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

["Alexandre Courbot" <acourbot@nvidia.com>]

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.