Re: [PATCH v2 03/14] rust: str: introduce `NullBorrowFormatter`

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alice Ryhl" <aliceryhl@google.com> writes:

> On Wed, Jul 09, 2025 at 05:49:57PM +0200, Andreas Hindborg wrote:
>> "Alice Ryhl" <aliceryhl@google.com> writes:
>>
>> > On Tue, Jul 08, 2025 at 09:44:58PM +0200, Andreas Hindborg wrote:
>> >> Add `NullBorrowFormatter`, a formatter that writes a null terminated string
>> >> to an array or slice buffer. Because this type needs to manage the trailing
>> >> null marker, the existing formatters cannot be used to implement this type.
>> >>
>> >> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>> >> ---
>> >>  rust/kernel/str.rs | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> >>  1 file changed, 59 insertions(+)
>> >>
>> >> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
>> >> index 78b2f95eb3171..05d79cf40c201 100644
>> >> --- a/rust/kernel/str.rs
>> >> +++ b/rust/kernel/str.rs
>> >> @@ -860,6 +860,65 @@ fn deref_mut(&mut self) -> &mut Self::Target {
>> >>      }
>> >>  }
>> >>
>> >> +/// A mutable reference to a byte buffer where a string can be written into.
>> >> +///
>> >> +/// The buffer will be automatically null terminated after the last written character.
>> >> +///
>> >> +/// # Invariants
>> >> +///
>> >> +/// `buffer` is always null terminated.
>> >> +pub(crate) struct NullBorrowFormatter<'a> {
>> >> +    buffer: &'a mut [u8],
>> >> +    pos: usize,
>> >> +}
>> >
>> > Do you need `pos`? Often I see this kind of code subslice `buffer`
>> > instead.
>>
>> How would that work? Can I move the start index of `buffer` in some way
>> without an unsafe block?
>
> Yes. I think this will work:
>
> let buffer = mem::take(&mut self.buffer);
> self.buffer = &mut buffer[pos..];
>
> Temporarily storing an empty slice avoids lifetime issues.

Ah, that is neat.


Best regards,
Andreas Hindborg