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

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 <[email protected]>
>> Signed-off-by: Eliot Courtney <[email protected]>
>
> 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/[email protected]/