linux-kernel - Re: [PATCH v1 19/28] rust: str: add `c

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Date:   Mon, 14 Nov 2022 14:39:28 +0000
From:   Gary Guo <gary@...yguo.net>
To:     Miguel Ojeda <ojeda@...nel.org>
Cc:     Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH v1 19/28] rust: str: add `c_str!` macro

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

> From: Gary Guo <gary@...yguo.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@...il.com>
> Signed-off-by: Alex Gaynor <alex.gaynor@...il.com>
> Signed-off-by: Gary Guo <gary@...yguo.net>
> [Reworded, adapted for upstream and applied latest changes]
> Signed-off-by: Miguel Ojeda <ojeda@...nel.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::*;