[PATCH v2 1/4] rust: add `CheckedAdd` trait

[Alexandre Courbot]

On Mon Aug 4, 2025 at 11:37 PM JST, Daniel Almeida wrote:
> Hi Alex,
>
>> On 4 Aug 2025, at 08:45, Alexandre Courbot <acourbot at nvidia.com> wrote:
>> 
>> Rust provides traits for standard arithmetic and logic operations, but
>> in the context of the kernel we often need to consider overflows. The
>> checked Rust arithmetic methods are unfortunately not behind a trait,
>> which makes them unavailable to generic code.
>> 
>> As a start, add the `CheckedAdd` trait providing the `checked_add`
>> operation and implement it for all integer types. Its name and location
>> are inspired by the user-space `num` crate.
>> 
>> This trait is to be first used by the `Alignment` type.
>> 
>> Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
>> ---
>> rust/kernel/lib.rs |  1 +
>> rust/kernel/num.rs | 28 ++++++++++++++++++++++++++++
>> 2 files changed, 29 insertions(+)
>> 
>> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
>> index 6b4774b2b1c37f4da1866e993be6230bc6715841..2955f65da1278dd4cba1e4272ff178b8211a892c 100644
>> --- a/rust/kernel/lib.rs
>> +++ b/rust/kernel/lib.rs
>> @@ -89,6 +89,7 @@
>> pub mod mm;
>> #[cfg(CONFIG_NET)]
>> pub mod net;
>> +pub mod num;
>> pub mod of;
>> #[cfg(CONFIG_PM_OPP)]
>> pub mod opp;
>> diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
>> new file mode 100644
>> index 0000000000000000000000000000000000000000..c81bb046078b70c321dd52aa9c2b5518be49d249
>> --- /dev/null
>> +++ b/rust/kernel/num.rs
>> @@ -0,0 +1,28 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +//! Numerical and binary utilities for primitive types.
>> +
>> +use core::ops::Add;
>> +
>> +/// Trait for performing a checked addition that returns `None` if the operation would overflow.
>
> nit: this can be [`None`] instead, which will let users click on it in the docs.
>
> This is of course pretty frivolous.

... but correct. Thanks.

>
>> +///
>> +/// This trait exists in order to represent scalar types already having a `checked_add` method in
>> +/// generic code.
>
> Maybe “scalar types that already have a `checked_add` method?
>
> But overall I feel like the whole sentence is a bit hard to parse, JFYI.

Let me rephrase this as "This trait exists to model scalar types with a
`checked_add` method in generic code." (provided this trait survives the
next revision).