Re: [PATCH v10 5/8] rust: clist: Add support to interface with C linked lists

[Alice Ryhl]

On Mon, Feb 23, 2026 at 2:13 AM Joel Fernandes <joelagnelf@xxxxxxxxxx> wrote:
>
> Hi Eliot,
>
> On 2/20/2026 3:16 AM, Eliot Courtney wrote:
> > On Thu Feb 19, 2026 at 5:55 AM JST, Joel Fernandes wrote:
> >> +/// Create a C doubly-circular linked list interface `CList` from a raw `list_head` pointer.
> >> +///
> >> +/// This macro creates a `CList<T, OFFSET>` that can iterate over items of type `$rust_type`
> >> +/// linked via the `$field` field in the underlying C struct `$c_type`.
> >> +///
> >> +/// # Arguments
> >> +///
> >> +/// - `$head`: Raw pointer to the sentinel `list_head` object (`*mut bindings::list_head`).
> >> +/// - `$rust_type`: Each item's rust wrapper type.
> >> +/// - `$c_type`: Each item's C struct type that contains the embedded `list_head`.
> >> +/// - `$field`: The name of the `list_head` field within the C struct.
> >> +///
> >> +/// # Safety
> >> +///
> >> +/// This is an unsafe macro. The caller must ensure:
> >> +///
> >> +/// - `$head` is a valid, initialized sentinel `list_head` pointing to a list that remains
> >> +///   unmodified for the lifetime of the rust `CList`.
> >> +/// - The list contains items of type `$c_type` linked via an embedded `$field`.
> >> +/// - `$rust_type` is `#[repr(transparent)]` over `$c_type` or has compatible layout.
> >> +///
> >> +/// # Examples
> >> +///
> >> +/// Refer to the examples in this module's documentation.
> >> +#[macro_export]
> >> +macro_rules! clist_create {
> >> +    ($head:expr, $rust_type:ty, $c_type:ty, $($field:tt).+) => {{
> >> +        // Compile-time check that field path is a list_head.
> >> +        let _: fn(*const $c_type) -> *const $crate::bindings::list_head =
> >> +            |p| &raw const (*p).$($field).+;
> >> +
> >> +        // Calculate offset and create `CList`.
> >> +        const OFFSET: usize = ::core::mem::offset_of!($c_type, $($field).+);
> >> +        $crate::ffi::clist::CList::<$rust_type, OFFSET>::from_raw($head)
> >> +    }};
> >> +}
> >
> > This uses offset_of! in a way that requires the offset_of_nested
> > feature, so it doesn't build in rust 1.78.0. The feature is already
> > added to rust_allowed_features, so I think it's ok to add
> > #![feature(offset_of_nested)].
>
> Maybe I am missing something, but why should the feature be gated behind
> that if all compiler versions (>= 1.78) support it either in a stable way
> or via an unstable feature flag?

The rust_allowed_features list only applies to drivers and such. It
doesn't apply to the Rust crates in the rust/ directory, which need to
use #![feature] annotations manually.

Alice