Re: [PATCH v2 2/5] rust: num: add `shr` and `shl` methods to `Bounded`

["Gary Guo" <gary@garyguo.net>]

On Wed Jan 21, 2026 at 7:23 AM GMT, Alexandre Courbot wrote:
> Shifting a `Bounded` left or right changes the number of bits required
> to represent the value. Add methods that perform the shift and return a
> `Bounded` with the appropriately adjusted bit width.
>
> These methods are particularly useful for bitfield extraction.
>
> Suggested-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
>  rust/kernel/num/bounded.rs | 40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
>
> diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
> index f870080af8ac..8782535770f1 100644
> --- a/rust/kernel/num/bounded.rs
> +++ b/rust/kernel/num/bounded.rs
> @@ -470,6 +470,46 @@ pub fn cast<U>(self) -> Bounded<U, N>
>          // `N` bits, and with the same signedness.
>          Bounded::__new(value)

This patch doesn't apply cleanly. Looks like you send it from the wrong base
commit.

The __new call here is still safe while your code has `unsafe {}` in it.

>      }
> +
> +    /// Right-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, { N - SHIFT }>`.

The returned bound can be larger given the assert below?

> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::num::Bounded;
> +    ///
> +    /// let v = Bounded::<u32, 16>::new::<0xff00>();
> +    /// let v_shifted: Bounded::<u32, 8> = v.shr::<8, _>();
> +    ///
> +    /// assert_eq!(v_shifted.get(), 0xff);
> +    /// ```
> +    pub fn shr<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
> +        const { assert!(RES >= N - SHIFT) }

Quite surprised that rustfmt didn't ask for the block to be expanded into
multiple lines.

I think we probably want to create a new assert macro for this pattern
(obviously this doesn't block this patch).

> +
> +        // SAFETY: we shift the value right by `SHIFT`, reducing the number of bits needed to
> +        // represent the shifted value by as much, and just asserted that `RES == N - SHIFT`.
> +        unsafe { Bounded::__new(self.0 >> SHIFT) }
> +    }
> +
> +    /// Left-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, { N + SHIFT }>`.

Same, the bound can be actually larger.

Best,
Gary

> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::num::Bounded;
> +    ///
> +    /// let v = Bounded::<u32, 8>::new::<0xff>();
> +    /// let v_shifted: Bounded::<u32, 16> = v.shl::<8, _>();
> +    ///
> +    /// assert_eq!(v_shifted.get(), 0xff00);
> +    /// ```
> +    pub fn shl<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
> +        const { assert!(RES >= N + SHIFT) }
> +
> +        // SAFETY: we shift the value left by `SHIFT`, augmenting the number of bits needed to
> +        // represent the shifted value by as much, and just asserted that `RES == N + SHIFT`.
> +        unsafe { Bounded::__new(self.0 << SHIFT) }
> +    }
>  }
>  
>  impl<T, const N: u32> Deref for Bounded<T, N>