Re: [PATCH v3] rust: check type of `$ptr` in `container_of!`

[Alice Ryhl <aliceryhl@google.com>]

On Sun, Apr 27, 2025 at 03:59:48PM -0700, John Hubbard wrote:
> On 4/23/25 10:40 AM, Tamir Duberstein wrote:
> ...
> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > index 1df11156302a..d14ed86efb68 100644
> > --- a/rust/kernel/lib.rs
> > +++ b/rust/kernel/lib.rs
> > @@ -198,9 +198,15 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
> >   /// ```
> >   #[macro_export]
> >   macro_rules! container_of {
> > -    ($ptr:expr, $type:ty, $($f:tt)*) => {{
> > -        let offset: usize = ::core::mem::offset_of!($type, $($f)*);
> > -        $ptr.byte_sub(offset).cast::<$type>()
> > +    ($field_ptr:expr, $Container:ty, $($fields:tt)*) => {{
> > +        let offset: usize = ::core::mem::offset_of!($Container, $($fields)*);
> > +        let field_ptr = $field_ptr;
> > +        let container_ptr = field_ptr.byte_sub(offset).cast::<$Container>();
> > +        if false {
> 
> This jumped out at me. It's something that I'd like to recommend NOT
> doing, here or anywhere else, because:
> 
>     a) Anything of the form "if false" will get removed by any compiler
>        worthy of the name, especially in kernel builds.

The `if false` branch is used to trigger a compilation failure when the
macro is used incorrectly. The intent is that the compiler should
optimize it out. I don't think there's anything wrong with that pattern.

Alice