#async #executor #futures


Implements Spawn, SpawnLocal and SpawnHandle for commonly used executors

12 unstable releases (4 breaking)

0.4.0 Jan 1, 2021
0.4.0-beta.2 Nov 5, 2020
0.3.0 Jun 11, 2020
0.3.0-beta.1 May 10, 2020
0.0.0 Oct 28, 2019

#132 in Asynchronous

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Abstract over different executors.

The aim of async_executors is to provide a uniform interface to the main async executors available in Rust. We provide wrapper types that always implement the Spawn and/or LocalSpawn traits from futures, making it easy to pass any supported executor to an API which requires exec: impl Spawn or exec: impl LocalSpawn.

A problem with these traits is that they do not provide an API for getting a JoinHandle to await completion of the task. The current trend in async is to favor joining tasks and thus certain executors now return JoinHandles from their spawn function. It's also a fundamental building block for structured concurrency. Async_executors provides an executor agnostic [JoinHandle] that wraps the framework native JoinHandle types.

SpawnHandle traits are also provided so API's can express you need to pass them an executor which allows spawning whilst returning a JoinHandle. Note that this was already provided by the futures in SpawnExt::spawn_with_handle, but this uses RemoteHandle which incurs a performance overhead. By wrapping the native JoinHandle types, we can avoid some of that overhead while still being executor agnostic.

The traits provided by this crate are also implemented for the Instrumented and WithDispatch wrappers from tracing-futures when the tracing feature is enabled. So you can pass an instrumented executor to an API requiring exec: impl SpawnHandle<SomeOutput>.

The currently supported executors are (file an issue on GitHub if you want to see another one supported):

  • async-global-executor - supports spawning !Send futures and works on Wasm.
  • async-std - supports spawning !Send futures and works on Wasm (uses async-global-executor and bindgen under the hood).
  • tokio CurrentThread - [tokio::runtime::Runtime] with basic scheduler and a LocalSet. (supports spawning !Send futures)
  • tokio ThreadPool - [tokio::runtime::Runtime] with threadpool scheduler.
  • wasm-bindgen-futures (only available on Wasm)
  • the futures-executor executors - They already implemented Spawn and SpawnLocal, but we implement the SpawnHandle family of traits for them as well. The types ThreadPool, LocalPool and LocalSpawner are re-exported for convenience.

All executors are behind feature flags: async_std, async_global, tokio_ct, tokio_tp, bindgen, localpool, threadpool.

Table of Contents


With cargo add: cargo add async_executors

With cargo yaml:


   async_executors: ^0.4

With Cargo.toml


    async_executors = "0.4"


Please check out the changelog when upgrading.


This crate has few dependencies. Cargo will automatically handle it's dependencies for you.

The only hard dependencies are futures-task and futures-util. The rest are the optional dependencies to turn on support for each executor.


The crate itself uses #[ forbid(unsafe_code) ].

Our dependencies use unsafe.


Most wrappers are very thin but the Spawn and LocalSpawn traits do imply boxing the future. With executors boxing futures to put them in a queue you probably get 2 heap allocations per spawn.

JoinHandle uses the native JoinHandle types from tokio and async-std to avoid the overhead from RemoteHandle, but for async-std, wrap the future in Abortable to create consistent behavior across all executors. The JoinHandle provided cancels it's future on drop unless you call detach on it.

SpawnHandle and LocalSpawnHandle require boxing the future twice, just like Spawn and LocalSpawn.

Existing benchmarks for all executors can be found in executor_benchmarks.

Missing features

These are some features that aren't provided yet but that are on the todo list:

  • an agnostic timeout mechanism.
  • an agnostic interface for spawn_blocking.


For API providers

When writing a library that needs to spawn, you probably shouldn't lock your clients into one framework or another. It's usually not appropriate to setup your own thread pool for spawning futures. It belongs to the application developer to decide where futures are spawned and it might not be appreciated if libraries bring in extra dependencies on a framework.

In order to get round this you can take an executor in as a parameter from client code and spawn your futures on the provided executor. Currently the only two traits that are kind of widely available for this are Spawn and LocalSpawn from the futures library. Unfortunately, other executor providers do not implement these traits. So by publishing an API that relies on these traits, you would have been restricting the clients to use the executors from futures, or start implementing their own wrappers that implement the traits.

Async_executors has wrappers providing impls on various executors, namely tokio, async_std, wasm_bindgen. As such you can just use the trait bounds and refer your users to this crate if they want to use any of the supported executors.

All wrappers also implement Clone, Debug and the zero sized ones also Copy. You can express you will need to clone in your API: impl Spawn + Clone.

Note that you should never use block_on inside async contexts. Some backends we use like tokio and RemoteHandle from futures use catch_unwind, so try to keep futures unwind safe.

Spawning with handles

You can use the SpawnHandle and LocalSpawnHandle traits as bounds for obtaining join handles.

  async_executors :: { JoinHandle, SpawnHandle, SpawnHandleExt       } ,
  std             :: { sync::Arc                                     } ,
  futures         :: { FutureExt, executor::{ ThreadPool, block_on } } ,

// Example of a library function that needs an executor. Just use impl Trait.
fn needs_exec( exec: impl SpawnHandle<()> )
   let handle = exec.spawn_handle( async {} );

// A type that needs to hold on to an executor during it's lifetime. Here it
// must be heap allocated.
struct SomeObj{ exec: Arc< dyn SpawnHandle<u8> > }

impl SomeObj
   pub fn new( exec: Arc< dyn SpawnHandle<u8> > ) -> SomeObj
      SomeObj{ exec }

   fn run( &self ) -> JoinHandle<u8>
      self.exec.spawn_handle( async{ 5 } ).expect( "spawn" )

fn main()
  let exec = ThreadPool::new().expect( "build threadpool" );
  let obj  = SomeObj::new( Arc::new(exec) );

  let x = block_on( obj.run() );

  assert_eq!( x, 5 );

For API consumers

You can basically pass the wrapper types provided in async_executors to API's that take any of the following. Traits are also implemented for Rc, Arc, &, &mut, Box and Instrumented and WithDispatch from tracing-futures wrappers:

  • impl Spawn
  • impl LocalSpawn
  • impl SpawnHandle<T>
  • impl LocalSpawnHandle<T>

All wrappers also implement Clone, Debug and the zero sized ones also Copy.

Some executors are a bit special, so make sure to check the API docs for the one you intend to use. Some also provide extra methods like block_on which will call a framework specific block_on rather than the one from futures.


  async_executors :: { AsyncStd, TokioTpBuilder, SpawnHandle } ,
  std             :: { convert::TryFrom                      } ,

fn needs_exec( exec: impl SpawnHandle<()> + SpawnHandle<String> ){};

// AsyncStd is zero sized, so it's easy to instantiate.
needs_exec( AsyncStd );

let tp = TokioTpBuilder::new().build().expect( "build threadpool" );

needs_exec( tp );

For more examples, check out the examples directory. If you want to get a more polished API for adhering to structured concurrency, check out async_nursery.


API documentation can be found on docs.rs.


Please check out the contribution guidelines.


Run ci/test.bash and ci/wasm.bash to run all tests.

Code of conduct

Any of the behaviors described in point 4 "Unacceptable Behavior" of the Citizens Code of Conduct are not welcome here and might get you banned. If anyone including maintainers and moderators of the project fail to respect these/your limits, you are entitled to call them out.




~50K SLoC