Re: [PATCH v3 2/4] rust: num: add Bounded integer wrapping type

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Nov 06, 2025 at 04:07:14PM +0900, Alexandre Courbot wrote:
> Add the `Bounded` integer wrapper type, which restricts the number of
> bits allowed to represent of value.
> 
> This is useful to e.g. enforce guarantees when working with bitfields
> that have an arbitrary number of bits.
> 
> Alongside this type, provide many `From` and `TryFrom` implementations
> are to reduce friction when using with regular integer types. Proxy
> implementations of common integer operations are also provided.
> 
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
>  rust/kernel/num.rs         |    3 +
>  rust/kernel/num/bounded.rs | 1045 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 1048 insertions(+)
> 
> diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
> index 3f85e50b8632..bc9abcc3a317 100644
> --- a/rust/kernel/num.rs
> +++ b/rust/kernel/num.rs
> @@ -4,6 +4,9 @@
>  
>  use core::ops;
>  
> +pub mod bounded;
> +pub use bounded::*;
> +
>  /// Designates unsigned primitive types.
>  pub struct Unsigned(());
>  
> diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
> new file mode 100644
> index 000000000000..2e4bc4ce9af5
> --- /dev/null
> +++ b/rust/kernel/num/bounded.rs
> @@ -0,0 +1,1045 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Implementation of [`Bounded`], a wrapper around integer types limiting the number of bits
> +//! usable for value representation.
> +
> +use core::{
> +    cmp,
> +    fmt,
> +    ops::{
> +        self,
> +        Deref, //
> +    }, //,
> +};
> +
> +use kernel::{
> +    num::Integer,
> +    prelude::*, //
> +};
> +
> +/// Evaluates to `true` if `$value` can be represented using at most `$n` bits in a `$type`.
> +///
> +/// Can be used in const context.
> +macro_rules! fits_within {
> +    ($value:expr, $type:ty, $n:expr) => {{
> +        let shift: u32 = <$type>::BITS - $n;
> +
> +        // `value` fits within `$n` bits if shifting it left by the number of unused bits, then
> +        // right by the same number, doesn't change it.
> +        //
> +        // This method has the benefit of working for both unsigned and signed values.
> +        ($value << shift) >> shift == $value

I'm still confused about whether this works or not for signed values.

I guess for a signed 4-bit int, the range of values is -8 to 7, so those
are the values that this shift should preserve the values of. Is that
what it does?

Alice