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</pre><pre class="rust"><code><span class="doccomment">//! The `select` macro.
</span><span class="macro">macro_rules! </span>document_select_macro {
<span class="comment">// This branch is required for `futures 0.3.1`, from before select_biased was introduced
</span>(<span class="macro-nonterminal">$select</span>:item) =&gt; {
<span class="doccomment">/// Polls multiple futures and streams simultaneously, executing the branch
/// for the future that finishes first. If multiple futures are ready,
/// one will be pseudo-randomly selected at runtime. Futures directly
/// passed to `select!` must be `Unpin` and implement `FusedFuture`.
///
/// If an expression which yields a `Future` is passed to `select!`
/// (e.g. an `async fn` call) instead of a `Future` by name the `Unpin`
/// requirement is relaxed, since the macro will pin the resulting `Future`
/// on the stack. However the `Future` returned by the expression must
/// still implement `FusedFuture`.
///
/// Futures and streams which are not already fused can be fused using the
/// `.fuse()` method. Note, though, that fusing a future or stream directly
/// in the call to `select!` will not be enough to prevent it from being
/// polled after completion if the `select!` call is in a loop, so when
/// `select!`ing in a loop, users should take care to `fuse()` outside of
/// the loop.
///
/// `select!` can be used as an expression and will return the return
/// value of the selected branch. For this reason the return type of every
/// branch in a `select!` must be the same.
///
/// This macro is only usable inside of async functions, closures, and blocks.
/// It is also gated behind the `async-await` feature of this library, which is
/// activated by default.
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::select;
/// let mut a = future::ready(4);
/// let mut b = future::pending::&lt;()&gt;();
///
/// let res = select! {
/// a_res = a =&gt; a_res + 1,
/// _ = b =&gt; 0,
/// };
/// assert_eq!(res, 5);
/// # });
/// ```
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::stream::{self, StreamExt};
/// use futures::select;
/// let mut st = stream::iter(vec![2]).fuse();
/// let mut fut = future::pending::&lt;()&gt;();
///
/// select! {
/// x = st.next() =&gt; assert_eq!(Some(2), x),
/// _ = fut =&gt; panic!(),
/// };
/// # });
/// ```
///
/// As described earlier, `select` can directly select on expressions
/// which return `Future`s - even if those do not implement `Unpin`:
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::FutureExt;
/// use futures::select;
///
/// // Calling the following async fn returns a Future which does not
/// // implement Unpin
/// async fn async_identity_fn(arg: usize) -&gt; usize {
/// arg
/// }
///
/// let res = select! {
/// a_res = async_identity_fn(62).fuse() =&gt; a_res + 1,
/// b_res = async_identity_fn(13).fuse() =&gt; b_res,
/// };
/// assert!(res == 63 || res == 13);
/// # });
/// ```
///
/// If a similar async function is called outside of `select` to produce
/// a `Future`, the `Future` must be pinned in order to be able to pass
/// it to `select`. This can be achieved via `Box::pin` for pinning a
/// `Future` on the heap or the `pin_mut!` macro for pinning a `Future`
/// on the stack.
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::FutureExt;
/// use futures::select;
/// use futures::pin_mut;
///
/// // Calling the following async fn returns a Future which does not
/// // implement Unpin
/// async fn async_identity_fn(arg: usize) -&gt; usize {
/// arg
/// }
///
/// let fut_1 = async_identity_fn(1).fuse();
/// let fut_2 = async_identity_fn(2).fuse();
/// let mut fut_1 = Box::pin(fut_1); // Pins the Future on the heap
/// pin_mut!(fut_2); // Pins the Future on the stack
///
/// let res = select! {
/// a_res = fut_1 =&gt; a_res,
/// b_res = fut_2 =&gt; b_res,
/// };
/// assert!(res == 1 || res == 2);
/// # });
/// ```
///
/// `select` also accepts a `complete` branch and a `default` branch.
/// `complete` will run if all futures and streams have already been
/// exhausted. `default` will run if no futures or streams are
/// immediately ready. `complete` takes priority over `default` in
/// the case where all futures have completed.
/// A motivating use-case for passing `Future`s by name as well as for
/// `complete` blocks is to call `select!` in a loop, which is
/// demonstrated in the following example:
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::select;
/// let mut a_fut = future::ready(4);
/// let mut b_fut = future::ready(6);
/// let mut total = 0;
///
/// loop {
/// select! {
/// a = a_fut =&gt; total += a,
/// b = b_fut =&gt; total += b,
/// complete =&gt; break,
/// default =&gt; panic!(), // never runs (futures run first, then complete)
/// };
/// }
/// assert_eq!(total, 10);
/// # });
/// ```
///
/// Note that the futures that have been matched over can still be mutated
/// from inside the `select!` block&#39;s branches. This can be used to implement
/// more complex behavior such as timer resets or writing into the head of
/// a stream.
</span><span class="macro-nonterminal">$select
</span>};
(<span class="macro-nonterminal">$select</span>:item <span class="macro-nonterminal">$select_biased</span>:item) =&gt; {
<span class="macro">document_select_macro!</span>(<span class="macro-nonterminal">$select</span>);
<span class="doccomment">/// Polls multiple futures and streams simultaneously, executing the branch
/// for the future that finishes first. Unlike [`select!`], if multiple futures are ready,
/// one will be selected in order of declaration. Futures directly
/// passed to `select_biased!` must be `Unpin` and implement `FusedFuture`.
///
/// If an expression which yields a `Future` is passed to `select_biased!`
/// (e.g. an `async fn` call) instead of a `Future` by name the `Unpin`
/// requirement is relaxed, since the macro will pin the resulting `Future`
/// on the stack. However the `Future` returned by the expression must
/// still implement `FusedFuture`.
///
/// Futures and streams which are not already fused can be fused using the
/// `.fuse()` method. Note, though, that fusing a future or stream directly
/// in the call to `select_biased!` will not be enough to prevent it from being
/// polled after completion if the `select_biased!` call is in a loop, so when
/// `select_biased!`ing in a loop, users should take care to `fuse()` outside of
/// the loop.
///
/// `select_biased!` can be used as an expression and will return the return
/// value of the selected branch. For this reason the return type of every
/// branch in a `select_biased!` must be the same.
///
/// This macro is only usable inside of async functions, closures, and blocks.
/// It is also gated behind the `async-await` feature of this library, which is
/// activated by default.
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::select_biased;
/// let mut a = future::ready(4);
/// let mut b = future::pending::&lt;()&gt;();
///
/// let res = select_biased! {
/// a_res = a =&gt; a_res + 1,
/// _ = b =&gt; 0,
/// };
/// assert_eq!(res, 5);
/// # });
/// ```
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::stream::{self, StreamExt};
/// use futures::select_biased;
/// let mut st = stream::iter(vec![2]).fuse();
/// let mut fut = future::pending::&lt;()&gt;();
///
/// select_biased! {
/// x = st.next() =&gt; assert_eq!(Some(2), x),
/// _ = fut =&gt; panic!(),
/// };
/// # });
/// ```
///
/// As described earlier, `select_biased` can directly select on expressions
/// which return `Future`s - even if those do not implement `Unpin`:
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::FutureExt;
/// use futures::select_biased;
///
/// // Calling the following async fn returns a Future which does not
/// // implement Unpin
/// async fn async_identity_fn(arg: usize) -&gt; usize {
/// arg
/// }
///
/// let res = select_biased! {
/// a_res = async_identity_fn(62).fuse() =&gt; a_res + 1,
/// b_res = async_identity_fn(13).fuse() =&gt; b_res,
/// };
/// assert!(res == 63 || res == 12);
/// # });
/// ```
///
/// If a similar async function is called outside of `select_biased` to produce
/// a `Future`, the `Future` must be pinned in order to be able to pass
/// it to `select_biased`. This can be achieved via `Box::pin` for pinning a
/// `Future` on the heap or the `pin_mut!` macro for pinning a `Future`
/// on the stack.
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::FutureExt;
/// use futures::select_biased;
/// use futures::pin_mut;
///
/// // Calling the following async fn returns a Future which does not
/// // implement Unpin
/// async fn async_identity_fn(arg: usize) -&gt; usize {
/// arg
/// }
///
/// let fut_1 = async_identity_fn(1).fuse();
/// let fut_2 = async_identity_fn(2).fuse();
/// let mut fut_1 = Box::pin(fut_1); // Pins the Future on the heap
/// pin_mut!(fut_2); // Pins the Future on the stack
///
/// let res = select_biased! {
/// a_res = fut_1 =&gt; a_res,
/// b_res = fut_2 =&gt; b_res,
/// };
/// assert!(res == 1 || res == 2);
/// # });
/// ```
///
/// `select_biased` also accepts a `complete` branch and a `default` branch.
/// `complete` will run if all futures and streams have already been
/// exhausted. `default` will run if no futures or streams are
/// immediately ready. `complete` takes priority over `default` in
/// the case where all futures have completed.
/// A motivating use-case for passing `Future`s by name as well as for
/// `complete` blocks is to call `select_biased!` in a loop, which is
/// demonstrated in the following example:
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::select_biased;
/// let mut a_fut = future::ready(4);
/// let mut b_fut = future::ready(6);
/// let mut total = 0;
///
/// loop {
/// select_biased! {
/// a = a_fut =&gt; total += a,
/// b = b_fut =&gt; total += b,
/// complete =&gt; break,
/// default =&gt; panic!(), // never runs (futures run first, then complete)
/// };
/// }
/// assert_eq!(total, 10);
/// # });
/// ```
///
/// Note that the futures that have been matched over can still be mutated
/// from inside the `select_biased!` block&#39;s branches. This can be used to implement
/// more complex behavior such as timer resets or writing into the head of
/// a stream.
///
/// [`select!`]: macro.select.html
</span><span class="macro-nonterminal">$select_biased
</span>};
}
<span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
#[allow(unreachable_pub)]
#[doc(hidden)]
</span><span class="kw">pub use </span>futures_macro::select_internal;
<span class="attribute">#[allow(unreachable_pub)]
#[doc(hidden)]
</span><span class="kw">pub use </span>futures_macro::select_biased_internal;
<span class="macro">document_select_macro! </span>{
<span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
#[macro_export]
</span><span class="macro">macro_rules! </span>select {
($(<span class="macro-nonterminal">$tokens</span>:tt)<span class="kw-2">*</span>) =&gt; {{
<span class="kw">use </span><span class="macro-nonterminal">$crate::__private </span><span class="kw">as </span>__futures_crate;
<span class="macro-nonterminal">$</span><span class="macro">crate::select_internal! </span>{
$( <span class="macro-nonterminal">$tokens </span>)*
}
}}
}
<span class="attribute">#[macro_export]
</span><span class="macro">macro_rules! </span>select_biased {
($(<span class="macro-nonterminal">$tokens</span>:tt)<span class="kw-2">*</span>) =&gt; {{
<span class="kw">use </span><span class="macro-nonterminal">$crate::__private </span><span class="kw">as </span>__futures_crate;
<span class="macro-nonterminal">$</span><span class="macro">crate::select_biased_internal! </span>{
$( <span class="macro-nonterminal">$tokens </span>)*
}
}}
}
}
</code></pre></div>
</section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="futures_util" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html>