Re: [PATCH v4 1/4] rust: macros: add derive macro for `Into`

["Benno Lossin" <lossin@kernel.org>]

On Sat Dec 27, 2025 at 11:45 AM CET, Jesung Yang wrote:
> On Sat, Dec 27, 2025 at 1:57 PM Benno Lossin <lossin@kernel.org> wrote:
>> On Fri Dec 26, 2025 at 10:36 AM CET, Jesung Yang wrote:
>> > On Fri, Dec 26, 2025 at 2:40 AM Benno Lossin <lossin@kernel.org> wrote:
>> >> On Thu Dec 25, 2025 at 9:37 AM CET, Jesung Yang via B4 Relay wrote:
>> >> > +    fn emit_overflow_assert(
>> >> > +        enum_ident: &Ident,
>> >> > +        variants: &[Ident],
>> >> > +        repr_ty: &syn::Path,
>> >> > +        input_ty: &ValidTy,
>> >> > +    ) -> TokenStream {
>> >>
>> >> I feel like we should track this via traits rather than using a const
>> >> assert. That approach will require & generate much less code.
>> >
>> > Sorry, but could you elaborate? A small example of what you have in
>> > mind would help a lot.
>>
>> Oh yeah sorry, I had something different in mind compared to what I'll
>> describe now, but it achieves the same thing without introducing new
>> traits:
>>
>> We have two options:
>> 1) We use `<input_ty as TryFrom<repr_ty>>::try_from` instead of writing
>>    the `fits` function ourself.
>> 2) We require `input_ty: From<repr_ty>`.
>>
>> The first option would still check every variant and should behave the
>> same as your current code.
>>
>> Option 2 allows us to avoid the const altogether, but requires us to
>> choose the smallest integer as the representation (and if we want to be
>> able to use both `i8` and `u8`, we can't). I missed this before, so
>> using option 1 might be the only way to allow conversions of this kind.
>
> AFAIK, `<input_ty as TryFrom<repr_ty>>::try_from` cannot be called in
> const contexts without `#![feature(const_trait_impl, const_convert)]`.
> I assume we want to keep this validation at compile-time? If so, we
> might need to stick with the custom `fits` check for now. Please let me
> know if I misunderstood you.

Oh yeah that doesn't work (yet) :(

I'll take another look at the function itself then.

Cheers,
Benno