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

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

On Tue Jun 3, 2025 at 7:15 PM CEST, Timur Tabi wrote:
> On Thu, 2025-05-29 at 22:14 +0200, Benno Lossin wrote:
>> > 
>> > The macro goes avoids str::rfind() because currently, that function is not
>> > const.  The compiler emits a call to memrchr, even when called on string
>> > literals.  Instead, the macro implements its own versions of rfind(),
>> > allowing the compiler to generate the slice at compile time.
>> > 
>> > Unfortunately, Rust does not consider the .. operator to be const either,
>> > so sfind!() cannot be assigned to consts.  For example, the following will
>> > not compile:
>> 
>> This is not the `..` operator, but rather the index operation of a str
>> (`&FILE[start..]`). It can be made const by using `from_utf8` [1] and
>> manually creating the correct byte slice using `unsafe` [2] instead.
>> 
>> [1]: https://doc.rust-lang.org/std/str/fn.from_utf8.html
>> [2]: https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html
>
>
> Ok, it took a lot of trial-and-error on my part, but I got this to work.

Great!

> It's not pretty, though.  Is this what you intended?
>
>         let start = find_last_or_zero(FILE, '/') + 1;
>         let end = find_last_or_len(FILE, '.');
>         
>         let base_ptr: *const u8 = FILE.as_ptr();
>         let ptr: *const u8 = unsafe { base_ptr.add(start) };
>         let slice = unsafe { core::slice::from_raw_parts(ptr, end - start) };
>         unsafe { core::str::from_utf8_unchecked(slice) }

Yeah that's roughly what I had in mind.

Maybe we should check that `end >= start` in case the filename is empty
(which shouldn't happen)? I'm not sure what happens with overflows /
other `unsafe` precondition violations...

> The good news is that this allows
>
> 	const SFILE: &'static str = sfile!();
>
> to work, so thank you.

That'll be useful :)

---
Cheers,
Benno