linux-kernel - Re: [PATCH] rust: str: add conversion from `CStr` to `CString`

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Message-ID: <CANeycqo+htksD_nj-eyCTOtE0TQHJR+dHZ1Hu8cL5eXNt4cd=A@mail.gmail.com>
Date:   Tue, 2 May 2023 13:59:56 -0300
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH] rust: str: add conversion from `CStr` to `CString`

On Tue, 2 May 2023 at 09:53, Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> These methods can be used to copy the data in a temporary c string into
> a separate allocation, so that it can be accessed later even if the
> original is deallocated.
>
> The API in this file mirrors the standard library API for the `&str` and
> `String` types. The `ToOwned` trait is not implemented because it
> assumes that allocations are infallible.
>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/str.rs | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index b771310fa4a4..54935ff3a610 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -2,6 +2,7 @@
>
>  //! String representations.
>
> +use alloc::collections::TryReserveError;
>  use alloc::vec::Vec;
>  use core::fmt::{self, Write};
>  use core::ops::{self, Deref, Index};
> @@ -199,6 +200,12 @@ impl CStr {
>      pub unsafe fn as_str_unchecked(&self) -> &str {
>          unsafe { core::str::from_utf8_unchecked(self.as_bytes()) }
>      }
> +
> +    /// Convert this [`CStr`] into a [`CString`] by allocating memory and
> +    /// copying over the string data.
> +    pub fn to_cstring(&self) -> Result<CString, TryReserveError> {
> +        CString::try_from(self)
> +    }
>  }
>
>  impl fmt::Display for CStr {
> @@ -584,6 +591,20 @@ impl Deref for CString {
>      }
>  }
>
> +impl<'a> TryFrom<&'a CStr> for CString {
> +    type Error = TryReserveError;

Wouldn't `AllocError` make more sense? Or even Error (with ENOMEM value).

`TryReserveError` is documented as "The error type for try_reserve
methods." -- that fact the we use a `Vec` is an implementation detail,
I feel it's better not to leak this fact through the public API.

> +
> +    fn try_from(cstr: &'a CStr) -> Result<CString, TryReserveError> {
> +        let len = cstr.len_with_nul();
> +        let mut buf = Vec::try_with_capacity(len)?;
> +        buf.try_extend_from_slice(cstr.as_bytes_with_nul())?;
> +
> +        // INVARIANT: The CStr and CString types have the same invariants for
> +        // the string data, and we copied it over without changes.
> +        Ok(CString { buf })
> +    }
> +}
> +
>  /// A convenience alias for [`core::format_args`].
>  #[macro_export]
>  macro_rules! fmt {
>
> base-commit: ea76e08f4d901a450619831a255e9e0a4c0ed162
> --
> 2.40.1.495.gc816e09b53d-goog
>