Re: [PATCH v10 4/5] rust: replace `kernel::c_str!` with C-Strings

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

On Sat May 24, 2025 at 10:33 PM CEST, Tamir Duberstein wrote:
> -/// Creates a new [`CStr`] from a string literal.
> +/// Creates a static C string wrapper at compile time.
>  ///
> -/// The string literal should not contain any `NUL` bytes.
> +/// Rust supports C string literals since Rust 1.77, and they should be used instead of this macro
> +/// where possible. This macro exists to allow static *non-literal* C strings to be created at
> +/// compile time. This is most often used in other macros.
> +///
> +/// # Panics
> +///
> +/// This macro panics if the operand contains an interior `NUL` byte.
>  ///
>  /// # Examples
>  ///
>  /// ```
> -/// # use kernel::c_str;
> +/// # use kernel::c_str_avoid_literals;
>  /// # use kernel::str::CStr;
> -/// const MY_CSTR: &CStr = c_str!("My awesome CStr!");
> +/// const MY_CSTR: &CStr = c_str_avoid_literals!(concat!(file!(), ":", line!(), ": My CStr!"));
>  /// ```
>  #[macro_export]
> -macro_rules! c_str {
> +macro_rules! c_str_avoid_literals {

I don't like this name, how about `concat_to_c_str` or
`concat_with_nul`?

This macro also is useful from macros that have a normal string literal,
but can't turn it into a `c""` one.

> +    // NB: we could write `($str:lit) => compile_error!("use a C string literal instead");` here but
> +    // that would trigger when the literal is at the top of several macro expansions. That would be
> +    // too limiting to macro authors, so we rely on the name as a hint instead.
>      ($str:expr) => {{
> -        const S: &str = concat!($str, "\0");
> -        const C: &$crate::str::CStr = match $crate::str::CStr::from_bytes_with_nul(S.as_bytes()) {
> -            Ok(v) => v,
> -            Err(_) => panic!("string contains interior NUL"),
> -        };
> +        const S: &'static str = concat!($str, "\0");
> +        const C: &'static $crate::str::CStr =
> +            match $crate::str::CStr::from_bytes_with_nul(S.as_bytes()) {

Why is this still our CStr?

> +                Ok(v) => v,
> +                Err(err) => {
> +                    let _: core::ffi::FromBytesWithNulError = err;

Is this really necessary?

---
Cheers,
Benno

> +                    panic!("string contains interior NUL")
> +                }
> +            };
>          C
>      }};
>  }