<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source to the Rust file `src/libcore/iter/sources.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>sources.rs.html -- source</title><link rel="stylesheet" type="text/css" href="../../../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../../../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../../../light1.35.0.css" id="themeStyle"><script src="../../../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../../../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../../../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../../../down-arrow1.35.0.svg");}</style></head><body class="rustdoc source"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../../core/index.html'><img src='../../../rust-logo1.35.0.png' alt='logo' width='100'></a></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../../theme1.35.0.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../../../settings.html"><img src="../../../wheel1.35.0.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><pre class="line-numbers"><span id="1"> 1</span>
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<span id="629">629</span>
<span id="630">630</span>
<span id="631">631</span>
<span id="632">632</span>
<span id="633">633</span>
<span id="634">634</span>
<span id="635">635</span>
<span id="636">636</span>
<span id="637">637</span>
<span id="638">638</span>
<span id="639">639</span>
</pre><div class="example-wrap"><pre class="rust ">
<span class="kw">use</span> <span class="ident">fmt</span>;
<span class="kw">use</span> <span class="ident">marker</span>;
<span class="kw">use</span> <span class="ident">usize</span>;
<span class="kw">use</span> <span class="kw">super</span>::{<span class="ident">FusedIterator</span>, <span class="ident">TrustedLen</span>};
<span class="doccomment">/// An iterator that repeats an element endlessly.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This `struct` is created by the [`repeat`] function. See its documentation for more.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`repeat`]: fn.repeat.html</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Clone</span>, <span class="ident">Debug</span>)]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Repeat</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="ident">element</span>: <span class="ident">A</span>
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>: <span class="ident">Clone</span><span class="op">></span> <span class="ident">Iterator</span> <span class="kw">for</span> <span class="ident">Repeat</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Item</span> <span class="op">=</span> <span class="ident">A</span>;
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">next</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> { <span class="prelude-val">Some</span>(<span class="self">self</span>.<span class="ident">element</span>.<span class="ident">clone</span>()) }
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">size_hint</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> (<span class="ident">usize</span>, <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">usize</span><span class="op">></span>) { (<span class="ident">usize</span>::<span class="ident">MAX</span>, <span class="prelude-val">None</span>) }
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>: <span class="ident">Clone</span><span class="op">></span> <span class="ident">DoubleEndedIterator</span> <span class="kw">for</span> <span class="ident">Repeat</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">next_back</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> { <span class="prelude-val">Some</span>(<span class="self">self</span>.<span class="ident">element</span>.<span class="ident">clone</span>()) }
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"fused"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.26.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>: <span class="ident">Clone</span><span class="op">></span> <span class="ident">FusedIterator</span> <span class="kw">for</span> <span class="ident">Repeat</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"trusted_len"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"37572"</span>)]</span>
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>: <span class="ident">Clone</span><span class="op">></span> <span class="ident">TrustedLen</span> <span class="kw">for</span> <span class="ident">Repeat</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {}
<span class="doccomment">/// Creates a new iterator that endlessly repeats a single element.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The `repeat()` function repeats a single value over and over again.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Infinite iterators like `repeat()` are often used with adapters like</span>
<span class="doccomment">/// [`take`], in order to make them finite.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`take`]: trait.Iterator.html#method.take</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the element type of the iterator you need does not implement `Clone`,</span>
<span class="doccomment">/// or if you do not want to keep the repeated element in memory, you can</span>
<span class="doccomment">/// instead use the [`repeat_with`] function.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`repeat_with`]: fn.repeat_with.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // the number four 4ever:</span>
<span class="doccomment">/// let mut fours = iter::repeat(4);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(Some(4), fours.next());</span>
<span class="doccomment">/// assert_eq!(Some(4), fours.next());</span>
<span class="doccomment">/// assert_eq!(Some(4), fours.next());</span>
<span class="doccomment">/// assert_eq!(Some(4), fours.next());</span>
<span class="doccomment">/// assert_eq!(Some(4), fours.next());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // yup, still four</span>
<span class="doccomment">/// assert_eq!(Some(4), fours.next());</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Going finite with [`take`]:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // that last example was too many fours. Let's only have four fours.</span>
<span class="doccomment">/// let mut four_fours = iter::repeat(4).take(4);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(Some(4), four_fours.next());</span>
<span class="doccomment">/// assert_eq!(Some(4), four_fours.next());</span>
<span class="doccomment">/// assert_eq!(Some(4), four_fours.next());</span>
<span class="doccomment">/// assert_eq!(Some(4), four_fours.next());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // ... and now we're done</span>
<span class="doccomment">/// assert_eq!(None, four_fours.next());</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">repeat</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Clone</span><span class="op">></span>(<span class="ident">elt</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="ident">Repeat</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">Repeat</span>{<span class="ident">element</span>: <span class="ident">elt</span>}
}
<span class="doccomment">/// An iterator that repeats elements of type `A` endlessly by</span>
<span class="doccomment">/// applying the provided closure `F: FnMut() -> A`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This `struct` is created by the [`repeat_with`] function.</span>
<span class="doccomment">/// See its documentation for more.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`repeat_with`]: fn.repeat_with.html</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Copy</span>, <span class="ident">Clone</span>, <span class="ident">Debug</span>)]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iterator_repeat_with"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.28.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">RepeatWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="ident">repeater</span>: <span class="ident">F</span>
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iterator_repeat_with"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.28.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnMut</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span> <span class="ident">Iterator</span> <span class="kw">for</span> <span class="ident">RepeatWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Item</span> <span class="op">=</span> <span class="ident">A</span>;
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">next</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> { <span class="prelude-val">Some</span>((<span class="self">self</span>.<span class="ident">repeater</span>)()) }
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">size_hint</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> (<span class="ident">usize</span>, <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">usize</span><span class="op">></span>) { (<span class="ident">usize</span>::<span class="ident">MAX</span>, <span class="prelude-val">None</span>) }
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iterator_repeat_with"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.28.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnMut</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span> <span class="ident">FusedIterator</span> <span class="kw">for</span> <span class="ident">RepeatWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"trusted_len"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"37572"</span>)]</span>
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnMut</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span> <span class="ident">TrustedLen</span> <span class="kw">for</span> <span class="ident">RepeatWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {}
<span class="doccomment">/// Creates a new iterator that repeats elements of type `A` endlessly by</span>
<span class="doccomment">/// applying the provided closure, the repeater, `F: FnMut() -> A`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The `repeat_with()` function calls the repeater over and over again.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Infinite iterators like `repeat_with()` are often used with adapters like</span>
<span class="doccomment">/// [`take`], in order to make them finite.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`take`]: trait.Iterator.html#method.take</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the element type of the iterator you need implements `Clone`, and</span>
<span class="doccomment">/// it is OK to keep the source element in memory, you should instead use</span>
<span class="doccomment">/// the [`repeat`] function.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`repeat`]: fn.repeat.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// An iterator produced by `repeat_with()` is not a `DoubleEndedIterator`.</span>
<span class="doccomment">/// If you need `repeat_with()` to return a `DoubleEndedIterator`,</span>
<span class="doccomment">/// please open a GitHub issue explaining your use case.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // let's assume we have some value of a type that is not `Clone`</span>
<span class="doccomment">/// // or which don't want to have in memory just yet because it is expensive:</span>
<span class="doccomment">/// #[derive(PartialEq, Debug)]</span>
<span class="doccomment">/// struct Expensive;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // a particular value forever:</span>
<span class="doccomment">/// let mut things = iter::repeat_with(|| Expensive);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(Some(Expensive), things.next());</span>
<span class="doccomment">/// assert_eq!(Some(Expensive), things.next());</span>
<span class="doccomment">/// assert_eq!(Some(Expensive), things.next());</span>
<span class="doccomment">/// assert_eq!(Some(Expensive), things.next());</span>
<span class="doccomment">/// assert_eq!(Some(Expensive), things.next());</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Using mutation and going finite:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```rust</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // From the zeroth to the third power of two:</span>
<span class="doccomment">/// let mut curr = 1;</span>
<span class="doccomment">/// let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp })</span>
<span class="doccomment">/// .take(4);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(Some(1), pow2.next());</span>
<span class="doccomment">/// assert_eq!(Some(2), pow2.next());</span>
<span class="doccomment">/// assert_eq!(Some(4), pow2.next());</span>
<span class="doccomment">/// assert_eq!(Some(8), pow2.next());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // ... and now we're done</span>
<span class="doccomment">/// assert_eq!(None, pow2.next());</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iterator_repeat_with"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.28.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">repeat_with</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnMut</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span>(<span class="ident">repeater</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">RepeatWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="ident">RepeatWith</span> { <span class="ident">repeater</span> }
}
<span class="doccomment">/// An iterator that yields nothing.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This `struct` is created by the [`empty`] function. See its documentation for more.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`empty`]: fn.empty.html</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_empty"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">marker</span>::<span class="ident">PhantomData</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>);
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"core_impl_debug"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.9.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">fmt</span>::<span class="ident">Debug</span> <span class="kw">for</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">f</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
<span class="ident">f</span>.<span class="ident">pad</span>(<span class="string">"Empty"</span>)
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_empty"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Iterator</span> <span class="kw">for</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Item</span> <span class="op">=</span> <span class="ident">T</span>;
<span class="kw">fn</span> <span class="ident">next</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="prelude-val">None</span>
}
<span class="kw">fn</span> <span class="ident">size_hint</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> (<span class="ident">usize</span>, <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">usize</span><span class="op">></span>){
(<span class="number">0</span>, <span class="prelude-val">Some</span>(<span class="number">0</span>))
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_empty"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">DoubleEndedIterator</span> <span class="kw">for</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">next_back</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="prelude-val">None</span>
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_empty"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">ExactSizeIterator</span> <span class="kw">for</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">len</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="ident">usize</span> {
<span class="number">0</span>
}
}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"trusted_len"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"37572"</span>)]</span>
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">TrustedLen</span> <span class="kw">for</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"fused"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.26.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">FusedIterator</span> <span class="kw">for</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
<span class="comment">// not #[derive] because that adds a Clone bound on T,</span>
<span class="comment">// which isn't necessary.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_empty"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Clone</span> <span class="kw">for</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">clone</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">Empty</span>(<span class="ident">marker</span>::<span class="ident">PhantomData</span>)
}
}
<span class="comment">// not #[derive] because that adds a Default bound on T,</span>
<span class="comment">// which isn't necessary.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_empty"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Default</span> <span class="kw">for</span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">default</span>() <span class="op">-></span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">Empty</span>(<span class="ident">marker</span>::<span class="ident">PhantomData</span>)
}
}
<span class="doccomment">/// Creates an iterator that yields nothing.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // this could have been an iterator over i32, but alas, it's just not.</span>
<span class="doccomment">/// let mut nope = iter::empty::<i32>();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(None, nope.next());</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_empty"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">const</span> <span class="kw">fn</span> <span class="ident">empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>() <span class="op">-></span> <span class="ident">Empty</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">Empty</span>(<span class="ident">marker</span>::<span class="ident">PhantomData</span>)
}
<span class="doccomment">/// An iterator that yields an element exactly once.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This `struct` is created by the [`once`] function. See its documentation for more.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`once`]: fn.once.html</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Clone</span>, <span class="ident">Debug</span>)]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Once</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">inner</span>: ::<span class="ident">option</span>::<span class="ident">IntoIter</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Iterator</span> <span class="kw">for</span> <span class="ident">Once</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Item</span> <span class="op">=</span> <span class="ident">T</span>;
<span class="kw">fn</span> <span class="ident">next</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="self">self</span>.<span class="ident">inner</span>.<span class="ident">next</span>()
}
<span class="kw">fn</span> <span class="ident">size_hint</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> (<span class="ident">usize</span>, <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">usize</span><span class="op">></span>) {
<span class="self">self</span>.<span class="ident">inner</span>.<span class="ident">size_hint</span>()
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">DoubleEndedIterator</span> <span class="kw">for</span> <span class="ident">Once</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">next_back</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="self">self</span>.<span class="ident">inner</span>.<span class="ident">next_back</span>()
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">ExactSizeIterator</span> <span class="kw">for</span> <span class="ident">Once</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">len</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="ident">usize</span> {
<span class="self">self</span>.<span class="ident">inner</span>.<span class="ident">len</span>()
}
}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"trusted_len"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"37572"</span>)]</span>
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">TrustedLen</span> <span class="kw">for</span> <span class="ident">Once</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"fused"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.26.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">FusedIterator</span> <span class="kw">for</span> <span class="ident">Once</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
<span class="doccomment">/// Creates an iterator that yields an element exactly once.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This is commonly used to adapt a single value into a [`chain`] of other</span>
<span class="doccomment">/// kinds of iteration. Maybe you have an iterator that covers almost</span>
<span class="doccomment">/// everything, but you need an extra special case. Maybe you have a function</span>
<span class="doccomment">/// which works on iterators, but you only need to process one value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`chain`]: trait.Iterator.html#method.chain</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // one is the loneliest number</span>
<span class="doccomment">/// let mut one = iter::once(1);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(Some(1), one.next());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // just one, that's all we get</span>
<span class="doccomment">/// assert_eq!(None, one.next());</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Chaining together with another iterator. Let's say that we want to iterate</span>
<span class="doccomment">/// over each file of the `.foo` directory, but also a configuration file,</span>
<span class="doccomment">/// `.foorc`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```no_run</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">/// use std::fs;</span>
<span class="doccomment">/// use std::path::PathBuf;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let dirs = fs::read_dir(".foo").unwrap();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // we need to convert from an iterator of DirEntry-s to an iterator of</span>
<span class="doccomment">/// // PathBufs, so we use map</span>
<span class="doccomment">/// let dirs = dirs.map(|file| file.unwrap().path());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // now, our iterator just for our config file</span>
<span class="doccomment">/// let config = iter::once(PathBuf::from(".foorc"));</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // chain the two iterators together into one big iterator</span>
<span class="doccomment">/// let files = dirs.chain(config);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // this will give us all of the files in .foo as well as .foorc</span>
<span class="doccomment">/// for f in files {</span>
<span class="doccomment">/// println!("{:?}", f);</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">once</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">value</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="ident">Once</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">Once</span> { <span class="ident">inner</span>: <span class="prelude-val">Some</span>(<span class="ident">value</span>).<span class="ident">into_iter</span>() }
}
<span class="doccomment">/// An iterator that yields a single element of type `A` by</span>
<span class="doccomment">/// applying the provided closure `F: FnOnce() -> A`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This `struct` is created by the [`once_with`] function.</span>
<span class="doccomment">/// See its documentation for more.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`once_with`]: fn.once_with.html</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Copy</span>, <span class="ident">Clone</span>, <span class="ident">Debug</span>)]</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once_with"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"57581"</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">OnceWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="ident">gen</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>,
}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once_with"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"57581"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span> <span class="ident">Iterator</span> <span class="kw">for</span> <span class="ident">OnceWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Item</span> <span class="op">=</span> <span class="ident">A</span>;
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">next</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="self">self</span>.<span class="ident">gen</span>.<span class="ident">take</span>().<span class="ident">map</span>(<span class="op">|</span><span class="ident">f</span><span class="op">|</span> <span class="ident">f</span>())
}
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">size_hint</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> (<span class="ident">usize</span>, <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">usize</span><span class="op">></span>) {
<span class="self">self</span>.<span class="ident">gen</span>.<span class="ident">iter</span>().<span class="ident">size_hint</span>()
}
}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once_with"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"57581"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span> <span class="ident">DoubleEndedIterator</span> <span class="kw">for</span> <span class="ident">OnceWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">next_back</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="self">self</span>.<span class="ident">next</span>()
}
}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once_with"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"57581"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span> <span class="ident">ExactSizeIterator</span> <span class="kw">for</span> <span class="ident">OnceWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">len</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="ident">usize</span> {
<span class="self">self</span>.<span class="ident">gen</span>.<span class="ident">iter</span>().<span class="ident">len</span>()
}
}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once_with"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"57581"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span> <span class="ident">FusedIterator</span> <span class="kw">for</span> <span class="ident">OnceWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {}
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once_with"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"57581"</span>)]</span>
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span> <span class="ident">TrustedLen</span> <span class="kw">for</span> <span class="ident">OnceWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {}
<span class="doccomment">/// Creates an iterator that lazily generates a value exactly once by invoking</span>
<span class="doccomment">/// the provided closure.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This is commonly used to adapt a single value generator into a [`chain`] of</span>
<span class="doccomment">/// other kinds of iteration. Maybe you have an iterator that covers almost</span>
<span class="doccomment">/// everything, but you need an extra special case. Maybe you have a function</span>
<span class="doccomment">/// which works on iterators, but you only need to process one value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Unlike [`once`], this function will lazily generate the value on request.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`once`]: fn.once.html</span>
<span class="doccomment">/// [`chain`]: trait.Iterator.html#method.chain</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(iter_once_with)]</span>
<span class="doccomment">///</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // one is the loneliest number</span>
<span class="doccomment">/// let mut one = iter::once_with(|| 1);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(Some(1), one.next());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // just one, that's all we get</span>
<span class="doccomment">/// assert_eq!(None, one.next());</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Chaining together with another iterator. Let's say that we want to iterate</span>
<span class="doccomment">/// over each file of the `.foo` directory, but also a configuration file,</span>
<span class="doccomment">/// `.foorc`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```no_run</span>
<span class="doccomment">/// #![feature(iter_once_with)]</span>
<span class="doccomment">///</span>
<span class="doccomment">/// use std::iter;</span>
<span class="doccomment">/// use std::fs;</span>
<span class="doccomment">/// use std::path::PathBuf;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let dirs = fs::read_dir(".foo").unwrap();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // we need to convert from an iterator of DirEntry-s to an iterator of</span>
<span class="doccomment">/// // PathBufs, so we use map</span>
<span class="doccomment">/// let dirs = dirs.map(|file| file.unwrap().path());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // now, our iterator just for our config file</span>
<span class="doccomment">/// let config = iter::once_with(|| PathBuf::from(".foorc"));</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // chain the two iterators together into one big iterator</span>
<span class="doccomment">/// let files = dirs.chain(config);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // this will give us all of the files in .foo as well as .foorc</span>
<span class="doccomment">/// for f in files {</span>
<span class="doccomment">/// println!("{:?}", f);</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_once_with"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"57581"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">once_with</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">F</span>: <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">A</span><span class="op">></span>(<span class="ident">gen</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">OnceWith</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="ident">OnceWith</span> { <span class="ident">gen</span>: <span class="prelude-val">Some</span>(<span class="ident">gen</span>) }
}
<span class="doccomment">/// Creates a new iterator where each iteration calls the provided closure</span>
<span class="doccomment">/// `F: FnMut() -> Option<T>`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This allows creating a custom iterator with any behavior</span>
<span class="doccomment">/// without using the more verbose syntax of creating a dedicated type</span>
<span class="doccomment">/// and implementing the `Iterator` trait for it.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Note that the `FromFn` iterator doesn’t make assumptions about the behavior of the closure,</span>
<span class="doccomment">/// and therefore conservatively does not implement [`FusedIterator`],</span>
<span class="doccomment">/// or override [`Iterator::size_hint`] from its default `(0, None)`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`FusedIterator`]: trait.FusedIterator.html</span>
<span class="doccomment">/// [`Iterator::size_hint`]: trait.Iterator.html#method.size_hint</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The closure can use captures and its environment to track state across iterations. Depending on</span>
<span class="doccomment">/// how the iterator is used, this may require specifying the `move` keyword on the closure.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Let’s re-implement the counter iterator from [module-level documentation]:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [module-level documentation]: index.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let mut count = 0;</span>
<span class="doccomment">/// let counter = std::iter::from_fn(move || {</span>
<span class="doccomment">/// // Increment our count. This is why we started at zero.</span>
<span class="doccomment">/// count += 1;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // Check to see if we've finished counting or not.</span>
<span class="doccomment">/// if count < 6 {</span>
<span class="doccomment">/// Some(count)</span>
<span class="doccomment">/// } else {</span>
<span class="doccomment">/// None</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// });</span>
<span class="doccomment">/// assert_eq!(counter.collect::<Vec<_>>(), &[1, 2, 3, 4, 5]);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_from_fn"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">from_fn</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span>(<span class="ident">f</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">FromFn</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnMut</span>() <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
{
<span class="ident">FromFn</span>(<span class="ident">f</span>)
}
<span class="doccomment">/// An iterator where each iteration calls the provided closure `F: FnMut() -> Option<T>`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This `struct` is created by the [`iter::from_fn`] function.</span>
<span class="doccomment">/// See its documentation for more.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`iter::from_fn`]: fn.from_fn.html</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Clone</span>)]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_from_fn"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">FromFn</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>(<span class="ident">F</span>);
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_from_fn"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span> <span class="ident">Iterator</span> <span class="kw">for</span> <span class="ident">FromFn</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnMut</span>() <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
{
<span class="kw">type</span> <span class="ident">Item</span> <span class="op">=</span> <span class="ident">T</span>;
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">next</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Item</span><span class="op">></span> {
(<span class="self">self</span>.<span class="number">0</span>)()
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_from_fn"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> <span class="ident">fmt</span>::<span class="ident">Debug</span> <span class="kw">for</span> <span class="ident">FromFn</span><span class="op"><</span><span class="ident">F</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">f</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
<span class="ident">f</span>.<span class="ident">debug_struct</span>(<span class="string">"FromFn"</span>).<span class="ident">finish</span>()
}
}
<span class="doccomment">/// Creates a new iterator where each successive item is computed based on the preceding one.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The iterator starts with the given first item (if any)</span>
<span class="doccomment">/// and calls the given `FnMut(&T) -> Option<T>` closure to compute each item’s successor.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::iter::successors;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let powers_of_10 = successors(Some(1_u16), |n| n.checked_mul(10));</span>
<span class="doccomment">/// assert_eq!(powers_of_10.collect::<Vec<_>>(), &[1, 10, 100, 1_000, 10_000]);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_successors"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">successors</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span>(<span class="ident">first</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>, <span class="ident">succ</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">Successors</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnMut</span>(<span class="kw-2">&</span><span class="ident">T</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
{
<span class="comment">// If this function returned `impl Iterator<Item=T>`</span>
<span class="comment">// it could be based on `unfold` and not need a dedicated type.</span>
<span class="comment">// However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.</span>
<span class="ident">Successors</span> {
<span class="ident">next</span>: <span class="ident">first</span>,
<span class="ident">succ</span>,
}
}
<span class="doccomment">/// An new iterator where each successive item is computed based on the preceding one.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This `struct` is created by the [`successors`] function.</span>
<span class="doccomment">/// See its documentation for more.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`successors`]: fn.successors.html</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Clone</span>)]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_successors"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Successors</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span> {
<span class="ident">next</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
<span class="ident">succ</span>: <span class="ident">F</span>,
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_successors"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span> <span class="ident">Iterator</span> <span class="kw">for</span> <span class="ident">Successors</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnMut</span>(<span class="kw-2">&</span><span class="ident">T</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
{
<span class="kw">type</span> <span class="ident">Item</span> <span class="op">=</span> <span class="ident">T</span>;
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">next</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Item</span><span class="op">></span> {
<span class="self">self</span>.<span class="ident">next</span>.<span class="ident">take</span>().<span class="ident">map</span>(<span class="op">|</span><span class="ident">item</span><span class="op">|</span> {
<span class="self">self</span>.<span class="ident">next</span> <span class="op">=</span> (<span class="self">self</span>.<span class="ident">succ</span>)(<span class="kw-2">&</span><span class="ident">item</span>);
<span class="ident">item</span>
})
}
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">size_hint</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> (<span class="ident">usize</span>, <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">usize</span><span class="op">></span>) {
<span class="kw">if</span> <span class="self">self</span>.<span class="ident">next</span>.<span class="ident">is_some</span>() {
(<span class="number">1</span>, <span class="prelude-val">None</span>)
} <span class="kw">else</span> {
(<span class="number">0</span>, <span class="prelude-val">Some</span>(<span class="number">0</span>))
}
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_successors"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span> <span class="ident">FusedIterator</span> <span class="kw">for</span> <span class="ident">Successors</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnMut</span>(<span class="kw-2">&</span><span class="ident">T</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
{}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_successors"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">fmt</span>::<span class="ident">Debug</span>, <span class="ident">F</span><span class="op">></span> <span class="ident">fmt</span>::<span class="ident">Debug</span> <span class="kw">for</span> <span class="ident">Successors</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">F</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">f</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
<span class="ident">f</span>.<span class="ident">debug_struct</span>(<span class="string">"Successors"</span>)
.<span class="ident">field</span>(<span class="string">"next"</span>, <span class="kw-2">&</span><span class="self">self</span>.<span class="ident">next</span>)
.<span class="ident">finish</span>()
}
}
</pre></div>
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