Add internal sync feature

Cargo.toml

@@ -45,8 +45,11 @@ tokio-serial = { version = "5.4.4", default-features = false }
 default = ["rtu", "tcp"]
 rtu = ["futures-util/sink"]
 tcp = ["tokio/net", "futures-util/sink"]
-rtu-sync = ["rtu", "dep:tokio-serial"]
-tcp-sync = ["tcp"]
+# The internal feature `sync` has no effect when used alone.
+# It is always enabled together with `rtu-sync` or `tcp-sync`.
+sync = []
+rtu-sync = ["rtu", "sync", "dep:tokio-serial"]
+tcp-sync = ["tcp", "sync"]
 server = ["dep:futures"]
 rtu-server = ["rtu", "server", "tokio/macros", "dep:tokio-serial"]
 tcp-server-unstable = [

src/client/mod.rs

@@ -18,7 +18,7 @@ pub mod rtu;
 #[cfg(feature = "tcp")]
 pub mod tcp;
 
-#[cfg(any(feature = "rtu-sync", feature = "tcp-sync"))]
+#[cfg(feature = "sync")]
 pub mod sync;
 
 /// Transport independent asynchronous client trait

src/prelude.rs

@@ -43,13 +43,13 @@ pub use crate::slave::{Slave, SlaveId};
 ///////////////////////////////////////////////////////////////////
 pub use crate::client::{Client, Reader, Writer};
 
-#[cfg(any(feature = "rtu-sync", feature = "tcp-sync"))]
+#[cfg(feature = "sync")]
 pub use crate::client::sync::Client as SyncClient;
 
-#[cfg(any(feature = "rtu-sync", feature = "tcp-sync"))]
+#[cfg(feature = "sync")]
 pub use crate::client::sync::Reader as SyncReader;
 
-#[cfg(any(feature = "rtu-sync", feature = "tcp-sync"))]
+#[cfg(feature = "sync")]
 pub use crate::client::sync::Writer as SyncWriter;
 
 pub use crate::slave::SlaveContext;