case.

> In any case, I hope we agree that this can be
> done separately.

Yes, since the feature does not exist today, it can happen separately.

Alice