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<span class="comment">// Copyright 2018 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="attribute">#![<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"futures_api"</span>,
<span class="ident">reason</span> <span class="op">=</span> <span class="string">"futures in libcore are unstable"</span>,
<span class="ident">issue</span> <span class="op">=</span> <span class="string">"50547"</span>)]</span>
<span class="doccomment">//! Asynchronous values.</span>
<span class="kw">use</span> <span class="ident">mem</span>::<span class="ident">PinMut</span>;
<span class="kw">use</span> <span class="ident">marker</span>::<span class="ident">Unpin</span>;
<span class="kw">use</span> <span class="ident">task</span>::{<span class="self">self</span>, <span class="ident">Poll</span>};
<span class="doccomment">/// A future represents an asychronous computation.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// A future is a value that may not have finished computing yet. This kind of</span>
<span class="doccomment">/// "asynchronous value" makes it possible for a thread to continue doing useful</span>
<span class="doccomment">/// work while it waits for the value to become available.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # The `poll` method</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The core method of future, `poll`, *attempts* to resolve the future into a</span>
<span class="doccomment">/// final value. This method does not block if the value is not ready. Instead,</span>
<span class="doccomment">/// the current task is scheduled to be woken up when it's possible to make</span>
<span class="doccomment">/// further progress by `poll`ing again. The wake up is performed using</span>
<span class="doccomment">/// `cx.waker()`, a handle for waking up the current task.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// When using a future, you generally won't call `poll` directly, but instead</span>
<span class="doccomment">/// `await!` the value.</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Future</span> {
<span class="doccomment">/// The result of the `Future`.</span>
<span class="kw">type</span> <span class="ident">Output</span>;
<span class="doccomment">/// Attempt to resolve the future to a final value, registering</span>
<span class="doccomment">/// the current task for wakeup if the value is not yet available.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Return value</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This function returns:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - `Poll::Pending` if the future is not ready yet</span>
<span class="doccomment">/// - `Poll::Ready(val)` with the result `val` of this future if it finished</span>
<span class="doccomment">/// successfully.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Once a future has finished, clients should not `poll` it again.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// When a future is not ready yet, `poll` returns</span>
<span class="doccomment">/// [`Poll::Pending`](::task::Poll). The future will *also* register the</span>
<span class="doccomment">/// interest of the current task in the value being produced. For example,</span>
<span class="doccomment">/// if the future represents the availability of data on a socket, then the</span>
<span class="doccomment">/// task is recorded so that when data arrives, it is woken up (via</span>
<span class="doccomment">/// [`cx.waker()`](::task::Context::waker)). Once a task has been woken up,</span>
<span class="doccomment">/// it should attempt to `poll` the future again, which may or may not</span>
<span class="doccomment">/// produce a final value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Note that if `Pending` is returned it only means that the *current* task</span>
<span class="doccomment">/// (represented by the argument `cx`) will receive a notification. Tasks</span>
<span class="doccomment">/// from previous calls to `poll` will *not* receive notifications.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Runtime characteristics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Futures alone are *inert*; they must be *actively* `poll`ed to make</span>
<span class="doccomment">/// progress, meaning that each time the current task is woken up, it should</span>
<span class="doccomment">/// actively re-`poll` pending futures that it still has an interest in.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The `poll` function is not called repeatedly in a tight loop for</span>
<span class="doccomment">/// futures, but only whenever the future itself is ready, as signaled via</span>
<span class="doccomment">/// the `Waker` inside `task::Context`. If you're familiar with the</span>
<span class="doccomment">/// `poll(2)` or `select(2)` syscalls on Unix it's worth noting that futures</span>
<span class="doccomment">/// typically do *not* suffer the same problems of "all wakeups must poll</span>
<span class="doccomment">/// all events"; they are more like `epoll(4)`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// An implementation of `poll` should strive to return quickly, and must</span>
<span class="doccomment">/// *never* block. Returning quickly prevents unnecessarily clogging up</span>
<span class="doccomment">/// threads or event loops. If it is known ahead of time that a call to</span>
<span class="doccomment">/// `poll` may end up taking awhile, the work should be offloaded to a</span>
<span class="doccomment">/// thread pool (or something similar) to ensure that `poll` can return</span>
<span class="doccomment">/// quickly.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Once a future has completed (returned `Ready` from `poll`),</span>
<span class="doccomment">/// then any future calls to `poll` may panic, block forever, or otherwise</span>
<span class="doccomment">/// cause bad behavior. The `Future` trait itself provides no guarantees</span>
<span class="doccomment">/// about the behavior of `poll` after a future has completed.</span>
<span class="kw">fn</span> <span class="ident">poll</span>(<span class="self">self</span>: <span class="ident">PinMut</span><span class="op"><</span><span class="self">Self</span><span class="op">></span>, <span class="ident">cx</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">task</span>::<span class="ident">Context</span>) <span class="op">-></span> <span class="ident">Poll</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Output</span><span class="op">></span>;
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">F</span>: <span class="question-mark">?</span><span class="ident">Sized</span> <span class="op">+</span> <span class="ident">Future</span> <span class="op">+</span> <span class="ident">Unpin</span><span class="op">></span> <span class="ident">Future</span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">F</span> {
<span class="kw">type</span> <span class="ident">Output</span> <span class="op">=</span> <span class="ident">F</span>::<span class="ident">Output</span>;
<span class="kw">fn</span> <span class="ident">poll</span>(<span class="kw-2">mut</span> <span class="self">self</span>: <span class="ident">PinMut</span><span class="op"><</span><span class="self">Self</span><span class="op">></span>, <span class="ident">cx</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">task</span>::<span class="ident">Context</span>) <span class="op">-></span> <span class="ident">Poll</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Output</span><span class="op">></span> {
<span class="ident">F</span>::<span class="ident">poll</span>(<span class="ident">PinMut</span>::<span class="ident">new</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="kw-2">*</span><span class="kw-2">*</span><span class="self">self</span>), <span class="ident">cx</span>)
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">F</span>: <span class="question-mark">?</span><span class="ident">Sized</span> <span class="op">+</span> <span class="ident">Future</span><span class="op">></span> <span class="ident">Future</span> <span class="kw">for</span> <span class="ident">PinMut</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">F</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Output</span> <span class="op">=</span> <span class="ident">F</span>::<span class="ident">Output</span>;
<span class="kw">fn</span> <span class="ident">poll</span>(<span class="kw-2">mut</span> <span class="self">self</span>: <span class="ident">PinMut</span><span class="op"><</span><span class="self">Self</span><span class="op">></span>, <span class="ident">cx</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">task</span>::<span class="ident">Context</span>) <span class="op">-></span> <span class="ident">Poll</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Output</span><span class="op">></span> {
<span class="ident">F</span>::<span class="ident">poll</span>((<span class="kw-2">*</span><span class="self">self</span>).<span class="ident">reborrow</span>(), <span class="ident">cx</span>)
}
}
</pre>
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