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

[Tamir Duberstein]

On Thu, Apr 17, 2025 at 1:29 PM Boqun Feng <boqun.feng at gmail.com> wrote:
>
> 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 at proton.me>
> > Link: https://lore.kernel.org/all/D8ORTXSUTKGL.1KOJAGBM8F8TN@proton.me/
> > Reviewed-by: Benno Lossin <benno.lossin at proton.me>
> > Signed-off-by: Tamir Duberstein <tamird at 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.

I changed the first two. This one I'll leave as `into()` because the
destination type is in the method signature, which is 5 lines above.

Thanks for the review!