Re: [PATCH v8 03/12] rust: num: add cv! macro to create values from constant expressions

["Eliot Courtney" <ecourtney@nvidia.com>]

On Thu Aug 27, 2026 at 8:12 PM JST, Alexandre Courbot wrote:
> On Thu Aug 27, 2026 at 7:42 PM JST, Alexandre Courbot wrote:
>> On Thu Aug 27, 2026 at 6:32 PM JST, Alice Ryhl wrote:
>>> On Thu, Aug 27, 2026 at 04:28:31PM +0900, Eliot Courtney wrote:
>>>> Currently, using NonZero/Bounded constants is quite verbose. It's
>>>> unfortunate because it disincentivizes using it in interface boundaries.
>>>> Introduce a macro to make it nicer to use. The macro `cv!` (for constant
>>>> value) takes a const integer expression and widens it to i128 (at build
>>>> time only) before passing it as a const generic value to a new trait
>>>> function `FromConst::from_const`. The trait is implemented by NonZero,
>>>> Bounded, and Alignment and lets values of each be constructed from
>>>> constants without a verbose turbofish syntax. For example,
>>>> `const { NonZero::new(1).unwrap() }` can be written as `cv!(1)`.
>>>> 
>>>> Suggested-by: Gary Guo <gary@garyguo.net>
>>>> Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
>>>
>>> This doesn't work in const context, so I don't think this is a great
>>> strategy.
>>>
>>> I would want to use it for cases like this:
>>>
>>> drivers/android/binder/netlink.rs
>>>         const BINDER_CMD_REPORT: u8 = kernel::uapi::BINDER_CMD_REPORT as u8;
>>>         const BINDER_A_REPORT_ERROR: c_int = kernel::uapi::BINDER_A_REPORT_ERROR as c_int;
>>>         const BINDER_A_REPORT_CONTEXT: c_int = kernel::uapi::BINDER_A_REPORT_CONTEXT as c_int;
>>>         const BINDER_A_REPORT_FROM_PID: c_int = kernel::uapi::BINDER_A_REPORT_FROM_PID as c_int;
>>>         const BINDER_A_REPORT_FROM_TID: c_int = kernel::uapi::BINDER_A_REPORT_FROM_TID as c_int;
>>>         const BINDER_A_REPORT_TO_PID: c_int = kernel::uapi::BINDER_A_REPORT_TO_PID as c_int;
>>>         const BINDER_A_REPORT_TO_TID: c_int = kernel::uapi::BINDER_A_REPORT_TO_TID as c_int;
>>>         const BINDER_A_REPORT_IS_REPLY: c_int = kernel::uapi::BINDER_A_REPORT_IS_REPLY as c_int;
>>>         const BINDER_A_REPORT_FLAGS: c_int = kernel::uapi::BINDER_A_REPORT_FLAGS as c_int;
>>>         const BINDER_A_REPORT_CODE: c_int = kernel::uapi::BINDER_A_REPORT_CODE as c_int;
>>>         const BINDER_A_REPORT_DATA_SIZE: c_int = kernel::uapi::BINDER_A_REPORT_DATA_SIZE as c_int;
>>
>> `const_as!` [1] should do the trick for this, provided you don't need to
>> create a const `NonZero`.
>>
>> [1] https://lore.kernel.org/all/20260825-const_as-v1-1-1ce712225fe2@nvidia.com/
>
> ... but I agree it would be nice to be able to use this in const
> context. And there is an overlap with `const_as!` that becomes more
> obvious the more I look at it.
>
> In for a penny, in for a pound of macro code as they say. Since we
> agreed on using macros, how about unifying both under the same `cv!`
> macro, with as many branches as we have types we want to initialize from
> a constant value? For instance:
>
>     // Does what `const_as!` currently does under the hood.
>     const BINDER_CMD_REPORT: u8 = cv!(u8::from(kernel::uapi::BINDER_CMD_REPORT));
>     // Calls `NonZero::new().unwrap()` under the hood.
>     const SOME_NONZERO: NonZero<u8> = cv!(NonZero::new(kernel::uapi::NONZERO_VALUE));
>     // Calls `Bounded::new::<{ ...}>()` under the hood.
>     const SOME_BOUNDED: Bounded<u32, 2> = cv!(Bounded::new(kernel::uapi::SMALL_VALUE));
>
> I.e. we would have one extra matching arm in `cv!` per type it handles
> instead of implementing a trait. The syntax of the macro would look more
> natural (bye bye `const_as`'s awkward `=>`), albeit it would have the
> limitations of such a semantic dispatch.
>
> Even the name `const_as!` wasn't really accurate to begin with: what it
> really emulates is a const `try_from`, and we even discussed
> implementing it in these terms in the future.
>
> I'm sure the idea needs more polishing but I think there's something to
> explore here.

Yeah I agree that const_as! is similar and if we had const traits we
could fully merge them and have it always work in a const context for
both duties (which are really a const tryfrom as you said).

I am not sure about the suggested syntax (e.g.
cv!(Bounded::new(kernel::uapi::SMALL_VALUE))), since it seems very
verbose.

Alternatively, what about just directly merging them so you use
cv!(5) (the non const context FromConst::from_const dispatcher in this
series) or cv!(value => u8) (exactly const_as!), with the =>
distinguishing between the two?

So this is what would work for Alice's example (const_as! but pushed
inside cv!):
```
const BINDER_CMD_REPORT: u8 = cv!(kernel::uapi::BINDER_CMD_REPORT => u8);
```

When we have const traits then we could remove the => syntax I think.