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

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Date: Fri, 16 Feb 2024 18:11:30 +0100
From: Danilo Krummrich <dakr@...hat.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: a.hindborg@...sung.com, alex.gaynor@...il.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

On 2/16/24 17:53, Alice Ryhl wrote:
>> +    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.

That would result into two copies if I actually want a CString, wouldn't it?

Also, what would be the use case? And even if someone wants to have a CStr
again, couldn't we just deref the resulting CString?

- Danilo

> 
>> +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
>