From 84613b26b047d5dcd8bd36ba7479d3aa89349619 Mon Sep 17 00:00:00 2001
From: Lewin Bormann <lewin@lewin-bormann.info>
Date: Mon, 11 Sep 2023 18:12:02 +0200
Subject: [PATCH] for #32: conditionally add Send requirement for
 AsyncRead/Write

---
 Cargo.toml    |  7 +++++--
 src/reader.rs | 18 ++++++++++++++----
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index 24341c9..e708041 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,7 +20,6 @@ bencher = "~0.1"
 
 [[example]]
 name = "encode_varint_from_stdin"
-required-features = ["tokio_async"]
 
 [[example]]
 name = "read_write_file"
@@ -31,11 +30,15 @@ name = "main"
 harness = false
 
 [features]
+default = ["sendable_io_traits"]
+
+# Requires that AsyncRead and AsyncWrite implementors are Send.
+sendable_io_traits = []
 # Enable one of these features if you want to use the AsyncRead/AsyncWrite traits from
 # the futures crate instead of those from tokio.
 tokio_async = ["tokio", "async-trait"]
 futures_async = ["futures-util", "async-trait"]
 
 [package.metadata.docs.rs]
-features = ["tokio_async"]
+features = ["tokio_async", "sendable_io_traits"]
 
diff --git a/src/reader.rs b/src/reader.rs
index 0ed63ac..b0ea6af 100644
--- a/src/reader.rs
+++ b/src/reader.rs
@@ -5,10 +5,20 @@ use crate::fixed::FixedInt;
 use crate::varint::{VarInt, VarIntMaxSize, MSB};
 
 #[cfg(feature = "tokio_async")]
-use tokio::io::{AsyncRead, AsyncReadExt};
+use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite};
 
 #[cfg(feature = "futures_async")]
-use futures_util::{io::AsyncRead, io::AsyncReadExt};
+use futures_util::{io::AsyncRead, io::AsyncReadExt, AsyncWrite};
+
+#[cfg(all(any(feature = "tokio_async", feature = "futures_async"), feature = "sendable_io_traits"))]
+trait AsyncReader : AsyncRead + Unpin + Send {}
+#[cfg(all(any(feature = "tokio_async", feature = "futures_async"), feature = "sendable_io_traits"))]
+trait AsyncWriter : AsyncWrite + Unpin + Send {}
+
+#[cfg(all(any(feature = "tokio_async", feature = "futures_async"), not(feature = "sendable_io_traits")))]
+trait AsyncReader : AsyncRead + Unpin + Send {}
+#[cfg(all(any(feature = "tokio_async", feature = "futures_async"), not(feature = "sendable_io_traits")))]
+trait AsyncWriter : AsyncWrite + Unpin + Send {}
 
 /// A trait for reading VarInts from any other `Reader`.
 ///
@@ -66,7 +76,7 @@ impl VarIntProcessor {
 
 #[cfg(any(feature = "tokio_async", feature = "futures_async"))]
 #[async_trait::async_trait(?Send)]
-impl<AR: AsyncRead + Unpin> VarIntAsyncReader for AR {
+impl<AR: AsyncReader> VarIntAsyncReader for AR {
     async fn read_varint_async<VI: VarInt>(&mut self) -> Result<VI> {
         let mut buf = [0_u8; 1];
         let mut p = VarIntProcessor::new::<VI>();
@@ -131,7 +141,7 @@ pub trait FixedIntAsyncReader {
 
 #[cfg(any(feature = "tokio_async", feature = "futures_async"))]
 #[async_trait::async_trait(?Send)]
-impl<AR: AsyncRead + Unpin> FixedIntAsyncReader for AR {
+impl<AR: AsyncReader> FixedIntAsyncReader for AR {
     async fn read_fixedint_async<FI: FixedInt>(&mut self) -> Result<FI> {
         let mut buf = [0_u8; 8];
         self.read_exact(&mut buf[0..std::mem::size_of::<FI>()])