Re: [PATCH v4] rust: introduce sfile macro for succinct code tracing

[Timur Tabi <ttabi@nvidia.com>]

On Tue, 2025-06-17 at 09:54 +0200, Miguel Ojeda wrote:
> Yeah, I meant the `const fn`, not the macro, i.e. returning `Option`
> (not `Result`) so that we keep the const block in the macro with an
> `unwrap()` in the macro (so no runtime panics).

I'm having trouble coding this up.  I have this so far, but it doesn't compile:

#[macro_export]
macro_rules! sfile {
    () => {{
        const fn shortname() -> Option<&'static str> {
            const FILE: &str = core::file!();
...

            unsafe { core::str::from_utf8_unchecked(p) }
        };

        shortname().unwrap()
    }};
}

I get a bunch of errors, starting with this:

error[E0308]: mismatched types
   --> /home/ttabi/linux/rust/kernel/print.rs:492:22
    |
456 | macro_rules! sfile {
    | ------------------ in this expansion of `sfile!`
457 |     () => {{
458 |         const fn shortname() -> Option<&'static str> {
    |                                 -------------------- expected `Option<&'static str>` because
of return type
...
492 |             unsafe { core::str::from_utf8_unchecked(p) }
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Option<&str>`, found `&str`
    |
   ::: samples/rust/rust_minimal.rs:25:37
    |
25  |         const SFILE: &'static str = sfile!();
    |                                     -------- in this macro invocation
    |
    = note:   expected enum `Option<&'static str>`
            found reference `&str`