linux-kernel - Re: [PATCH v5 02/18] rust: str: allow `str::Formatter` to format into `&mut [u8]`.

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Message-Id: <15990453-889F-40C3-863D-DB41306E2A84@collabora.com>
Date: Wed, 27 Aug 2025 09:48:08 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>,
 Jens Axboe <axboe@...nel.dk>,
 Breno Leitao <leitao@...ian.org>,
 linux-block@...r.kernel.org,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 02/18] rust: str: allow `str::Formatter` to format into
 `&mut [u8]`.

Hi Andreas,

> On 15 Aug 2025, at 04:30, Andreas Hindborg <a.hindborg@...nel.org> wrote:
> 
> Improve `Formatter` so that it can write to an array or slice buffer.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
> rust/kernel/str.rs | 23 +++++++++++++++++------
> 1 file changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 082790b7a621..76632da357a6 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -7,7 +7,10 @@
>     fmt::{self, Write},
>     prelude::*,
> };
> -use core::ops::{self, Deref, DerefMut, Index};
> +use core::{
> +    marker::PhantomData,
> +    ops::{self, Deref, DerefMut, Index},
> +};
> 
> /// Byte string without UTF-8 validity guarantee.
> #[repr(transparent)]
> @@ -825,9 +828,9 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
> /// Allows formatting of [`fmt::Arguments`] into a raw buffer.
> ///
> /// Fails if callers attempt to write more than will fit in the buffer.
> -pub(crate) struct Formatter(RawFormatter);
> +pub(crate) struct Formatter<'a>(RawFormatter, PhantomData<&'a mut ()>);
> 
> -impl Formatter {
> +impl Formatter<'_> {
>     /// Creates a new instance of [`Formatter`] with the given buffer.
>     ///
>     /// # Safety
> @@ -836,11 +839,19 @@ impl Formatter {
>     /// for the lifetime of the returned [`Formatter`].
>     pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
>         // SAFETY: The safety requirements of this function satisfy those of the callee.
> -        Self(unsafe { RawFormatter::from_buffer(buf, len) })
> +        Self(unsafe { RawFormatter::from_buffer(buf, len) }, PhantomData)
> +    }
> +
> +    /// Create a new [`Self`] instance.
> +    #[expect(dead_code)]
> +    pub(crate) fn new(buffer: &mut [u8]) -> Self {
> +        // SAFETY: `buffer` is valid for writes for the entire length for
> +        // the lifetime of `Self`.
> +        unsafe { Formatter::from_buffer(buffer.as_mut_ptr(), buffer.len()) }
>     }
> }
> 
> -impl Deref for Formatter {
> +impl Deref for Formatter<'_> {
>     type Target = RawFormatter;
> 
>     fn deref(&self) -> &Self::Target {
> @@ -848,7 +859,7 @@ fn deref(&self) -> &Self::Target {
>     }
> }
> 
> -impl fmt::Write for Formatter {
> +impl fmt::Write for Formatter<'_> {
>     fn write_str(&mut self, s: &str) -> fmt::Result {
>         self.0.write_str(s)?;
> 
> 
> -- 
> 2.47.2
> 
> 
> 

I gave my r-b on v3 for this patch IIRC. What happened?

— Daniel