1//! Abstractions for asynchronous programming.
2//!
3//! This crate provides a number of core abstractions for writing asynchronous
4//! code:
5//!
6//! - [Futures](crate::future) are single eventual values produced by
7//! asynchronous computations. Some programming languages (e.g. JavaScript)
8//! call this concept "promise".
9//! - [Streams](crate::stream) represent a series of values
10//! produced asynchronously.
11//! - [Sinks](crate::sink) provide support for asynchronous writing of
12//! data.
13//! - [Executors](crate::executor) are responsible for running asynchronous
14//! tasks.
15//!
16//! The crate also contains abstractions for [asynchronous I/O](crate::io) and
17//! [cross-task communication](crate::channel).
18//!
19//! Underlying all of this is the *task system*, which is a form of lightweight
20//! threading. Large asynchronous computations are built up using futures,
21//! streams and sinks, and then spawned as independent tasks that are run to
22//! completion, but *do not block* the thread running them.
23//!
24//! The following example describes how the task system context is built and used
25//! within macros and keywords such as async and await!.
26//!
27//! ```rust
28//! # use futures::channel::mpsc;
29//! # use futures::executor; ///standard executors to provide a context for futures and streams
30//! # use futures::executor::ThreadPool;
31//! # use futures::StreamExt;
32//! #
33//! fn main() {
34//! # {
35//! let pool = ThreadPool::new().expect("Failed to build pool");
36//! let (tx, rx) = mpsc::unbounded::<i32>();
37//!
38//! // Create a future by an async block, where async is responsible for an
39//! // implementation of Future. At this point no executor has been provided
40//! // to this future, so it will not be running.
41//! let fut_values = async {
42//! // Create another async block, again where the Future implementation
43//! // is generated by async. Since this is inside of a parent async block,
44//! // it will be provided with the executor of the parent block when the parent
45//! // block is executed.
46//! //
47//! // This executor chaining is done by Future::poll whose second argument
48//! // is a std::task::Context. This represents our executor, and the Future
49//! // implemented by this async block can be polled using the parent async
50//! // block's executor.
51//! let fut_tx_result = async move {
52//! (0..100).for_each(|v| {
53//! tx.unbounded_send(v).expect("Failed to send");
54//! })
55//! };
56//!
57//! // Use the provided thread pool to spawn the generated future
58//! // responsible for transmission
59//! pool.spawn_ok(fut_tx_result);
60//!
61//! let fut_values = rx
62//! .map(|v| v * 2)
63//! .collect();
64//!
65//! // Use the executor provided to this async block to wait for the
66//! // future to complete.
67//! fut_values.await
68//! };
69//!
70//! // Actually execute the above future, which will invoke Future::poll and
71//! // subsequently chain appropriate Future::poll and methods needing executors
72//! // to drive all futures. Eventually fut_values will be driven to completion.
73//! let values: Vec<i32> = executor::block_on(fut_values);
74//!
75//! println!("Values={:?}", values);
76//! # }
77//! # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
78//! }
79//! ```
80//!
81//! The majority of examples and code snippets in this crate assume that they are
82//! inside an async block as written above.
83
84#![no_std]
85#![doc(test(
86 no_crate_inject,
87 attr(
88 deny(warnings, rust_2018_idioms, single_use_lifetimes),
89 allow(dead_code, unused_assignments, unused_variables)
90 )
91))]
92#![warn(missing_docs, unsafe_op_in_unsafe_fn)]
93#![cfg_attr(docsrs, feature(doc_cfg))]
94
95#[cfg(all(feature = "bilock", not(feature = "unstable")))]
96compile_error!("The `bilock` feature requires the `unstable` feature as an explicit opt-in to unstable features");
97
98#[doc(no_inline)]
99pub use futures_core::future::{Future, TryFuture};
100#[doc(no_inline)]
101pub use futures_util::future::{FutureExt, TryFutureExt};
102
103#[doc(no_inline)]
104pub use futures_core::stream::{Stream, TryStream};
105#[doc(no_inline)]
106pub use futures_util::stream::{StreamExt, TryStreamExt};
107
108#[doc(no_inline)]
109pub use futures_sink::Sink;
110#[doc(no_inline)]
111pub use futures_util::sink::SinkExt;
112
113#[cfg(feature = "std")]
114#[doc(no_inline)]
115pub use futures_io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite};
116#[cfg(feature = "std")]
117#[doc(no_inline)]
118pub use futures_util::{AsyncBufReadExt, AsyncReadExt, AsyncSeekExt, AsyncWriteExt};
119
120// Macro reexports
121pub use futures_core::ready; // Readiness propagation
122pub use futures_util::pin_mut;
123#[cfg(feature = "std")]
124#[cfg(feature = "async-await")]
125pub use futures_util::select;
126#[cfg(feature = "async-await")]
127pub use futures_util::{join, pending, poll, select_biased, try_join}; // Async-await
128
129// Module reexports
130#[doc(inline)]
131pub use futures_util::{future, never, sink, stream, task};
132
133#[cfg(feature = "std")]
134#[cfg(feature = "async-await")]
135pub use futures_util::stream_select;
136
137#[cfg(feature = "alloc")]
138#[doc(inline)]
139pub use futures_channel as channel;
140#[cfg(feature = "alloc")]
141#[doc(inline)]
142pub use futures_util::lock;
143
144#[cfg(feature = "std")]
145#[doc(inline)]
146pub use futures_util::io;
147
148#[cfg(feature = "executor")]
149#[cfg_attr(docsrs, doc(cfg(feature = "executor")))]
150pub mod executor {
151 //! Built-in executors and related tools.
152 //!
153 //! All asynchronous computation occurs within an executor, which is
154 //! capable of spawning futures as tasks. This module provides several
155 //! built-in executors, as well as tools for building your own.
156 //!
157 //!
158 //! This module is only available when the `executor` feature of this
159 //! library is activated.
160 //!
161 //! # Using a thread pool (M:N task scheduling)
162 //!
163 //! Most of the time tasks should be executed on a [thread pool](ThreadPool).
164 //! A small set of worker threads can handle a very large set of spawned tasks
165 //! (which are much lighter weight than threads). Tasks spawned onto the pool
166 //! with the [`spawn_ok`](ThreadPool::spawn_ok) function will run ambiently on
167 //! the created threads.
168 //!
169 //! # Spawning additional tasks
170 //!
171 //! Tasks can be spawned onto a spawner by calling its [`spawn_obj`] method
172 //! directly. In the case of `!Send` futures, [`spawn_local_obj`] can be used
173 //! instead.
174 //!
175 //! # Single-threaded execution
176 //!
177 //! In addition to thread pools, it's possible to run a task (and the tasks
178 //! it spawns) entirely within a single thread via the [`LocalPool`] executor.
179 //! Aside from cutting down on synchronization costs, this executor also makes
180 //! it possible to spawn non-`Send` tasks, via [`spawn_local_obj`]. The
181 //! [`LocalPool`] is best suited for running I/O-bound tasks that do relatively
182 //! little work between I/O operations.
183 //!
184 //! There is also a convenience function [`block_on`] for simply running a
185 //! future to completion on the current thread.
186 //!
187 //! [`spawn_obj`]: https://docs.rs/futures/0.3/futures/task/trait.Spawn.html#tymethod.spawn_obj
188 //! [`spawn_local_obj`]: https://docs.rs/futures/0.3/futures/task/trait.LocalSpawn.html#tymethod.spawn_local_obj
189
190 pub use futures_executor::{
191 block_on, block_on_stream, enter, BlockingStream, Enter, EnterError, LocalPool,
192 LocalSpawner,
193 };
194
195 #[cfg(feature = "thread-pool")]
196 #[cfg_attr(docsrs, doc(cfg(feature = "thread-pool")))]
197 pub use futures_executor::{ThreadPool, ThreadPoolBuilder};
198}
199
200#[cfg(feature = "compat")]
201#[cfg_attr(docsrs, doc(cfg(feature = "compat")))]
202pub mod compat {
203 //! Interop between `futures` 0.1 and 0.3.
204 //!
205 //! This module is only available when the `compat` feature of this
206 //! library is activated.
207
208 pub use futures_util::compat::{
209 Compat, Compat01As03, Compat01As03Sink, CompatSink, Executor01As03, Executor01CompatExt,
210 Executor01Future, Future01CompatExt, Sink01CompatExt, Stream01CompatExt,
211 };
212
213 #[cfg(feature = "io-compat")]
214 #[cfg_attr(docsrs, doc(cfg(feature = "io-compat")))]
215 pub use futures_util::compat::{AsyncRead01CompatExt, AsyncWrite01CompatExt};
216}
217
218pub mod prelude {
219 //! A "prelude" for crates using the `futures` crate.
220 //!
221 //! This prelude is similar to the standard library's prelude in that you'll
222 //! almost always want to import its entire contents, but unlike the
223 //! standard library's prelude you'll have to do so manually:
224 //!
225 //! ```
226 //! # #[allow(unused_imports)]
227 //! use futures::prelude::*;
228 //! ```
229 //!
230 //! The prelude may grow over time as additional items see ubiquitous use.
231
232 pub use crate::future::{self, Future, TryFuture};
233 pub use crate::sink::{self, Sink};
234 pub use crate::stream::{self, Stream, TryStream};
235
236 #[doc(no_inline)]
237 #[allow(unreachable_pub)]
238 pub use crate::future::{FutureExt as _, TryFutureExt as _};
239 #[doc(no_inline)]
240 pub use crate::sink::SinkExt as _;
241 #[doc(no_inline)]
242 #[allow(unreachable_pub)]
243 pub use crate::stream::{StreamExt as _, TryStreamExt as _};
244
245 #[cfg(feature = "std")]
246 pub use crate::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite};
247
248 #[cfg(feature = "std")]
249 #[doc(no_inline)]
250 #[allow(unreachable_pub)]
251 pub use crate::io::{
252 AsyncBufReadExt as _, AsyncReadExt as _, AsyncSeekExt as _, AsyncWriteExt as _,
253 };
254}
255