Re: [PATCH v2] rust: check type of `$ptr` in `container_of!`

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

On Sat Apr 12, 2025 at 8:16 PM CEST, Tamir Duberstein wrote:
> Add a compile-time check that `*$ptr` is of the type of `$type->$($f)*`.
>
> Given the incorrect usage:
>
> ; diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> ; index 8d978c896747..6a7089149878 100644
> ; --- a/rust/kernel/rbtree.rs
> ; +++ b/rust/kernel/rbtree.rs
> ; @@ -329,7 +329,7 @@ fn raw_entry(&mut self, key: &K) -> RawEntry<'_, K, V> {
> ;          while !(*child_field_of_parent).is_null() {
> ;              let curr = *child_field_of_parent;
> ;              // SAFETY: All links fields we create are in a `Node<K, V>`.
> ; -            let node = unsafe { container_of!(curr, Node<K, V>, links) };
> ; +            let node = unsafe { container_of!(curr, Node<K, V>, key) };
> ;
> ;              // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> ;              match key.cmp(unsafe { &(*node).key }) {
>
> this patch produces the compilation error:
>
> ; error[E0308]: mismatched types
> ;    --> rust/kernel/lib.rs:206:26
> ;     |
> ; 206 |             [$field_ptr, field_ptr]; // typeof(`$ptr_to_field`) == typeof(`$Container.$($fields)*`)
> ;     |                          ^^^^^^^^^ expected `*mut rb_node`, found `*mut K`
> ;     |
> ;    ::: rust/kernel/rbtree.rs:270:6
> ;     |
> ; 270 | impl<K, V> RBTree<K, V>
> ;     |      - found this type parameter
> ; ...
> ; 332 |             let node = unsafe { container_of!(curr, Node<K, V>, key) };
> ;     |                                 ------------------------------------ in this macro invocation
> ;     |
> ;     = note: expected raw pointer `*mut bindings::rb_node`
> ;                found raw pointer `*mut K`
> ;     = note: this error originates in the macro `container_of` (in Nightly builds, run with -Z macro-backtrace for more info)
> ;
> ; error: aborting due to 1 previous error
>
> Suggested-by: Alice Ryhl <aliceryhl@google.com>
> Link: https://lore.kernel.org/all/CAH5fLgh6gmqGBhPMi2SKn7mCmMWfOSiS0WP5wBuGPYh9ZTAiww@mail.gmail.com/
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

> ---
> I also considered an implementation using a function, but the resulting
> compilation error was not as concise:

Thanks for checking :)

---
Cheers,
Benno