linux-kernel - Re: [PATCH v3 02/16] rust: str: allow `str::Formatter` to format into `&mut [u8]`.

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Message-Id: <37C6B308-202A-48A3-9DD2-5997E0EE73C1@collabora.com>
Date: Wed, 6 Aug 2025 10:05:22 -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>,
 linux-block@...r.kernel.org,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 02/16] rust: str: allow `str::Formatter` to format into
 `&mut [u8]`.

Hi Andreas,

> On 11 Jul 2025, at 08:43, Andreas Hindborg <a.hindborg@...nel.org> wrote:
> 
> Improve `Formatter` so that it can write to an array or slice buffer.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
> rust/kernel/str.rs | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 488b0e97004e..41af456a46c8 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -6,6 +6,7 @@
> use crate::prelude::*;
> use core::{
>     fmt::{self, Write},
> +    marker::PhantomData,
>     ops::{self, Deref, DerefMut, Index},
> };
> 
> @@ -794,9 +795,9 @@ 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
> @@ -805,11 +806,19 @@ 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<'a>(buffer: &'a mut [u8]) -> Formatter<'a> {

nit: the function above this one returns Self, and this one returns Formatter.
Perhaps this one should also return Self for consistency?
 
> +        // 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 {
> @@ -817,7 +826,7 @@ 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
> 
> 
> 

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>