[PATCH v7 6/7] rust: enable `clippy::cast_lossless` lint

Tamir Duberstein
2025-03-25T20:08:00Z

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 | 10 +++++-----
rust/bindings/lib.rs | 1 +
rust/kernel/net/phy.rs | 4 ++--
rust/uapi/lib.rs | 1 +
5 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index 2af40bfed9ce..2e9eca8b7671 100644
--- a/Makefile
+++ b/Makefile
@@ -479,6 +479,7 @@ 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 ecd87e8ffe05..01337ce896df 100644
--- a/drivers/gpu/drm/drm_panic_qr.rs
+++ b/drivers/gpu/drm/drm_panic_qr.rs
@@ -305,15 +305,15 @@ fn get_next_13b(data: &[u8], offset: usize) -> Option<(u16, usize)> {
// `b` is 20 at max (`bit_off` <= 7 and `size` <= 13).
let b = (bit_off + size) as u16;

- let first_byte = (data[byte_off] << bit_off >> bit_off) as u16;
+ let first_byte = u16::from(data[byte_off] << bit_off >> bit_off);

let number = match b {
0..=8 => first_byte >> (8 - b),
- 9..=16 => (first_byte << (b - 8)) + (data[byte_off + 1] >> (16 - b)) as u16,
+ 9..=16 => (first_byte << (b - 8)) + u16::from(data[byte_off + 1] >> (16 - b)),
_ => {
(first_byte << (b - 8))
- + ((data[byte_off + 1] as u16) << (b - 16))
- + (data[byte_off + 2] >> (24 - b)) as u16
+ + u16::from(data[byte_off + 1] << (b - 16))
+ + u16::from(data[byte_off + 2] >> (24 - b))
}
};
Some((number, size))
@@ -414,7 +414,7 @@ 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();
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()
}

/// 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