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]
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..7555513a4fd8 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 = u16::from(data[self.offset]);
                     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..f821480ad72b 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) == 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()
     }
 
     /// # 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

On Fri, Apr 18, 2025 at 11:37:21AM -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]
> 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>

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

Regards,
Boqun

> 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..7555513a4fd8 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 = u16::from(data[self.offset]);
>                      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..f821480ad72b 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) == 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()
>      }
>  
>      /// # 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
>