Re: [PATCH] [v2] rust: introduce sfile macro for succinct code tracing

[Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>]

On Sat, Jun 7, 2025 at 12:11 AM Timur Tabi <ttabi@nvidia.com> wrote:
>
> I don't understand why, when I use "&FILE[start..]", this compiles as inline:
>
>         pr_err!("{}:{}\n", sfile!(), line!());
>
> but this doesn't:
>
>         const SFILE: &'static str = sfile!();
>
> It seems to me that the second line is "more const" than the first line, if that makes any sense.

The second call is in a const context, the first is just a normal
line. So the second must be a constant expression, which is a big
restriction on what you can write there, but the first doesn't require
that.

Hmm... Perhaps you are thinking that the compiler (or optimizer)
should know that it all resolves to a constant, and thus it should
allow it anyway.

However, constant evaluation doesn't work like that, i.e. even if some
expressions may happen to be able to be evaluated at compile-time, it
doesn't make them constant expressions.

It is similar to how C++'s `constexpr` works, if you have done that.
The languages keep expanding what can be done within const contexts
over time, but it is not based on whether a given instance could be.

(One place where we do use "this particular case must be X" kind of
logic is `build_assert!`).

> You mean you don't like [PATCH][V2]?  I've been doing that for 20 years, and this is the first time
> anyone's complained.

Sorry, I should have said "ideally" there too, since I intended it
mostly as a suggestion like the one in parenthesis. Neither of those
are blockers, of course, and we regularly take different styles of
patches. The `--base` thing is more "important" (useful).

Cheers,
Miguel