Merge pull request #2305 from bit-aloo/2026-08-18-return-unclaimed-error

return unclaimed bytes and not remove them

Author: bit-aloo

sv2/binary-sv2/src/lib.rs

@@ -87,11 +87,14 @@ pub fn to_bytes<T: Encodable + GetSize>(src: T) -> Result<Vec<u8>, Error> {
     Ok(result)
 }
 
-/// Encodes the SV2 data type to the provided byte slice.
+/// Encodes the SV2 data type to the provided byte slice and returns the number of bytes
+/// written.
+///
+/// `dst` may be larger than the encoded value; the bytes past the returned length are left
+/// untouched, so a caller reusing a buffer must only transmit `&dst[..written]`.
 #[allow(clippy::wrong_self_convention)]
-pub fn to_writer<T: Encodable>(src: T, dst: &mut [u8]) -> Result<(), Error> {
-    src.to_bytes(dst)?;
-    Ok(())
+pub fn to_writer<T: Encodable>(src: T, dst: &mut [u8]) -> Result<usize, Error> {
+    src.to_bytes(dst)
 }
 
 /// Decodes an SV2-encoded byte slice into the specified data type.

sv2/binary-sv2/tests/test.rs

@@ -732,3 +732,18 @@ mod test_b064k_length_prefix {
         );
     }
 }
+
+mod test_to_writer_len {
+    use super::*;
+
+    #[test]
+    fn to_writer_reports_bytes_written_into_oversized_buffer() {
+        let mut oversized = [0xAAu8; 16];
+
+        let written = to_writer(0x0102_0304_u32, &mut oversized[..]).unwrap();
+
+        assert_eq!(written, 4);
+        assert_eq!(&oversized[..written], &[0x04, 0x03, 0x02, 0x01]);
+        assert!(oversized[written..].iter().all(|b| *b == 0xAA));
+    }
+}