Merge pull request #982 from wvwwvwwv/main

Change: #959 ditch async_trait for the singlethreade feature

macros/Cargo.toml

@@ -17,6 +17,5 @@ proc-macro = true
 
 [features]
 
-# Passes `?Send` to `async_trait` to force affected tasks to be spawned in the current thread.
+# Do not use `async_trait` and do not add `Send` bounds.
 singlethreaded = []
-

macros/src/lib.rs

@@ -1,16 +1,23 @@
 use proc_macro::TokenStream;
 
-/// This macro either emits `#[async_trait::async_trait]` or `#[async_trait::async_trait(?Send)]`
-/// based on the activated feature set.
+/// This macro emits `#[async_trait::async_trait]` if the `singlethreaded` feature is disabled.
 ///
-/// This assumes that the `[async_trait](https://crates.io/crates/async-trait)` crate is imported
-/// as `async_trait`. If the `singlethreaded` feature is enabled, `?Send` is passed to
-/// `async_trait`, thereby forcing the affected asynchronous trait functions and methods to be run
-/// in the same thread.
+/// Starting from Rust 1.75.0, the `async_fn_in_trait` language feature is generally available
+/// that allows traits to contain asynchronous methods and associated functions. However, the
+/// feature has several known issues that are mainly related to Rust compiler's abilities to infer
+/// `Send` bounds of associated types: [`90696``](https://github.com/rust-lang/rust/issues/90696).
+///
+/// Therefore, if a trait requires `Send` bounds in its associated data types, this macro
+/// circumvents the compiler shortcomings by using the
+/// [`async-trait`](https://crates.io/crates/async-trait) crate which boxes return
+/// [`Future`](https://doc.rust-lang.org/std/future/trait.Future.html) types of all the
+/// asynchronous methods and associated functions of the trait.
 #[proc_macro_attribute]
 pub fn add_async_trait(_attr: TokenStream, item: TokenStream) -> TokenStream {
     if cfg!(feature = "singlethreaded") {
-        let mut output = "#[async_trait::async_trait(?Send)]".parse::<TokenStream>().unwrap();
+        // `async_fn_in_trait` requires the user to explicitly specify the `Send` bound for public
+        // trait methods, however the `singlethreaded` feature renders the requirement irrelevant.
+        let mut output = "#[allow(async_fn_in_trait)]".parse::<TokenStream>().unwrap();
         output.extend(item);
         output
     } else {

openraft/src/docs/feature_flags/feature-flags.md

@@ -47,6 +47,8 @@ By default openraft enables no features.
 - `singlethreaded`: removes `Send` and `Sync` bounds from `AppData`, `AppDataResponse`, `RaftEntry`, `SnapshotData`
   and other types to force the  asynchronous runtime to spawn any tasks in the current thread.
   This is for any single-threaded application that never allows a raft instance to be shared among multiple threads.
+  This feature relies on the `async_fn_in_trait` language feature that is officially supported from Rust 1.75.0.
+  If the feature is enabled, affected asynchronous trait methods and associated functions no longer use `async_trait`.
   In order to use the feature, `AsyncRuntime::spawn` should invoke `tokio::task::spawn_local` or equivalents.
   <br/><br/>