Re: [PATCH v3 1/3] rust: num: add cv! macro to create values from constant expressions

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

On Fri Sep 18, 2026 at 7:03 PM BST, Alexandre Courbot wrote:
> On Thu Sep 17, 2026 at 9:53 AM BST, Gary Guo wrote:
>> On Tue Sep 15, 2026 at 8:39 AM BST, Alexandre Courbot wrote:
>>> On Wed Sep 2, 2026 at 6:16 PM JST, 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
>>>> `FromConst`. The value is then converted and appears in the
>>>> associated constant `FromConst::VALUE`. 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>
>>>
>>> I don't think we have any user but nova-core at the moment, and it
>>> benefits from this in several series (patch 3 here, but also ID pool and
>>> later r000). Miguel, is this ok if we take it (i.e. the next revision)
>>> through drm-rust-next?
>>
>> How about a topic tag shared between Rust and drm-rust-next? We could also
>> remove the old num casts API as part of topic tag and fix all existing users as
>> part of merge.
>
> Not opposed to the idea but IIRC there was already some pushback to the
> idea. There doesn't seem to be an urgency to remove the old num casts
> API that justifies doing it now with a more complicated merge path. 

Sorry, what pushbacks? I thought that `cv!` is what everyone agrees on. I see no
downside of this compared to the turbofish const conversion APIs.

I am not opposed to delaying removal for the next cycle, but IMO the `cv!` macro
should be present on Rust tree so it's visible and improvements can be made from
there, so I think we should do a topic tag regardless whether the old API
removal happens or not.

Best,
Gary