From b083d5d294f591ae4dbb349a3aef8a3f6b2acf90 Mon Sep 17 00:00:00 2001
From: tobias-wilfert <tobias.wilfert@sentry.io>
Date: Wed, 12 Aug 2026 09:49:34 +0200
Subject: [PATCH 1/2] add `format_into`

---
 src/lib.rs                | 46 ++++++++++++++++++++++++++++++++++-----
 tests/test_format_into.rs | 21 ++++++++++++++++++
 2 files changed, 61 insertions(+), 6 deletions(-)
 create mode 100644 tests/test_format_into.rs

diff --git a/src/lib.rs b/src/lib.rs
index 13711b8..e479e7b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -663,19 +663,53 @@ pub trait Format<'f> {
             return Ok(Cow::Borrowed(format));
         }
 
-        let mut access = ArgumentAccess::new(arguments);
         let mut buffer = Vec::with_capacity(format.len());
+        self.format_into(&mut buffer, format, arguments)?;
+
+        Ok(Cow::Owned(unsafe { String::from_utf8_unchecked(buffer) }))
+    }
+
+    /// Formats the given string with the specified arguments and writes the result into the given writer.
+    ///
+    /// Individual arguments must implement [`Debug`] and [`serde::Serialize`]. The arguments
+    /// container must implement the [`FormatArgs`] trait.
+    ///
+    /// ```rust
+    /// use dynfmt::{Format, NoopFormat};
+    ///
+    /// let mut buffer = Vec::new();
+    /// NoopFormat.format_into(&mut buffer, "hello, world", &["unused"]).expect("formatting failed");
+    /// assert_eq!(b"hello, world", buffer.as_slice());
+    /// ```
+    ///
+    /// [`Debug`]: https://doc.rust-lang.org/stable/std/fmt/trait.Debug.html
+    /// [`serde::Serialize`]: https://docs.rs/serde/latest/serde/trait.Serialize.html
+    /// [`FormatArgs`]: trait.FormatArgs.html
+    fn format_into<W, A>(
+        &self,
+        mut write: W,
+        format: &'f str,
+        arguments: A,
+    ) -> Result<(), Error<'f>>
+    where
+        W: io::Write,
+        A: FormatArgs,
+    {
+        let mut access = ArgumentAccess::new(arguments);
         let mut last_match = 0;
 
-        for spec in iter {
+        for spec in self.iter_args(format)? {
             let spec = spec?;
-            buffer.extend(format[last_match..spec.start()].as_bytes());
-            spec.format_into(&mut buffer, &mut access)?;
+            write
+                .write_all(format[last_match..spec.start()].as_bytes())
+                .map_err(Error::Io)?;
+            spec.format_into(&mut write, &mut access)?;
             last_match = spec.end();
         }
 
-        buffer.extend(format[last_match..].as_bytes());
-        Ok(Cow::Owned(unsafe { String::from_utf8_unchecked(buffer) }))
+        write
+            .write_all(format[last_match..].as_bytes())
+            .map_err(Error::Io)
     }
 }
 
diff --git a/tests/test_format_into.rs b/tests/test_format_into.rs
new file mode 100644
index 0000000..682d97b
--- /dev/null
+++ b/tests/test_format_into.rs
@@ -0,0 +1,21 @@
+#![cfg(feature = "python")]
+
+use std::io::ErrorKind;
+
+use dynfmt::{Error, Format, PythonFormat};
+
+#[test]
+fn writes_formatted_output() {
+    let mut buffer = Vec::new();
+    PythonFormat
+        .format_into(&mut buffer, "hello, %s!", &["world"])
+        .expect("formatting failed");
+    assert_eq!(b"hello, world!", buffer.as_slice());
+}
+
+#[test]
+fn aborts_on_full_writer() {
+    let mut buffer = [0u8; 8];
+    let result = PythonFormat.format_into(&mut buffer.as_mut_slice(), "hello, %s!", &["world"]);
+    assert!(matches!(result.unwrap_err(), Error::BadData(..)));
+}

From e8a6e882621a61696537ff6621cad7bfad23ce3e Mon Sep 17 00:00:00 2001
From: Jan Michael Auer <mail@jauer.org>
Date: Wed, 12 Aug 2026 14:56:11 +0200
Subject: [PATCH 2/2] Fix clippy::needless_borrows_for_generic_args in
 test_format_into

The stricter clippy config merged from master flags the unnecessary
& borrows on array literals and an unused import.
---
 tests/test_format_into.rs | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/tests/test_format_into.rs b/tests/test_format_into.rs
index 682d97b..c539e98 100644
--- a/tests/test_format_into.rs
+++ b/tests/test_format_into.rs
@@ -1,14 +1,12 @@
 #![cfg(feature = "python")]
 
-use std::io::ErrorKind;
-
 use dynfmt::{Error, Format, PythonFormat};
 
 #[test]
 fn writes_formatted_output() {
     let mut buffer = Vec::new();
     PythonFormat
-        .format_into(&mut buffer, "hello, %s!", &["world"])
+        .format_into(&mut buffer, "hello, %s!", ["world"])
         .expect("formatting failed");
     assert_eq!(b"hello, world!", buffer.as_slice());
 }
@@ -16,6 +14,6 @@ fn writes_formatted_output() {
 #[test]
 fn aborts_on_full_writer() {
     let mut buffer = [0u8; 8];
-    let result = PythonFormat.format_into(&mut buffer.as_mut_slice(), "hello, %s!", &["world"]);
+    let result = PythonFormat.format_into(buffer.as_mut_slice(), "hello, %s!", ["world"]);
     assert!(matches!(result.unwrap_err(), Error::BadData(..)));
 }