On 15 Aug 2025, at 04:30, Andreas Hindborg [off-list ref] wrote:

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

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.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