Re: [PATCH v1 19/28] rust: str: add `c_str!` macro

Gary Guo
2022-11-14T14:40:18Z

On Thu, 10 Nov 2022 17:41:31 +0100
Miguel Ojeda <ojeda@kernel.org> wrote:

> From: Gary Guo <gary@garyguo.net>
>
> Add `c_str!`, which is a convenience macro that creates a new `CStr`
> from a string literal.
>
> It is designed to be similar to a `str` in usage, and it is usable
> in const contexts, for instance:
>
> const X: &CStr = c_str!("Example");
>
> Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
> Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> [Reworded, adapted for upstream and applied latest changes]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
> rust/kernel/str.rs | 32 ++++++++++++++++++++++++++++++++
> 1 file changed, 32 insertions(+)
>
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 3ed685cb5a3c..3fb73b888dce 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -128,6 +128,18 @@ impl CStr {
> Ok(unsafe { Self::from_bytes_with_nul_unchecked(bytes) })
> }
>
> + /// Creates a [`CStr`] from a `[u8]`, panic if input is not valid.
> + ///
> + /// This function is only meant to be used by `c_str!` macro, so
> + /// crates using `c_str!` macro don't have to enable `const_panic` feature.

I wrote this code when `const_panic` was not yet stable. Now we have
stable const panic this function could be removed.

> + #[doc(hidden)]
> + pub const fn from_bytes_with_nul_unwrap(bytes: &[u8]) -> &Self {
> + match Self::from_bytes_with_nul(bytes) {
> + Ok(v) => v,
> + Err(_) => panic!("string contains interior NUL"),
> + }
> + }
> +
> /// Creates a [`CStr`] from a `[u8]` without performing any additional
> /// checks.
> ///
> @@ -321,6 +333,26 @@ where
> }
> }
>
> +/// Creates a new [`CStr`] from a string literal.
> +///
> +/// The string literal should not contain any `NUL` bytes.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// # use kernel::c_str;
> +/// # use kernel::str::CStr;
> +/// const MY_CSTR: &CStr = c_str!("My awesome CStr!");
> +/// ```
> +#[macro_export]
> +macro_rules! c_str {
> + ($str:expr) => {{
> + const S: &str = concat!($str, "\0");
> + const C: &$crate::str::CStr = $crate::str::CStr::from_bytes_with_nul_unwrap(S.as_bytes());
> + C
> + }};
> +}

+#[macro_export]
+macro_rules! c_str {
+ ($str:expr) => {{
+ const S: &str = concat!($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"),
+ };
+ C
+ }};
+}

> +
> #[cfg(test)]
> mod tests {
> use super::*;