xref: /aosp_15_r20/external/rust/android-crates-io/crates/futures/src/lib.rs

//! Abstractions for asynchronous programming.
//!
//! This crate provides a number of core abstractions for writing asynchronous
//! code:
//!
//! - [Futures](crate::future) are single eventual values produced by
//!   asynchronous computations. Some programming languages (e.g. JavaScript)
//!   call this concept "promise".
//! - [Streams](crate::stream) represent a series of values
//!   produced asynchronously.
//! - [Sinks](crate::sink) provide support for asynchronous writing of
//!   data.
//! - [Executors](crate::executor) are responsible for running asynchronous
//!   tasks.
//!
//! The crate also contains abstractions for [asynchronous I/O](crate::io) and
//! [cross-task communication](crate::channel).
//!
//! Underlying all of this is the *task system*, which is a form of lightweight
//! threading. Large asynchronous computations are built up using futures,
//! streams and sinks, and then spawned as independent tasks that are run to
//! completion, but *do not block* the thread running them.
//!
//! The following example describes how the task system context is built and used
//! within macros and keywords such as async and await!.
//!
//! ```rust
//! # use futures::channel::mpsc;
//! # use futures::executor; ///standard executors to provide a context for futures and streams
//! # use futures::executor::ThreadPool;
//! # use futures::StreamExt;
//! #
//! fn main() {
//!     # {
//!     let pool = ThreadPool::new().expect("Failed to build pool");
//!     let (tx, rx) = mpsc::unbounded::<i32>();
//!
//!     // Create a future by an async block, where async is responsible for an
//!     // implementation of Future. At this point no executor has been provided
//!     // to this future, so it will not be running.
//!     let fut_values = async {
//!         // Create another async block, again where the Future implementation
//!         // is generated by async. Since this is inside of a parent async block,
//!         // it will be provided with the executor of the parent block when the parent
//!         // block is executed.
//!         //
//!         // This executor chaining is done by Future::poll whose second argument
//!         // is a std::task::Context. This represents our executor, and the Future
//!         // implemented by this async block can be polled using the parent async
//!         // block's executor.
//!         let fut_tx_result = async move {
//!             (0..100).for_each(|v| {
//!                 tx.unbounded_send(v).expect("Failed to send");
//!             })
//!         };
//!
//!         // Use the provided thread pool to spawn the generated future
//!         // responsible for transmission
//!         pool.spawn_ok(fut_tx_result);
//!
//!         let fut_values = rx
//!             .map(|v| v * 2)
//!             .collect();
//!
//!         // Use the executor provided to this async block to wait for the
//!         // future to complete.
//!         fut_values.await
//!     };
//!
//!     // Actually execute the above future, which will invoke Future::poll and
//!     // subsequently chain appropriate Future::poll and methods needing executors
//!     // to drive all futures. Eventually fut_values will be driven to completion.
//!     let values: Vec<i32> = executor::block_on(fut_values);
//!
//!     println!("Values={:?}", values);
//!     # }
//!     # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
//! }
//! ```
//!
//! The majority of examples and code snippets in this crate assume that they are
//! inside an async block as written above.

#![no_std]
#![doc(test(
    no_crate_inject,
    attr(
        deny(warnings, rust_2018_idioms, single_use_lifetimes),
        allow(dead_code, unused_assignments, unused_variables)
    )
))]
#![warn(missing_docs, unsafe_op_in_unsafe_fn)]
#![cfg_attr(docsrs, feature(doc_cfg))]

#[cfg(all(feature = "bilock", not(feature = "unstable")))]
compile_error!("The `bilock` feature requires the `unstable` feature as an explicit opt-in to unstable features");

#[doc(no_inline)]
pub use futures_core::future::{Future, TryFuture};
#[doc(no_inline)]
pub use futures_util::future::{FutureExt, TryFutureExt};

#[doc(no_inline)]
pub use futures_core::stream::{Stream, TryStream};
#[doc(no_inline)]
pub use futures_util::stream::{StreamExt, TryStreamExt};

#[doc(no_inline)]
pub use futures_sink::Sink;
#[doc(no_inline)]
pub use futures_util::sink::SinkExt;

#[cfg(feature = "std")]
#[doc(no_inline)]
pub use futures_io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite};
#[cfg(feature = "std")]
#[doc(no_inline)]
pub use futures_util::{AsyncBufReadExt, AsyncReadExt, AsyncSeekExt, AsyncWriteExt};

// Macro reexports
pub use futures_core::ready; // Readiness propagation
pub use futures_util::pin_mut;
#[cfg(feature = "std")]
#[cfg(feature = "async-await")]
pub use futures_util::select;
#[cfg(feature = "async-await")]
pub use futures_util::{join, pending, poll, select_biased, try_join}; // Async-await

// Module reexports
#[doc(inline)]
pub use futures_util::{future, never, sink, stream, task};

#[cfg(feature = "std")]
#[cfg(feature = "async-await")]
pub use futures_util::stream_select;

#[cfg(feature = "alloc")]
#[doc(inline)]
pub use futures_channel as channel;
#[cfg(feature = "alloc")]
#[doc(inline)]
pub use futures_util::lock;

#[cfg(feature = "std")]
#[doc(inline)]
pub use futures_util::io;

#[cfg(feature = "executor")]
#[cfg_attr(docsrs, doc(cfg(feature = "executor")))]
pub mod executor {
    //! Built-in executors and related tools.
    //!
    //! All asynchronous computation occurs within an executor, which is
    //! capable of spawning futures as tasks. This module provides several
    //! built-in executors, as well as tools for building your own.
    //!
    //!
    //! This module is only available when the `executor` feature of this
    //! library is activated.
    //!
    //! # Using a thread pool (M:N task scheduling)
    //!
    //! Most of the time tasks should be executed on a [thread pool](ThreadPool).
    //! A small set of worker threads can handle a very large set of spawned tasks
    //! (which are much lighter weight than threads). Tasks spawned onto the pool
    //! with the [`spawn_ok`](ThreadPool::spawn_ok) function will run ambiently on
    //! the created threads.
    //!
    //! # Spawning additional tasks
    //!
    //! Tasks can be spawned onto a spawner by calling its [`spawn_obj`] method
    //! directly. In the case of `!Send` futures, [`spawn_local_obj`] can be used
    //! instead.
    //!
    //! # Single-threaded execution
    //!
    //! In addition to thread pools, it's possible to run a task (and the tasks
    //! it spawns) entirely within a single thread via the [`LocalPool`] executor.
    //! Aside from cutting down on synchronization costs, this executor also makes
    //! it possible to spawn non-`Send` tasks, via [`spawn_local_obj`]. The
    //! [`LocalPool`] is best suited for running I/O-bound tasks that do relatively
    //! little work between I/O operations.
    //!
    //! There is also a convenience function [`block_on`] for simply running a
    //! future to completion on the current thread.
    //!
    //! [`spawn_obj`]: https://docs.rs/futures/0.3/futures/task/trait.Spawn.html#tymethod.spawn_obj
    //! [`spawn_local_obj`]: https://docs.rs/futures/0.3/futures/task/trait.LocalSpawn.html#tymethod.spawn_local_obj

    pub use futures_executor::{
        block_on, block_on_stream, enter, BlockingStream, Enter, EnterError, LocalPool,
        LocalSpawner,
    };

    #[cfg(feature = "thread-pool")]
    #[cfg_attr(docsrs, doc(cfg(feature = "thread-pool")))]
    pub use futures_executor::{ThreadPool, ThreadPoolBuilder};
}

#[cfg(feature = "compat")]
#[cfg_attr(docsrs, doc(cfg(feature = "compat")))]
pub mod compat {
    //! Interop between `futures` 0.1 and 0.3.
    //!
    //! This module is only available when the `compat` feature of this
    //! library is activated.

    pub use futures_util::compat::{
        Compat, Compat01As03, Compat01As03Sink, CompatSink, Executor01As03, Executor01CompatExt,
        Executor01Future, Future01CompatExt, Sink01CompatExt, Stream01CompatExt,
    };

    #[cfg(feature = "io-compat")]
    #[cfg_attr(docsrs, doc(cfg(feature = "io-compat")))]
    pub use futures_util::compat::{AsyncRead01CompatExt, AsyncWrite01CompatExt};
}

pub mod prelude {
    //! A "prelude" for crates using the `futures` crate.
    //!
    //! This prelude is similar to the standard library's prelude in that you'll
    //! almost always want to import its entire contents, but unlike the
    //! standard library's prelude you'll have to do so manually:
    //!
    //! ```
    //! # #[allow(unused_imports)]
    //! use futures::prelude::*;
    //! ```
    //!
    //! The prelude may grow over time as additional items see ubiquitous use.

    pub use crate::future::{self, Future, TryFuture};
    pub use crate::sink::{self, Sink};
    pub use crate::stream::{self, Stream, TryStream};

    #[doc(no_inline)]
    #[allow(unreachable_pub)]
    pub use crate::future::{FutureExt as _, TryFutureExt as _};
    #[doc(no_inline)]
    pub use crate::sink::SinkExt as _;
    #[doc(no_inline)]
    #[allow(unreachable_pub)]
    pub use crate::stream::{StreamExt as _, TryStreamExt as _};

    #[cfg(feature = "std")]
    pub use crate::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite};

    #[cfg(feature = "std")]
    #[doc(no_inline)]
    #[allow(unreachable_pub)]
    pub use crate::io::{
        AsyncBufReadExt as _, AsyncReadExt as _, AsyncSeekExt as _, AsyncWriteExt as _,
    };
}