use crate::runtime;
use crate::task::JoinHandle;
use std::future::Future;
doc_rt_core! {
/// Spawns a new asynchronous task, returning a
/// [`JoinHandle`](super::JoinHandle) for it.
///
/// Spawning a task enables the task to execute concurrently to other tasks. The
/// spawned task may execute on the current thread, or it may be sent to a
/// different thread to be executed. The specifics depend on the current
/// [`Runtime`](crate::runtime::Runtime) configuration.
///
/// There is no guarantee that a spawned task will execute to completion.
/// When a runtime is shutdown, all outstanding tasks are dropped,
/// regardless of the lifecycle of that task.
///
/// This function must be called from the context of a Tokio runtime. Tasks running on
/// the Tokio runtime are always inside its context, but you can also enter the context
/// using the [`Handle::enter`](crate::runtime::Handle::enter()) method.
///
/// # Examples
///
/// In this example, a server is started and `spawn` is used to start a new task
/// that processes each received connection.
///
/// ```no_run
/// use tokio::net::{TcpListener, TcpStream};
///
/// use std::io;
///
/// async fn process(socket: TcpStream) {
/// // ...
/// # drop(socket);
/// }
///
/// #[tokio::main]
/// async fn main() -> io::Result<()> {
/// let mut listener = TcpListener::bind("127.0.0.1:8080").await?;
///
/// loop {
/// let (socket, _) = listener.accept().await?;
///
/// tokio::spawn(async move {
/// // Process each socket concurrently.
/// process(socket).await
/// });
/// }
/// }
/// ```
///
/// # Panics
///
/// Panics if called from **outside** of the Tokio runtime.
///
/// # Using `!Send` values from a task
///
/// The task supplied to `spawn` must implement `Send`. However, it is
/// possible to **use** `!Send` values from the task as long as they only
/// exist between calls to `.await`.
///
/// For example, this will work:
///
/// ```
/// use tokio::task;
///
/// use std::rc::Rc;
///
/// fn use_rc(rc: Rc<()>) {
/// // Do stuff w/ rc
/// # drop(rc);
/// }
///
/// #[tokio::main]
/// async fn main() {
/// tokio::spawn(async {
/// // Force the `Rc` to stay in a scope with no `.await`
/// {
/// let rc = Rc::new(());
/// use_rc(rc.clone());
/// }
///
/// task::yield_now().await;
/// }).await.unwrap();
/// }
/// ```
///
/// This will **not** work:
///
/// ```compile_fail
/// use tokio::task;
///
/// use std::rc::Rc;
///
/// fn use_rc(rc: Rc<()>) {
/// // Do stuff w/ rc
/// # drop(rc);
/// }
///
/// #[tokio::main]
/// async fn main() {
/// tokio::spawn(async {
/// let rc = Rc::new(());
///
/// task::yield_now().await;
///
/// use_rc(rc.clone());
/// }).await.unwrap();
/// }
/// ```
///
/// Holding on to a `!Send` value across calls to `.await` will result in
/// an unfriendly compile error message similar to:
///
/// ```text
/// `[... some type ...]` cannot be sent between threads safely
/// ```
///
/// or:
///
/// ```text
/// error[E0391]: cycle detected when processing `main`
/// ```
pub fn spawn<T>(task: T) -> JoinHandle<T::Output>
where
T: Future + Send + 'static,
T::Output: Send + 'static,
{
let spawn_handle = runtime::context::spawn_handle()
.expect("must be called from the context of Tokio runtime configured with either `basic_scheduler` or `threaded_scheduler`");
spawn_handle.spawn(task)
}
}