Re: [PATCH v5 1/7] rust: time: make Delta generic over its time unit

[FUJITA Tomonori <tomo@flapping.org>]

On Sun, 09 Aug 2026 11:26:49 +0900
"Alexandre Courbot" <acourbot@nvidia.com> wrote:

> On Fri Aug 7, 2026 at 9:10 PM JST, FUJITA Tomonori wrote:
>>>> +
>>>> +impl TimeUnit for Nsec {
>>>> +    type Repr = i64;
>>>> +}
>>>> +
>>>>  /// A span of time.
>>>>  ///
>>>> -/// This struct represents a span of time, with its value stored as nanoseconds.
>>>> -/// The value can represent any valid i64 value, including negative, zero, and
>>>> -/// positive numbers.
>>>> +/// The span is stored in the unit given by the type parameter `U` (see
>>>> +/// [`TimeUnit`]); its value has type `U::Repr`. `U` defaults to [`Nsec`], so a
>>>> +/// plain [`Delta`] is a span in nanoseconds. The value can be negative, zero, or
>>>> +/// positive.
>>>>  #[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord, Debug)]
>>>> -pub struct Delta {
>>>> -    nanos: i64,
>>>> +pub struct Delta<U: TimeUnit = Nsec> {
>>>> +    value: U::Repr,
>>>>  }
>>>>  
>>>>  impl ops::Add for Delta {
>>> 
>>> When you add `Jiffy` later, this impl block will only cover
>>> `Delta<Nsec>`. Is that intentional, or did you intend to support
>>> all these operations operations for `Delta<Jiffy>` as well?
>>
>> Intentional. Delta<Jiffy> exists to carry a jiffies-valued timeout
>> across the C boundary; it is not meant as a general arithmetic type. I
>> can add them if a user needs them.
> 
> Is there a reason for not doing it now? Common arithmetic sounds useful
> for any unit, and since they are already agreed to panic at the bounds
> we can constrain them on `U` implementing the corresponding traits.

I was planning to do that in a separate patch on top of this series.

You are right that `Add`, `Sub`, `Mul` and the `*Assign` variants are
straightforward. `Div` isn't; unlike `Delta<Nsec>`, `Delta<Jiffy>`
needs no CONFIG_64BIT split, as a single shared impl doesn't work.
This part needs some discussion, so I'd rather keep it out of this
series.