[PATCH v9 1/6] rust: enable `clippy::ptr_as_ptr` lint

[Boqun Feng]

On Wed, Apr 16, 2025 at 01:36:05PM -0400, Tamir Duberstein wrote:
> In Rust 1.51.0, Clippy introduced the `ptr_as_ptr` lint [1]:
> 
> > Though `as` casts between raw pointers are not terrible,
> > `pointer::cast` is safer because it cannot accidentally change the
> > pointer's mutability, nor cast the pointer to other types like `usize`.
> 
> There are a few classes of changes required:
> - Modules generated by bindgen are marked
>   `#[allow(clippy::ptr_as_ptr)]`.
> - Inferred casts (` as _`) are replaced with `.cast()`.
> - Ascribed casts (` as *... T`) are replaced with `.cast::<T>()`.
> - Multistep casts from references (` as *const _ as *const T`) are
>   replaced with `core::ptr::from_ref(&x).cast()` with or without `::<T>`
>   according to the previous rules. The `core::ptr::from_ref` call is
>   required because `(x as *const _).cast::<T>()` results in inference
>   failure.
> - Native literal C strings are replaced with `c_str!().as_char_ptr()`.
> - `*mut *mut T as _` is replaced with `let *mut *const T = (*mut *mut
>   T)`.cast();` since pointer to pointer can be confusing.
> 
> Apply these changes and enable the lint -- no functional change
> intended.
> 
> Link: https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr [1]
> Reviewed-by: Benno Lossin <benno.lossin at proton.me>
> Signed-off-by: Tamir Duberstein <tamird at gmail.com>

Reviewed-by: Boqun Feng <boqun.feng at gmail.com>

A few nits below though...

> ---
>  Makefile                               |  1 +
>  rust/bindings/lib.rs                   |  1 +
>  rust/kernel/alloc/allocator_test.rs    |  2 +-
>  rust/kernel/alloc/kvec.rs              |  4 ++--
>  rust/kernel/device.rs                  |  4 ++--
>  rust/kernel/devres.rs                  |  2 +-
>  rust/kernel/dma.rs                     |  4 ++--
>  rust/kernel/error.rs                   |  2 +-
>  rust/kernel/firmware.rs                |  3 ++-
>  rust/kernel/fs/file.rs                 |  2 +-
>  rust/kernel/kunit.rs                   | 11 +++++++----
>  rust/kernel/list/impl_list_item_mod.rs |  2 +-
>  rust/kernel/pci.rs                     |  2 +-
>  rust/kernel/platform.rs                |  4 +++-
>  rust/kernel/print.rs                   |  6 +++---
>  rust/kernel/seq_file.rs                |  2 +-
>  rust/kernel/str.rs                     |  2 +-
>  rust/kernel/sync/poll.rs               |  2 +-
>  rust/kernel/time/hrtimer/pin.rs        |  2 +-
>  rust/kernel/time/hrtimer/pin_mut.rs    |  2 +-
>  rust/kernel/workqueue.rs               | 10 +++++-----
>  rust/uapi/lib.rs                       |  1 +
>  22 files changed, 40 insertions(+), 31 deletions(-)
> 
[...]
> diff --git a/rust/kernel/list/impl_list_item_mod.rs b/rust/kernel/list/impl_list_item_mod.rs
> index a0438537cee1..1f9498c1458f 100644
> --- a/rust/kernel/list/impl_list_item_mod.rs
> +++ b/rust/kernel/list/impl_list_item_mod.rs
> @@ -34,7 +34,7 @@ pub unsafe trait HasListLinks<const ID: u64 = 0> {
>      unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut ListLinks<ID> {
>          // SAFETY: The caller promises that the pointer is valid. The implementer promises that the
>          // `OFFSET` constant is correct.
> -        unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut ListLinks<ID> }
> +        unsafe { ptr.cast::<u8>().add(Self::OFFSET).cast() }

I think we better do:

	unsafe { ptr.byte_add(Self::OFFSET).cast() }

here, similar for a few instances below. Maybe in a follow-up patch?
byte_add() is way more clear about what is done here.

Regards,
Boqun

>      }
>  }
>  
[...]
> @@ -457,7 +457,7 @@ fn get_work_offset(&self) -> usize {
>      #[inline]
>      unsafe fn raw_get_work(ptr: *mut Self) -> *mut Work<T, ID> {
>          // SAFETY: The caller promises that the pointer is valid.
> -        unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut Work<T, ID> }
> +        unsafe { ptr.cast::<u8>().add(Self::OFFSET).cast::<Work<T, ID>>() }
>      }
>  
>      /// Returns a pointer to the struct containing the [`Work<T, ID>`] field.
> @@ -472,7 +472,7 @@ unsafe fn work_container_of(ptr: *mut Work<T, ID>) -> *mut Self
>      {
>          // SAFETY: The caller promises that the pointer points at a field of the right type in the
>          // right kind of struct.
> -        unsafe { (ptr as *mut u8).sub(Self::OFFSET) as *mut Self }
> +        unsafe { ptr.cast::<u8>().sub(Self::OFFSET).cast::<Self>() }
>      }
>  }
>  
[...]