Re: [PATCH v2 0/3] rust: const_eval: add a mechanism to do const trait calls

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

On Thu Sep 3, 2026 at 4:34 PM BST, Miguel Ojeda wrote:
> On Thu, Sep 3, 2026 at 5:21 PM Gary Guo <gary@garyguo.net> wrote:
>>
>> Add a mechanism to perform const trait method calls, with the same type
>> inference capabilities. This would only work for concrete types (not inside
>> generic functions).
>>
>> As an example, convert `as_char_ptr_in_const_context`, so instead of
>>
>>     kernel::str::as_char_ptr_in_const_context(c_str)
>>
>> one may write
>>
>>     const_call!(c_str.as_char_ptr())
>>
>> instead.
>>
>> Signed-off-by: Gary Guo <gary@garyguo.net>
>
> Thanks for resending this!
>
> So I definitely like having `#[const_eval_only]`, because we do want
> to limit those calls sometimes and it is nice to have C++'s equivalent
> to `consteval` (perhaps this could be called just `#[consteval]` to
> match?); i.e. what we talked about in the call.
>
> On `Const` and `const_call!`, i.e. the other two patches: I am not
> sure if we care that much to introduce a macro for it, but it doesn't
> look bad either. I guess perhaps it helps to have a consistent way to
> do it and to easily remove it later when the actual feature lands in
> Rust. I would like to hear some thoughts on it from others.

I think it's good to have such macro compared to our current approach; it can be
made into a identity macro once we have const trait impl.

FWIW, I even considered an extra macro that allow you to write

    #[const_trait_impl] // #[const_trait_impl(foreign)] for core types
    impl MyTrait for MyType {}

and generate the `impl Const` part too (and also add a line to the documentation
that the method is const-callable with `const_call!()`).

Best,
Gary