linux-kernel - Re: [PATCH v3] rust: str: add {make,to}

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Date: Fri, 16 Feb 2024 16:53:20 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: dakr@...hat.com
Cc: a.hindborg@...sung.com, alex.gaynor@...il.com, aliceryhl@...gle.com, 
	benno.lossin@...ton.me, bjorn3_gh@...tonmail.com, boqun.feng@...il.com, 
	gary@...yguo.net, linux-kernel@...r.kernel.org, ojeda@...nel.org, 
	rust-for-linux@...r.kernel.org, wedsonaf@...il.com
Subject: Re: [PATCH v3] rust: str: add {make,to}_{upper,lower}case() to CString

> +    pub fn make_ascii_lowercase(&mut self) {
> +        self.0.make_ascii_lowercase();
> +    }

It's important to note here that this doesn't remove or introduce NUL
bytes.

pub fn make_ascii_lowercase(&mut self) {
    // INVARIANT: This doesn't introduce or remove NUL bytes in the c
    // string.
    self.0.make_ascii_lowercase();
}

Ditto for make_ascii_uppercase. (But not the to_* methods.)

> +    /// Returns a copy of this [`CString`] where each character is mapped to its
> +    /// ASCII lower case equivalent.
> +    ///
> +    /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
> +    /// but non-ASCII letters are unchanged.
> +    ///
> +    /// To lowercase the value in-place, use [`make_ascii_lowercase`].
> +    ///
> +    /// [`make_ascii_lowercase`]: str::make_ascii_lowercase
> +    pub fn to_ascii_lowercase(&self) -> Result<CString, AllocError> {
> +        let mut s = (*self).to_cstring()?;
> +
> +        s.make_ascii_lowercase();
> +
> +        return Ok(s);
> +    }
> +
> +    /// Returns a copy of this [`CString`] where each character is mapped to its
> +    /// ASCII upper case equivalent.
> +    ///
> +    /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
> +    /// but non-ASCII letters are unchanged.
> +    ///
> +    /// To uppercase the value in-place, use [`make_ascii_uppercase`].
> +    ///
> +    /// [`make_ascii_uppercase`]: str::make_ascii_uppercase
> +    pub fn to_ascii_uppercase(&self) -> Result<CString, AllocError> {
> +        let mut s = (*self).to_cstring()?;
> +
> +        s.make_ascii_uppercase();
> +
> +        return Ok(s);
> +    }

Please move these to `CStr` as well.

> +impl DerefMut for CString {
> +    fn deref_mut(&mut self) -> &mut Self::Target {
> +        unsafe { CStr::from_bytes_with_nul_unchecked_mut(&mut *self.buf) }
> +    }
> +}

Needs a safety comment.

impl DerefMut for CString {
    fn deref_mut(&mut self) -> &mut Self::Target {
        // SAFETY: A `CString` is always NUL-terminated and contains no
	// other NUL bytes.
        unsafe { CStr::from_bytes_with_nul_unchecked_mut(&mut *self.buf) }
    }
}

Alice