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

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Message-ID: <87cy988qr2.fsf@kernel.org>
Date: Wed, 06 Aug 2025 16:32:49 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Daniel Almeida <daniel.almeida@...labora.com>
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]`.

"Daniel Almeida" <daniel.almeida@...labora.com> writes:

> 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?

Thanks. Not sure about the explicit lifetime either, I'll
reformat:

@@ -844,7 +844,7 @@ pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
 
     /// Create a new [`Self`] instance.
     #[expect(dead_code)]
-    pub(crate) fn new<'a>(buffer: &'a mut [u8]) -> Formatter<'a> {
+    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()) }


Best regards,
Andreas Hindborg