[PATCH] rust: error: allow `useless_conversion` for 32-bit builds

[Alice Ryhl]

On Tue, Jul 30, 2024 at 5:57 PM Miguel Ojeda <ojeda at kernel.org> wrote:
>
> For the new Rust support for 32-bit arm [1], Clippy warns:
>
>     error: useless conversion to the same type: `i32`
>        --> rust/kernel/error.rs:139:36
>         |
>     139 |         unsafe { bindings::ERR_PTR(self.0.into()) as *mut _ }
>         |                                    ^^^^^^^^^^^^^ help: consider removing `.into()`: `self.0`
>         |
>         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
>         = note: `-D clippy::useless-conversion` implied by `-D warnings`
>         = help: to override `-D warnings` add `#[allow(clippy::useless_conversion)]`
>
> The `self.0.into()` converts an `c_int` into `ERR_PTR`'s parameter
> which is a `c_long`. Thus, both types are `i32` in 32-bit. Therefore,
> allow it for those architectures.
>
> Link: https://lore.kernel.org/rust-for-linux/2dbd1491-149d-443c-9802-75786a6a3b73@gmail.com/ [1]
> Signed-off-by: Miguel Ojeda <ojeda at kernel.org>
> ---
>  rust/kernel/error.rs | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
> index 145f5c397009..6f1587a2524e 100644
> --- a/rust/kernel/error.rs
> +++ b/rust/kernel/error.rs
> @@ -135,8 +135,11 @@ pub(crate) fn to_blk_status(self) -> bindings::blk_status_t {
>      /// Returns the error encoded as a pointer.
>      #[allow(dead_code)]
>      pub(crate) fn to_ptr<T>(self) -> *mut T {
> +        #[cfg_attr(target_pointer_width = "32", allow(clippy::useless_conversion))]
>          // SAFETY: `self.0` is a valid error due to its invariant.
> -        unsafe { bindings::ERR_PTR(self.0.into()) as *mut _ }
> +        unsafe {
> +            bindings::ERR_PTR(self.0.into()) as *mut _
> +        }
>      }

The formatting here is a bit weird. Perhaps we should swap the cfg and
the comment?

Either way, it looks good to me.

Reviewed-by: Alice Ryhl <aliceryhl at google.com>