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

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

On Thu Aug 27, 2026 at 3:29 PM BST, Alexandre Courbot wrote:
> On Thu Aug 27, 2026 at 10:48 PM JST, Eliot Courtney wrote:
>> 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.
>
> A bit, but what I like is that it looks very close to what you would
> naturally write if you had const traits (minus the unwraps), so you
> don't have to learn a new syntax. As long as it's not *more* verbose
> than natural Rust, I think it's fine.
>
> It also has the benefit of relying less on type inference, i.e. `cv!(5)`
> requires the caller to specify the type even with a `let` statement,
> whereas you could do `let v = cv!(NonZero::new(5));` and it would work
> as expected.

Even with const try_from we'd still want `cv!()` to avoid having to write

    const { Type::try_from(...).unwrap() }

I think having `=>` syntax is great because it is a good place to *optionally*
require type annotation.

For enum repr for example, I think it'd be great that

    const BINDER_CMD_REPORT: u8 = cv!(kernel::uapi::BINDER_CMD_REPORT);

would work directly. It might need some tricks, which I have hard time coming up
as I'm not feeling very well today, but I'll give it a shot over the weekend...

Best,
Gary