Re: [PATCH v9 5/6] rust: enable `clippy::cast_lossless` lint

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Apr 16, 2025 at 01:36:09PM -0400, Tamir Duberstein wrote:
> Before Rust 1.29.0, Clippy introduced the `cast_lossless` lint [1]:
> 
> > Rust’s `as` keyword will perform many kinds of conversions, including
> > silently lossy conversions. Conversion functions such as `i32::from`
> > will only perform lossless conversions. Using the conversion functions
> > prevents conversions from becoming silently lossy if the input types
> > ever change, and makes it clear for people reading the code that the
> > conversion is lossless.
> 
> While this doesn't eliminate unchecked `as` conversions, it makes such
> conversions easier to scrutinize.  It also has the slight benefit of
> removing a degree of freedom on which to bikeshed. Thus apply the
> changes and enable the lint -- no functional change intended.
> 
> Link: https://rust-lang.github.io/rust-clippy/master/index.html#cast_lossless [1]

Hmm.. I agree with the solution mentioned from the lint URL, using
`from()` is better, so..

> Suggested-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/all/D8ORTXSUTKGL.1KOJAGBM8F8TN@proton.me/
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> ---
>  Makefile                        | 1 +
>  drivers/gpu/drm/drm_panic_qr.rs | 2 +-
>  rust/bindings/lib.rs            | 1 +
>  rust/kernel/net/phy.rs          | 4 ++--
>  rust/uapi/lib.rs                | 1 +
>  5 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 57080a64913f..eb5a942241a2 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -476,6 +476,7 @@ export rust_common_flags := --edition=2021 \
>  			    -Wclippy::all \
>  			    -Wclippy::as_ptr_cast_mut \
>  			    -Wclippy::as_underscore \
> +			    -Wclippy::cast_lossless \
>  			    -Wclippy::ignored_unit_patterns \
>  			    -Wclippy::mut_mut \
>  			    -Wclippy::needless_bitwise_bool \
> diff --git a/drivers/gpu/drm/drm_panic_qr.rs b/drivers/gpu/drm/drm_panic_qr.rs
> index f2a99681b998..d28e8f199d11 100644
> --- a/drivers/gpu/drm/drm_panic_qr.rs
> +++ b/drivers/gpu/drm/drm_panic_qr.rs
> @@ -386,7 +386,7 @@ fn next(&mut self) -> Option<Self::Item> {
>          match self.segment {
>              Segment::Binary(data) => {
>                  if self.offset < data.len() {
> -                    let byte = data[self.offset] as u16;
> +                    let byte = data[self.offset].into();

	let byte = u16::from(data[self.offset]);

otherwise, the code has not local indicator saying what type the byte
is, and given its name is "byte" but it's really a `u16`, I think it's
better we mention the type here.

>                      self.offset += 1;
>                      Some((byte, 8))
>                  } else {
> diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs
> index 0486a32ed314..b105a0d899cc 100644
> --- a/rust/bindings/lib.rs
> +++ b/rust/bindings/lib.rs
> @@ -25,6 +25,7 @@
>  )]
>  
>  #[allow(dead_code)]
> +#[allow(clippy::cast_lossless)]
>  #[allow(clippy::ptr_as_ptr)]
>  #[allow(clippy::undocumented_unsafe_blocks)]
>  mod bindings_raw {
> diff --git a/rust/kernel/net/phy.rs b/rust/kernel/net/phy.rs
> index a59469c785e3..abc58b4d1bf4 100644
> --- a/rust/kernel/net/phy.rs
> +++ b/rust/kernel/net/phy.rs
> @@ -142,7 +142,7 @@ pub fn is_autoneg_enabled(&self) -> bool {
>          // SAFETY: The struct invariant ensures that we may access
>          // this field without additional synchronization.
>          let bit_field = unsafe { &(*self.0.get())._bitfield_1 };
> -        bit_field.get(13, 1) == bindings::AUTONEG_ENABLE as u64
> +        bit_field.get(13, 1) == bindings::AUTONEG_ENABLE.into()

        bit_field.get(13, 1) == u64::from(bindings::AUTONEG_ENABLE)

>      }
>  
>      /// Gets the current auto-negotiation state.
> @@ -426,7 +426,7 @@ impl<T: Driver> Adapter<T> {
>          // where we hold `phy_device->lock`, so the accessors on
>          // `Device` are okay to call.
>          let dev = unsafe { Device::from_raw(phydev) };
> -        T::match_phy_device(dev) as i32
> +        T::match_phy_device(dev).into()

	i32::from(T::match_phy_device(dev))

Thoughts? Better be explicit in these cases, IMO.

Regards,
Boqun

>      }
>  
>      /// # Safety
> diff --git a/rust/uapi/lib.rs b/rust/uapi/lib.rs
> index f03b7aead35a..d5dab4dfabec 100644
> --- a/rust/uapi/lib.rs
> +++ b/rust/uapi/lib.rs
> @@ -14,6 +14,7 @@
>  #![cfg_attr(test, allow(unsafe_op_in_unsafe_fn))]
>  #![allow(
>      clippy::all,
> +    clippy::cast_lossless,
>      clippy::ptr_as_ptr,
>      clippy::undocumented_unsafe_blocks,
>      dead_code,
> 
> -- 
> 2.49.0
>