rust: str: allow str::Formatter to format into &mut [u8].

Improve `Formatter` so that it can write to an array or slice buffer.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250902-rnull-up-v6-16-v7-2-b5212cc89b98@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Andreas Hindborg

rust/kernel/str.rs

@@ -7,7 +7,10 @@ use crate::{
     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 @@ impl fmt::Write for RawFormatter {
 /// 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
@@ -836,19 +839,27 @@ 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 {
         &self.0
     }
 }
 
-impl fmt::Write for Formatter {
+impl fmt::Write for Formatter<'_> {
     fn write_str(&mut self, s: &str) -> fmt::Result {
         self.0.write_str(s)?;