Response::Custom: Return payload as Bytes instead of Vec

CHANGELOG.md

@@ -5,11 +5,13 @@
 
 ## v0.9.0 (Unreleased)
 
-- Avoid allocations when writing multiple coils/registers
+- Optimization: Avoid allocations when writing multiple coils/registers.
+- Optimization: Avoid allocations when receiving custom responses.
 
 ### Breaking Changes
 
-- `Request` requires a lifetime and stores multiple coils/registers wrapped into `Cow`.
+- `Request`: Requires a lifetime and stores multiple coils/registers wrapped into `Cow` to avoid allocations.
+- `Response::Custom`: The payload is returned as `Bytes` instead of `Vec` to avoid allocations.
 
 ## v0.8.2 (2023-06-15)
 

src/codec/mod.rs

@@ -1,19 +1,24 @@
 // SPDX-FileCopyrightText: Copyright (c) 2017-2023 slowtec GmbH <post@slowtec.de>
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
+use std::{
+    convert::TryFrom,
+    io::{self, Cursor, Error, ErrorKind},
+};
+
+use byteorder::{BigEndian, ReadBytesExt as _};
+
+use crate::{
+    bytes::{BufMut, Bytes, BytesMut},
+    frame::*,
+};
+
 #[cfg(feature = "rtu")]
 pub(crate) mod rtu;
 
 #[cfg(feature = "tcp")]
 pub(crate) mod tcp;
 
-use crate::frame::*;
-
-use byteorder::{BigEndian, ReadBytesExt as _};
-use bytes::{BufMut, Bytes, BytesMut};
-use std::convert::TryFrom;
-use std::io::{self, Cursor, Error, ErrorKind};
-
 #[allow(clippy::cast_possible_truncation)]
 fn u16_len(len: usize) -> u16 {
     // This type conversion should always be safe, because either
@@ -328,7 +333,10 @@ impl TryFrom<Bytes> for Response {
                 }
                 ReadWriteMultipleRegisters(data)
             }
-            _ => Custom(fn_code, bytes[1..].into()),
+            _ => {
+                let mut bytes = bytes;
+                Custom(fn_code, bytes.split_off(1))
+            }
         };
         Ok(rsp)
     }
@@ -581,7 +589,7 @@ mod tests {
         assert_eq!(rsp_to_fn_code(&WriteMultipleRegisters(0, 0)), 0x10);
         assert_eq!(rsp_to_fn_code(&MaskWriteRegister(0, 0, 0)), 0x16);
         assert_eq!(rsp_to_fn_code(&ReadWriteMultipleRegisters(vec![])), 0x17);
-        assert_eq!(rsp_to_fn_code(&Custom(99, vec![])), 99);
+        assert_eq!(rsp_to_fn_code(&Custom(99, Bytes::from_static(&[]))), 99);
     }
 
     #[test]
@@ -1064,7 +1072,8 @@ mod tests {
 
         #[test]
         fn custom() {
-            let bytes: Bytes = Response::Custom(0x55, vec![0xCC, 0x88, 0xAA, 0xFF]).into();
+            let bytes: Bytes =
+                Response::Custom(0x55, Bytes::from_static(&[0xCC, 0x88, 0xAA, 0xFF])).into();
             assert_eq!(bytes[0], 0x55);
             assert_eq!(bytes[1], 0xCC);
             assert_eq!(bytes[2], 0x88);
@@ -1186,7 +1195,10 @@ mod tests {
         fn custom() {
             let bytes = Bytes::from(vec![0x55, 0xCC, 0x88, 0xAA, 0xFF]);
             let rsp = Response::try_from(bytes).unwrap();
-            assert_eq!(rsp, Response::Custom(0x55, vec![0xCC, 0x88, 0xAA, 0xFF]));
+            assert_eq!(
+                rsp,
+                Response::Custom(0x55, Bytes::from_static(&[0xCC, 0x88, 0xAA, 0xFF]))
+            );
         }
     }
 }

src/codec/rtu.rs

@@ -1,16 +1,20 @@
 // SPDX-FileCopyrightText: Copyright (c) 2017-2023 slowtec GmbH <post@slowtec.de>
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
-use super::*;
-
-use crate::{frame::rtu::*, slave::SlaveId};
+use std::io::{Cursor, Error, ErrorKind, Result};
 
 use byteorder::BigEndian;
-use bytes::{Buf, BufMut, Bytes, BytesMut};
 use smallvec::SmallVec;
-use std::io::{Cursor, Error, ErrorKind, Result};
 use tokio_util::codec::{Decoder, Encoder};
 
+use crate::{
+    bytes::{Buf, BufMut, Bytes, BytesMut},
+    frame::rtu::*,
+    slave::SlaveId,
+};
+
+use super::*;
+
 // [Modbus over Serial Line Specification and Implementation Guide V1.02](http://modbus.org/docs/Modbus_over_serial_line_V1_02.pdf), page 13
 // "The maximum size of a Modbus RTU frame is 256 bytes."
 const MAX_FRAME_LEN: usize = 256;
@@ -364,7 +368,7 @@ impl Encoder<ResponseAdu> for ServerCodec {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use bytes::Bytes;
+    use crate::bytes::Bytes;
 
     #[test]
     fn test_calc_crc() {

src/codec/tcp.rs

@@ -1,15 +1,18 @@
 // SPDX-FileCopyrightText: Copyright (c) 2017-2023 slowtec GmbH <post@slowtec.de>
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
-use super::*;
-
-use crate::frame::tcp::*;
+use std::io::{Error, ErrorKind, Result};
 
 use byteorder::{BigEndian, ByteOrder};
-use bytes::{BufMut, Bytes, BytesMut};
-use std::io::{Error, ErrorKind, Result};
 use tokio_util::codec::{Decoder, Encoder};
 
+use crate::{
+    bytes::{BufMut, Bytes, BytesMut},
+    frame::tcp::*,
+};
+
+use super::*;
+
 const HEADER_LEN: usize = 7;
 
 const PROTOCOL_ID: u16 = 0x0000; // TCP
@@ -174,7 +177,7 @@ impl Encoder<ResponseAdu> for ServerCodec {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use bytes::Bytes;
+    use crate::bytes::Bytes;
 
     mod client {
 

src/frame/mod.rs

@@ -9,6 +9,8 @@ pub(crate) mod tcp;
 
 use std::{borrow::Cow, error, fmt};
 
+use crate::bytes::Bytes;
+
 /// A Modbus function code is represented by an unsigned 8 bit integer.
 pub type FunctionCode = u8;
 
@@ -220,7 +222,7 @@ pub enum Response {
     /// Response to a raw Modbus request
     /// The first parameter contains the returned Modbus function code
     /// The second parameter contains the bytes read following the function code
-    Custom(FunctionCode, Vec<u8>),
+    Custom(FunctionCode, Bytes),
 }
 
 /// A server (slave) exception.

src/lib.rs

@@ -16,6 +16,13 @@
 #![warn(rustdoc::broken_intra_doc_links)]
 #![doc = include_str!("../README.md")]
 
+/// Re-export the `bytes` crate
+///
+/// Needed to prevent version conflicts with types that are exposed by the public API.
+///
+/// Used by [`Response::Custom`].
+pub use bytes;
+
 pub mod prelude;
 
 pub mod client;