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core/iter/traits/
iterator.rs

1use super::super::{
2    ArrayChunks, ByRefSized, Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, FlatMap,
3    Flatten, Fuse, Inspect, Intersperse, IntersperseWith, Map, MapWhile, MapWindows, Peekable,
4    Product, Rev, Scan, Skip, SkipWhile, StepBy, Sum, Take, TakeWhile, TrustedRandomAccessNoCoerce,
5    Zip, try_process,
6};
7use crate::array;
8use crate::cmp::{self, Ordering};
9use crate::num::NonZero;
10use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
11
12fn _assert_is_dyn_compatible(_: &dyn Iterator<Item = ()>) {}
13
14/// A trait for dealing with iterators.
15///
16/// This is the main iterator trait. For more about the concept of iterators
17/// generally, please see the [module-level documentation]. In particular, you
18/// may want to know how to [implement `Iterator`][impl].
19///
20/// [module-level documentation]: crate::iter
21/// [impl]: crate::iter#implementing-iterator
22#[stable(feature = "rust1", since = "1.0.0")]
23#[rustc_on_unimplemented(
24    on(
25        Self = "core::ops::range::RangeTo<Idx>",
26        note = "you might have meant to use a bounded `Range`"
27    ),
28    on(
29        Self = "core::ops::range::RangeToInclusive<Idx>",
30        note = "you might have meant to use a bounded `RangeInclusive`"
31    ),
32    label = "`{Self}` is not an iterator",
33    message = "`{Self}` is not an iterator"
34)]
35#[doc(notable_trait)]
36#[lang = "iterator"]
37#[rustc_diagnostic_item = "Iterator"]
38#[must_use = "iterators are lazy and do nothing unless consumed"]
39pub trait Iterator {
40    /// The type of the elements being iterated over.
41    #[rustc_diagnostic_item = "IteratorItem"]
42    #[stable(feature = "rust1", since = "1.0.0")]
43    type Item;
44
45    /// Advances the iterator and returns the next value.
46    ///
47    /// Returns [`None`] when iteration is finished. Individual iterator
48    /// implementations may choose to resume iteration, and so calling `next()`
49    /// again may or may not eventually start returning [`Some(Item)`] again at some
50    /// point.
51    ///
52    /// [`Some(Item)`]: Some
53    ///
54    /// # Examples
55    ///
56    /// ```
57    /// let a = [1, 2, 3];
58    ///
59    /// let mut iter = a.into_iter();
60    ///
61    /// // A call to next() returns the next value...
62    /// assert_eq!(Some(1), iter.next());
63    /// assert_eq!(Some(2), iter.next());
64    /// assert_eq!(Some(3), iter.next());
65    ///
66    /// // ... and then None once it's over.
67    /// assert_eq!(None, iter.next());
68    ///
69    /// // More calls may or may not return `None`. Here, they always will.
70    /// assert_eq!(None, iter.next());
71    /// assert_eq!(None, iter.next());
72    /// ```
73    #[lang = "next"]
74    #[stable(feature = "rust1", since = "1.0.0")]
75    fn next(&mut self) -> Option<Self::Item>;
76
77    /// Advances the iterator and returns an array containing the next `N` values.
78    ///
79    /// If there are not enough elements to fill the array then `Err` is returned
80    /// containing an iterator over the remaining elements.
81    ///
82    /// # Examples
83    ///
84    /// Basic usage:
85    ///
86    /// ```
87    /// #![feature(iter_next_chunk)]
88    ///
89    /// let mut iter = "lorem".chars();
90    ///
91    /// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']);              // N is inferred as 2
92    /// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']);         // N is inferred as 3
93    /// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4
94    /// ```
95    ///
96    /// Split a string and get the first three items.
97    ///
98    /// ```
99    /// #![feature(iter_next_chunk)]
100    ///
101    /// let quote = "not all those who wander are lost";
102    /// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap();
103    /// assert_eq!(first, "not");
104    /// assert_eq!(second, "all");
105    /// assert_eq!(third, "those");
106    /// ```
107    #[inline]
108    #[unstable(feature = "iter_next_chunk", reason = "recently added", issue = "98326")]
109    fn next_chunk<const N: usize>(
110        &mut self,
111    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
112    where
113        Self: Sized,
114    {
115        array::iter_next_chunk(self)
116    }
117
118    /// Returns the bounds on the remaining length of the iterator.
119    ///
120    /// Specifically, `size_hint()` returns a tuple where the first element
121    /// is the lower bound, and the second element is the upper bound.
122    ///
123    /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
124    /// A [`None`] here means that either there is no known upper bound, or the
125    /// upper bound is larger than [`usize`].
126    ///
127    /// # Implementation notes
128    ///
129    /// It is not enforced that an iterator implementation yields the declared
130    /// number of elements. A buggy iterator may yield less than the lower bound
131    /// or more than the upper bound of elements.
132    ///
133    /// `size_hint()` is primarily intended to be used for optimizations such as
134    /// reserving space for the elements of the iterator, but must not be
135    /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
136    /// implementation of `size_hint()` should not lead to memory safety
137    /// violations.
138    ///
139    /// That said, the implementation should provide a correct estimation,
140    /// because otherwise it would be a violation of the trait's protocol.
141    ///
142    /// The default implementation returns <code>(0, [None])</code> which is correct for any
143    /// iterator.
144    ///
145    /// # Examples
146    ///
147    /// Basic usage:
148    ///
149    /// ```
150    /// let a = [1, 2, 3];
151    /// let mut iter = a.iter();
152    ///
153    /// assert_eq!((3, Some(3)), iter.size_hint());
154    /// let _ = iter.next();
155    /// assert_eq!((2, Some(2)), iter.size_hint());
156    /// ```
157    ///
158    /// A more complex example:
159    ///
160    /// ```
161    /// // The even numbers in the range of zero to nine.
162    /// let iter = (0..10).filter(|x| x % 2 == 0);
163    ///
164    /// // We might iterate from zero to ten times. Knowing that it's five
165    /// // exactly wouldn't be possible without executing filter().
166    /// assert_eq!((0, Some(10)), iter.size_hint());
167    ///
168    /// // Let's add five more numbers with chain()
169    /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
170    ///
171    /// // now both bounds are increased by five
172    /// assert_eq!((5, Some(15)), iter.size_hint());
173    /// ```
174    ///
175    /// Returning `None` for an upper bound:
176    ///
177    /// ```
178    /// // an infinite iterator has no upper bound
179    /// // and the maximum possible lower bound
180    /// let iter = 0..;
181    ///
182    /// assert_eq!((usize::MAX, None), iter.size_hint());
183    /// ```
184    #[inline]
185    #[stable(feature = "rust1", since = "1.0.0")]
186    fn size_hint(&self) -> (usize, Option<usize>) {
187        (0, None)
188    }
189
190    /// Consumes the iterator, counting the number of iterations and returning it.
191    ///
192    /// This method will call [`next`] repeatedly until [`None`] is encountered,
193    /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
194    /// called at least once even if the iterator does not have any elements.
195    ///
196    /// [`next`]: Iterator::next
197    ///
198    /// # Overflow Behavior
199    ///
200    /// The method does no guarding against overflows, so counting elements of
201    /// an iterator with more than [`usize::MAX`] elements either produces the
202    /// wrong result or panics. If overflow checks are enabled, a panic is
203    /// guaranteed.
204    ///
205    /// # Panics
206    ///
207    /// This function might panic if the iterator has more than [`usize::MAX`]
208    /// elements.
209    ///
210    /// # Examples
211    ///
212    /// ```
213    /// let a = [1, 2, 3];
214    /// assert_eq!(a.iter().count(), 3);
215    ///
216    /// let a = [1, 2, 3, 4, 5];
217    /// assert_eq!(a.iter().count(), 5);
218    /// ```
219    #[inline]
220    #[stable(feature = "rust1", since = "1.0.0")]
221    fn count(self) -> usize
222    where
223        Self: Sized,
224    {
225        self.fold(
226            0,
227            #[rustc_inherit_overflow_checks]
228            |count, _| count + 1,
229        )
230    }
231
232    /// Consumes the iterator, returning the last element.
233    ///
234    /// This method will evaluate the iterator until it returns [`None`]. While
235    /// doing so, it keeps track of the current element. After [`None`] is
236    /// returned, `last()` will then return the last element it saw.
237    ///
238    /// # Examples
239    ///
240    /// ```
241    /// let a = [1, 2, 3];
242    /// assert_eq!(a.into_iter().last(), Some(3));
243    ///
244    /// let a = [1, 2, 3, 4, 5];
245    /// assert_eq!(a.into_iter().last(), Some(5));
246    /// ```
247    #[inline]
248    #[stable(feature = "rust1", since = "1.0.0")]
249    fn last(self) -> Option<Self::Item>
250    where
251        Self: Sized,
252    {
253        #[inline]
254        fn some<T>(_: Option<T>, x: T) -> Option<T> {
255            Some(x)
256        }
257
258        self.fold(None, some)
259    }
260
261    /// Advances the iterator by `n` elements.
262    ///
263    /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
264    /// times until [`None`] is encountered.
265    ///
266    /// `advance_by(n)` will return `Ok(())` if the iterator successfully advances by
267    /// `n` elements, or a `Err(NonZero<usize>)` with value `k` if [`None`] is encountered,
268    /// where `k` is remaining number of steps that could not be advanced because the iterator ran out.
269    /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
270    /// Otherwise, `k` is always less than `n`.
271    ///
272    /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
273    /// can advance its outer iterator until it finds an inner iterator that is not empty, which
274    /// then often allows it to return a more accurate `size_hint()` than in its initial state.
275    ///
276    /// [`Flatten`]: crate::iter::Flatten
277    /// [`next`]: Iterator::next
278    ///
279    /// # Examples
280    ///
281    /// ```
282    /// #![feature(iter_advance_by)]
283    ///
284    /// use std::num::NonZero;
285    ///
286    /// let a = [1, 2, 3, 4];
287    /// let mut iter = a.into_iter();
288    ///
289    /// assert_eq!(iter.advance_by(2), Ok(()));
290    /// assert_eq!(iter.next(), Some(3));
291    /// assert_eq!(iter.advance_by(0), Ok(()));
292    /// assert_eq!(iter.advance_by(100), Err(NonZero::new(99).unwrap())); // only `4` was skipped
293    /// ```
294    #[inline]
295    #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
296    fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
297        /// Helper trait to specialize `advance_by` via `try_fold` for `Sized` iterators.
298        trait SpecAdvanceBy {
299            fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>>;
300        }
301
302        impl<I: Iterator + ?Sized> SpecAdvanceBy for I {
303            default fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
304                for i in 0..n {
305                    if self.next().is_none() {
306                        // SAFETY: `i` is always less than `n`.
307                        return Err(unsafe { NonZero::new_unchecked(n - i) });
308                    }
309                }
310                Ok(())
311            }
312        }
313
314        impl<I: Iterator> SpecAdvanceBy for I {
315            fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
316                let Some(n) = NonZero::new(n) else {
317                    return Ok(());
318                };
319
320                let res = self.try_fold(n, |n, _| NonZero::new(n.get() - 1));
321
322                match res {
323                    None => Ok(()),
324                    Some(n) => Err(n),
325                }
326            }
327        }
328
329        self.spec_advance_by(n)
330    }
331
332    /// Returns the `n`th element of the iterator.
333    ///
334    /// Like most indexing operations, the count starts from zero, so `nth(0)`
335    /// returns the first value, `nth(1)` the second, and so on.
336    ///
337    /// Note that all preceding elements, as well as the returned element, will be
338    /// consumed from the iterator. That means that the preceding elements will be
339    /// discarded, and also that calling `nth(0)` multiple times on the same iterator
340    /// will return different elements.
341    ///
342    /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
343    /// iterator.
344    ///
345    /// # Examples
346    ///
347    /// Basic usage:
348    ///
349    /// ```
350    /// let a = [1, 2, 3];
351    /// assert_eq!(a.into_iter().nth(1), Some(2));
352    /// ```
353    ///
354    /// Calling `nth()` multiple times doesn't rewind the iterator:
355    ///
356    /// ```
357    /// let a = [1, 2, 3];
358    ///
359    /// let mut iter = a.into_iter();
360    ///
361    /// assert_eq!(iter.nth(1), Some(2));
362    /// assert_eq!(iter.nth(1), None);
363    /// ```
364    ///
365    /// Returning `None` if there are less than `n + 1` elements:
366    ///
367    /// ```
368    /// let a = [1, 2, 3];
369    /// assert_eq!(a.into_iter().nth(10), None);
370    /// ```
371    #[inline]
372    #[stable(feature = "rust1", since = "1.0.0")]
373    fn nth(&mut self, n: usize) -> Option<Self::Item> {
374        self.advance_by(n).ok()?;
375        self.next()
376    }
377
378    /// Creates an iterator starting at the same point, but stepping by
379    /// the given amount at each iteration.
380    ///
381    /// Note 1: The first element of the iterator will always be returned,
382    /// regardless of the step given.
383    ///
384    /// Note 2: The time at which ignored elements are pulled is not fixed.
385    /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
386    /// `self.nth(step-1)`, …, but is also free to behave like the sequence
387    /// `advance_n_and_return_first(&mut self, step)`,
388    /// `advance_n_and_return_first(&mut self, step)`, …
389    /// Which way is used may change for some iterators for performance reasons.
390    /// The second way will advance the iterator earlier and may consume more items.
391    ///
392    /// `advance_n_and_return_first` is the equivalent of:
393    /// ```
394    /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
395    /// where
396    ///     I: Iterator,
397    /// {
398    ///     let next = iter.next();
399    ///     if n > 1 {
400    ///         iter.nth(n - 2);
401    ///     }
402    ///     next
403    /// }
404    /// ```
405    ///
406    /// # Panics
407    ///
408    /// The method will panic if the given step is `0`.
409    ///
410    /// # Examples
411    ///
412    /// ```
413    /// let a = [0, 1, 2, 3, 4, 5];
414    /// let mut iter = a.into_iter().step_by(2);
415    ///
416    /// assert_eq!(iter.next(), Some(0));
417    /// assert_eq!(iter.next(), Some(2));
418    /// assert_eq!(iter.next(), Some(4));
419    /// assert_eq!(iter.next(), None);
420    /// ```
421    #[inline]
422    #[stable(feature = "iterator_step_by", since = "1.28.0")]
423    fn step_by(self, step: usize) -> StepBy<Self>
424    where
425        Self: Sized,
426    {
427        StepBy::new(self, step)
428    }
429
430    /// Takes two iterators and creates a new iterator over both in sequence.
431    ///
432    /// `chain()` will return a new iterator which will first iterate over
433    /// values from the first iterator and then over values from the second
434    /// iterator.
435    ///
436    /// In other words, it links two iterators together, in a chain. 🔗
437    ///
438    /// [`once`] is commonly used to adapt a single value into a chain of
439    /// other kinds of iteration.
440    ///
441    /// # Examples
442    ///
443    /// Basic usage:
444    ///
445    /// ```
446    /// let s1 = "abc".chars();
447    /// let s2 = "def".chars();
448    ///
449    /// let mut iter = s1.chain(s2);
450    ///
451    /// assert_eq!(iter.next(), Some('a'));
452    /// assert_eq!(iter.next(), Some('b'));
453    /// assert_eq!(iter.next(), Some('c'));
454    /// assert_eq!(iter.next(), Some('d'));
455    /// assert_eq!(iter.next(), Some('e'));
456    /// assert_eq!(iter.next(), Some('f'));
457    /// assert_eq!(iter.next(), None);
458    /// ```
459    ///
460    /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
461    /// anything that can be converted into an [`Iterator`], not just an
462    /// [`Iterator`] itself. For example, arrays (`[T]`) implement
463    /// [`IntoIterator`], and so can be passed to `chain()` directly:
464    ///
465    /// ```
466    /// let a1 = [1, 2, 3];
467    /// let a2 = [4, 5, 6];
468    ///
469    /// let mut iter = a1.into_iter().chain(a2);
470    ///
471    /// assert_eq!(iter.next(), Some(1));
472    /// assert_eq!(iter.next(), Some(2));
473    /// assert_eq!(iter.next(), Some(3));
474    /// assert_eq!(iter.next(), Some(4));
475    /// assert_eq!(iter.next(), Some(5));
476    /// assert_eq!(iter.next(), Some(6));
477    /// assert_eq!(iter.next(), None);
478    /// ```
479    ///
480    /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
481    ///
482    /// ```
483    /// #[cfg(windows)]
484    /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
485    ///     use std::os::windows::ffi::OsStrExt;
486    ///     s.encode_wide().chain(std::iter::once(0)).collect()
487    /// }
488    /// ```
489    ///
490    /// [`once`]: crate::iter::once
491    /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
492    #[inline]
493    #[stable(feature = "rust1", since = "1.0.0")]
494    fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
495    where
496        Self: Sized,
497        U: IntoIterator<Item = Self::Item>,
498    {
499        Chain::new(self, other.into_iter())
500    }
501
502    /// 'Zips up' two iterators into a single iterator of pairs.
503    ///
504    /// `zip()` returns a new iterator that will iterate over two other
505    /// iterators, returning a tuple where the first element comes from the
506    /// first iterator, and the second element comes from the second iterator.
507    ///
508    /// In other words, it zips two iterators together, into a single one.
509    ///
510    /// If either iterator returns [`None`], [`next`] from the zipped iterator
511    /// will return [`None`].
512    /// If the zipped iterator has no more elements to return then each further attempt to advance
513    /// it will first try to advance the first iterator at most one time and if it still yielded an item
514    /// try to advance the second iterator at most one time.
515    ///
516    /// To 'undo' the result of zipping up two iterators, see [`unzip`].
517    ///
518    /// [`unzip`]: Iterator::unzip
519    ///
520    /// # Examples
521    ///
522    /// Basic usage:
523    ///
524    /// ```
525    /// let s1 = "abc".chars();
526    /// let s2 = "def".chars();
527    ///
528    /// let mut iter = s1.zip(s2);
529    ///
530    /// assert_eq!(iter.next(), Some(('a', 'd')));
531    /// assert_eq!(iter.next(), Some(('b', 'e')));
532    /// assert_eq!(iter.next(), Some(('c', 'f')));
533    /// assert_eq!(iter.next(), None);
534    /// ```
535    ///
536    /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
537    /// anything that can be converted into an [`Iterator`], not just an
538    /// [`Iterator`] itself. For example, arrays (`[T]`) implement
539    /// [`IntoIterator`], and so can be passed to `zip()` directly:
540    ///
541    /// ```
542    /// let a1 = [1, 2, 3];
543    /// let a2 = [4, 5, 6];
544    ///
545    /// let mut iter = a1.into_iter().zip(a2);
546    ///
547    /// assert_eq!(iter.next(), Some((1, 4)));
548    /// assert_eq!(iter.next(), Some((2, 5)));
549    /// assert_eq!(iter.next(), Some((3, 6)));
550    /// assert_eq!(iter.next(), None);
551    /// ```
552    ///
553    /// `zip()` is often used to zip an infinite iterator to a finite one.
554    /// This works because the finite iterator will eventually return [`None`],
555    /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
556    ///
557    /// ```
558    /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
559    ///
560    /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
561    ///
562    /// assert_eq!((0, 'f'), enumerate[0]);
563    /// assert_eq!((0, 'f'), zipper[0]);
564    ///
565    /// assert_eq!((1, 'o'), enumerate[1]);
566    /// assert_eq!((1, 'o'), zipper[1]);
567    ///
568    /// assert_eq!((2, 'o'), enumerate[2]);
569    /// assert_eq!((2, 'o'), zipper[2]);
570    /// ```
571    ///
572    /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
573    ///
574    /// ```
575    /// use std::iter::zip;
576    ///
577    /// let a = [1, 2, 3];
578    /// let b = [2, 3, 4];
579    ///
580    /// let mut zipped = zip(
581    ///     a.into_iter().map(|x| x * 2).skip(1),
582    ///     b.into_iter().map(|x| x * 2).skip(1),
583    /// );
584    ///
585    /// assert_eq!(zipped.next(), Some((4, 6)));
586    /// assert_eq!(zipped.next(), Some((6, 8)));
587    /// assert_eq!(zipped.next(), None);
588    /// ```
589    ///
590    /// compared to:
591    ///
592    /// ```
593    /// # let a = [1, 2, 3];
594    /// # let b = [2, 3, 4];
595    /// #
596    /// let mut zipped = a
597    ///     .into_iter()
598    ///     .map(|x| x * 2)
599    ///     .skip(1)
600    ///     .zip(b.into_iter().map(|x| x * 2).skip(1));
601    /// #
602    /// # assert_eq!(zipped.next(), Some((4, 6)));
603    /// # assert_eq!(zipped.next(), Some((6, 8)));
604    /// # assert_eq!(zipped.next(), None);
605    /// ```
606    ///
607    /// [`enumerate`]: Iterator::enumerate
608    /// [`next`]: Iterator::next
609    /// [`zip`]: crate::iter::zip
610    #[inline]
611    #[stable(feature = "rust1", since = "1.0.0")]
612    fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
613    where
614        Self: Sized,
615        U: IntoIterator,
616    {
617        Zip::new(self, other.into_iter())
618    }
619
620    /// Creates a new iterator which places a copy of `separator` between adjacent
621    /// items of the original iterator.
622    ///
623    /// In case `separator` does not implement [`Clone`] or needs to be
624    /// computed every time, use [`intersperse_with`].
625    ///
626    /// # Examples
627    ///
628    /// Basic usage:
629    ///
630    /// ```
631    /// #![feature(iter_intersperse)]
632    ///
633    /// let mut a = [0, 1, 2].into_iter().intersperse(100);
634    /// assert_eq!(a.next(), Some(0));   // The first element from `a`.
635    /// assert_eq!(a.next(), Some(100)); // The separator.
636    /// assert_eq!(a.next(), Some(1));   // The next element from `a`.
637    /// assert_eq!(a.next(), Some(100)); // The separator.
638    /// assert_eq!(a.next(), Some(2));   // The last element from `a`.
639    /// assert_eq!(a.next(), None);       // The iterator is finished.
640    /// ```
641    ///
642    /// `intersperse` can be very useful to join an iterator's items using a common element:
643    /// ```
644    /// #![feature(iter_intersperse)]
645    ///
646    /// let words = ["Hello", "World", "!"];
647    /// let hello: String = words.into_iter().intersperse(" ").collect();
648    /// assert_eq!(hello, "Hello World !");
649    /// ```
650    ///
651    /// [`Clone`]: crate::clone::Clone
652    /// [`intersperse_with`]: Iterator::intersperse_with
653    #[inline]
654    #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
655    fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
656    where
657        Self: Sized,
658        Self::Item: Clone,
659    {
660        Intersperse::new(self, separator)
661    }
662
663    /// Creates a new iterator which places an item generated by `separator`
664    /// between adjacent items of the original iterator.
665    ///
666    /// The closure will be called exactly once each time an item is placed
667    /// between two adjacent items from the underlying iterator; specifically,
668    /// the closure is not called if the underlying iterator yields less than
669    /// two items and after the last item is yielded.
670    ///
671    /// If the iterator's item implements [`Clone`], it may be easier to use
672    /// [`intersperse`].
673    ///
674    /// # Examples
675    ///
676    /// Basic usage:
677    ///
678    /// ```
679    /// #![feature(iter_intersperse)]
680    ///
681    /// #[derive(PartialEq, Debug)]
682    /// struct NotClone(usize);
683    ///
684    /// let v = [NotClone(0), NotClone(1), NotClone(2)];
685    /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
686    ///
687    /// assert_eq!(it.next(), Some(NotClone(0)));  // The first element from `v`.
688    /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
689    /// assert_eq!(it.next(), Some(NotClone(1)));  // The next element from `v`.
690    /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
691    /// assert_eq!(it.next(), Some(NotClone(2)));  // The last element from `v`.
692    /// assert_eq!(it.next(), None);               // The iterator is finished.
693    /// ```
694    ///
695    /// `intersperse_with` can be used in situations where the separator needs
696    /// to be computed:
697    /// ```
698    /// #![feature(iter_intersperse)]
699    ///
700    /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
701    ///
702    /// // The closure mutably borrows its context to generate an item.
703    /// let mut happy_emojis = [" ❤️ ", " 😀 "].into_iter();
704    /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
705    ///
706    /// let result = src.intersperse_with(separator).collect::<String>();
707    /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
708    /// ```
709    /// [`Clone`]: crate::clone::Clone
710    /// [`intersperse`]: Iterator::intersperse
711    #[inline]
712    #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
713    fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
714    where
715        Self: Sized,
716        G: FnMut() -> Self::Item,
717    {
718        IntersperseWith::new(self, separator)
719    }
720
721    /// Takes a closure and creates an iterator which calls that closure on each
722    /// element.
723    ///
724    /// `map()` transforms one iterator into another, by means of its argument:
725    /// something that implements [`FnMut`]. It produces a new iterator which
726    /// calls this closure on each element of the original iterator.
727    ///
728    /// If you are good at thinking in types, you can think of `map()` like this:
729    /// If you have an iterator that gives you elements of some type `A`, and
730    /// you want an iterator of some other type `B`, you can use `map()`,
731    /// passing a closure that takes an `A` and returns a `B`.
732    ///
733    /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
734    /// lazy, it is best used when you're already working with other iterators.
735    /// If you're doing some sort of looping for a side effect, it's considered
736    /// more idiomatic to use [`for`] than `map()`.
737    ///
738    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
739    ///
740    /// # Examples
741    ///
742    /// Basic usage:
743    ///
744    /// ```
745    /// let a = [1, 2, 3];
746    ///
747    /// let mut iter = a.iter().map(|x| 2 * x);
748    ///
749    /// assert_eq!(iter.next(), Some(2));
750    /// assert_eq!(iter.next(), Some(4));
751    /// assert_eq!(iter.next(), Some(6));
752    /// assert_eq!(iter.next(), None);
753    /// ```
754    ///
755    /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
756    ///
757    /// ```
758    /// # #![allow(unused_must_use)]
759    /// // don't do this:
760    /// (0..5).map(|x| println!("{x}"));
761    ///
762    /// // it won't even execute, as it is lazy. Rust will warn you about this.
763    ///
764    /// // Instead, use a for-loop:
765    /// for x in 0..5 {
766    ///     println!("{x}");
767    /// }
768    /// ```
769    #[rustc_diagnostic_item = "IteratorMap"]
770    #[inline]
771    #[stable(feature = "rust1", since = "1.0.0")]
772    fn map<B, F>(self, f: F) -> Map<Self, F>
773    where
774        Self: Sized,
775        F: FnMut(Self::Item) -> B,
776    {
777        Map::new(self, f)
778    }
779
780    /// Calls a closure on each element of an iterator.
781    ///
782    /// This is equivalent to using a [`for`] loop on the iterator, although
783    /// `break` and `continue` are not possible from a closure. It's generally
784    /// more idiomatic to use a `for` loop, but `for_each` may be more legible
785    /// when processing items at the end of longer iterator chains. In some
786    /// cases `for_each` may also be faster than a loop, because it will use
787    /// internal iteration on adapters like `Chain`.
788    ///
789    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
790    ///
791    /// # Examples
792    ///
793    /// Basic usage:
794    ///
795    /// ```
796    /// use std::sync::mpsc::channel;
797    ///
798    /// let (tx, rx) = channel();
799    /// (0..5).map(|x| x * 2 + 1)
800    ///       .for_each(move |x| tx.send(x).unwrap());
801    ///
802    /// let v: Vec<_> = rx.iter().collect();
803    /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
804    /// ```
805    ///
806    /// For such a small example, a `for` loop may be cleaner, but `for_each`
807    /// might be preferable to keep a functional style with longer iterators:
808    ///
809    /// ```
810    /// (0..5).flat_map(|x| x * 100 .. x * 110)
811    ///       .enumerate()
812    ///       .filter(|&(i, x)| (i + x) % 3 == 0)
813    ///       .for_each(|(i, x)| println!("{i}:{x}"));
814    /// ```
815    #[inline]
816    #[stable(feature = "iterator_for_each", since = "1.21.0")]
817    fn for_each<F>(self, f: F)
818    where
819        Self: Sized,
820        F: FnMut(Self::Item),
821    {
822        #[inline]
823        fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
824            move |(), item| f(item)
825        }
826
827        self.fold((), call(f));
828    }
829
830    /// Creates an iterator which uses a closure to determine if an element
831    /// should be yielded.
832    ///
833    /// Given an element the closure must return `true` or `false`. The returned
834    /// iterator will yield only the elements for which the closure returns
835    /// `true`.
836    ///
837    /// # Examples
838    ///
839    /// Basic usage:
840    ///
841    /// ```
842    /// let a = [0i32, 1, 2];
843    ///
844    /// let mut iter = a.into_iter().filter(|x| x.is_positive());
845    ///
846    /// assert_eq!(iter.next(), Some(1));
847    /// assert_eq!(iter.next(), Some(2));
848    /// assert_eq!(iter.next(), None);
849    /// ```
850    ///
851    /// Because the closure passed to `filter()` takes a reference, and many
852    /// iterators iterate over references, this leads to a possibly confusing
853    /// situation, where the type of the closure is a double reference:
854    ///
855    /// ```
856    /// let s = &[0, 1, 2];
857    ///
858    /// let mut iter = s.iter().filter(|x| **x > 1); // needs two *s!
859    ///
860    /// assert_eq!(iter.next(), Some(&2));
861    /// assert_eq!(iter.next(), None);
862    /// ```
863    ///
864    /// It's common to instead use destructuring on the argument to strip away one:
865    ///
866    /// ```
867    /// let s = &[0, 1, 2];
868    ///
869    /// let mut iter = s.iter().filter(|&x| *x > 1); // both & and *
870    ///
871    /// assert_eq!(iter.next(), Some(&2));
872    /// assert_eq!(iter.next(), None);
873    /// ```
874    ///
875    /// or both:
876    ///
877    /// ```
878    /// let s = &[0, 1, 2];
879    ///
880    /// let mut iter = s.iter().filter(|&&x| x > 1); // two &s
881    ///
882    /// assert_eq!(iter.next(), Some(&2));
883    /// assert_eq!(iter.next(), None);
884    /// ```
885    ///
886    /// of these layers.
887    ///
888    /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
889    #[inline]
890    #[stable(feature = "rust1", since = "1.0.0")]
891    #[rustc_diagnostic_item = "iter_filter"]
892    fn filter<P>(self, predicate: P) -> Filter<Self, P>
893    where
894        Self: Sized,
895        P: FnMut(&Self::Item) -> bool,
896    {
897        Filter::new(self, predicate)
898    }
899
900    /// Creates an iterator that both filters and maps.
901    ///
902    /// The returned iterator yields only the `value`s for which the supplied
903    /// closure returns `Some(value)`.
904    ///
905    /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
906    /// concise. The example below shows how a `map().filter().map()` can be
907    /// shortened to a single call to `filter_map`.
908    ///
909    /// [`filter`]: Iterator::filter
910    /// [`map`]: Iterator::map
911    ///
912    /// # Examples
913    ///
914    /// Basic usage:
915    ///
916    /// ```
917    /// let a = ["1", "two", "NaN", "four", "5"];
918    ///
919    /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
920    ///
921    /// assert_eq!(iter.next(), Some(1));
922    /// assert_eq!(iter.next(), Some(5));
923    /// assert_eq!(iter.next(), None);
924    /// ```
925    ///
926    /// Here's the same example, but with [`filter`] and [`map`]:
927    ///
928    /// ```
929    /// let a = ["1", "two", "NaN", "four", "5"];
930    /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
931    /// assert_eq!(iter.next(), Some(1));
932    /// assert_eq!(iter.next(), Some(5));
933    /// assert_eq!(iter.next(), None);
934    /// ```
935    #[inline]
936    #[stable(feature = "rust1", since = "1.0.0")]
937    fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
938    where
939        Self: Sized,
940        F: FnMut(Self::Item) -> Option<B>,
941    {
942        FilterMap::new(self, f)
943    }
944
945    /// Creates an iterator which gives the current iteration count as well as
946    /// the next value.
947    ///
948    /// The iterator returned yields pairs `(i, val)`, where `i` is the
949    /// current index of iteration and `val` is the value returned by the
950    /// iterator.
951    ///
952    /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
953    /// different sized integer, the [`zip`] function provides similar
954    /// functionality.
955    ///
956    /// # Overflow Behavior
957    ///
958    /// The method does no guarding against overflows, so enumerating more than
959    /// [`usize::MAX`] elements either produces the wrong result or panics. If
960    /// overflow checks are enabled, a panic is guaranteed.
961    ///
962    /// # Panics
963    ///
964    /// The returned iterator might panic if the to-be-returned index would
965    /// overflow a [`usize`].
966    ///
967    /// [`zip`]: Iterator::zip
968    ///
969    /// # Examples
970    ///
971    /// ```
972    /// let a = ['a', 'b', 'c'];
973    ///
974    /// let mut iter = a.into_iter().enumerate();
975    ///
976    /// assert_eq!(iter.next(), Some((0, 'a')));
977    /// assert_eq!(iter.next(), Some((1, 'b')));
978    /// assert_eq!(iter.next(), Some((2, 'c')));
979    /// assert_eq!(iter.next(), None);
980    /// ```
981    #[inline]
982    #[stable(feature = "rust1", since = "1.0.0")]
983    #[rustc_diagnostic_item = "enumerate_method"]
984    fn enumerate(self) -> Enumerate<Self>
985    where
986        Self: Sized,
987    {
988        Enumerate::new(self)
989    }
990
991    /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
992    /// to look at the next element of the iterator without consuming it. See
993    /// their documentation for more information.
994    ///
995    /// Note that the underlying iterator is still advanced when [`peek`] or
996    /// [`peek_mut`] are called for the first time: In order to retrieve the
997    /// next element, [`next`] is called on the underlying iterator, hence any
998    /// side effects (i.e. anything other than fetching the next value) of
999    /// the [`next`] method will occur.
1000    ///
1001    ///
1002    /// # Examples
1003    ///
1004    /// Basic usage:
1005    ///
1006    /// ```
1007    /// let xs = [1, 2, 3];
1008    ///
1009    /// let mut iter = xs.into_iter().peekable();
1010    ///
1011    /// // peek() lets us see into the future
1012    /// assert_eq!(iter.peek(), Some(&1));
1013    /// assert_eq!(iter.next(), Some(1));
1014    ///
1015    /// assert_eq!(iter.next(), Some(2));
1016    ///
1017    /// // we can peek() multiple times, the iterator won't advance
1018    /// assert_eq!(iter.peek(), Some(&3));
1019    /// assert_eq!(iter.peek(), Some(&3));
1020    ///
1021    /// assert_eq!(iter.next(), Some(3));
1022    ///
1023    /// // after the iterator is finished, so is peek()
1024    /// assert_eq!(iter.peek(), None);
1025    /// assert_eq!(iter.next(), None);
1026    /// ```
1027    ///
1028    /// Using [`peek_mut`] to mutate the next item without advancing the
1029    /// iterator:
1030    ///
1031    /// ```
1032    /// let xs = [1, 2, 3];
1033    ///
1034    /// let mut iter = xs.into_iter().peekable();
1035    ///
1036    /// // `peek_mut()` lets us see into the future
1037    /// assert_eq!(iter.peek_mut(), Some(&mut 1));
1038    /// assert_eq!(iter.peek_mut(), Some(&mut 1));
1039    /// assert_eq!(iter.next(), Some(1));
1040    ///
1041    /// if let Some(p) = iter.peek_mut() {
1042    ///     assert_eq!(*p, 2);
1043    ///     // put a value into the iterator
1044    ///     *p = 1000;
1045    /// }
1046    ///
1047    /// // The value reappears as the iterator continues
1048    /// assert_eq!(iter.collect::<Vec<_>>(), vec![1000, 3]);
1049    /// ```
1050    /// [`peek`]: Peekable::peek
1051    /// [`peek_mut`]: Peekable::peek_mut
1052    /// [`next`]: Iterator::next
1053    #[inline]
1054    #[stable(feature = "rust1", since = "1.0.0")]
1055    fn peekable(self) -> Peekable<Self>
1056    where
1057        Self: Sized,
1058    {
1059        Peekable::new(self)
1060    }
1061
1062    /// Creates an iterator that [`skip`]s elements based on a predicate.
1063    ///
1064    /// [`skip`]: Iterator::skip
1065    ///
1066    /// `skip_while()` takes a closure as an argument. It will call this
1067    /// closure on each element of the iterator, and ignore elements
1068    /// until it returns `false`.
1069    ///
1070    /// After `false` is returned, `skip_while()`'s job is over, and the
1071    /// rest of the elements are yielded.
1072    ///
1073    /// # Examples
1074    ///
1075    /// Basic usage:
1076    ///
1077    /// ```
1078    /// let a = [-1i32, 0, 1];
1079    ///
1080    /// let mut iter = a.into_iter().skip_while(|x| x.is_negative());
1081    ///
1082    /// assert_eq!(iter.next(), Some(0));
1083    /// assert_eq!(iter.next(), Some(1));
1084    /// assert_eq!(iter.next(), None);
1085    /// ```
1086    ///
1087    /// Because the closure passed to `skip_while()` takes a reference, and many
1088    /// iterators iterate over references, this leads to a possibly confusing
1089    /// situation, where the type of the closure argument is a double reference:
1090    ///
1091    /// ```
1092    /// let s = &[-1, 0, 1];
1093    ///
1094    /// let mut iter = s.iter().skip_while(|x| **x < 0); // need two *s!
1095    ///
1096    /// assert_eq!(iter.next(), Some(&0));
1097    /// assert_eq!(iter.next(), Some(&1));
1098    /// assert_eq!(iter.next(), None);
1099    /// ```
1100    ///
1101    /// Stopping after an initial `false`:
1102    ///
1103    /// ```
1104    /// let a = [-1, 0, 1, -2];
1105    ///
1106    /// let mut iter = a.into_iter().skip_while(|&x| x < 0);
1107    ///
1108    /// assert_eq!(iter.next(), Some(0));
1109    /// assert_eq!(iter.next(), Some(1));
1110    ///
1111    /// // while this would have been false, since we already got a false,
1112    /// // skip_while() isn't used any more
1113    /// assert_eq!(iter.next(), Some(-2));
1114    ///
1115    /// assert_eq!(iter.next(), None);
1116    /// ```
1117    #[inline]
1118    #[doc(alias = "drop_while")]
1119    #[stable(feature = "rust1", since = "1.0.0")]
1120    fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1121    where
1122        Self: Sized,
1123        P: FnMut(&Self::Item) -> bool,
1124    {
1125        SkipWhile::new(self, predicate)
1126    }
1127
1128    /// Creates an iterator that yields elements based on a predicate.
1129    ///
1130    /// `take_while()` takes a closure as an argument. It will call this
1131    /// closure on each element of the iterator, and yield elements
1132    /// while it returns `true`.
1133    ///
1134    /// After `false` is returned, `take_while()`'s job is over, and the
1135    /// rest of the elements are ignored.
1136    ///
1137    /// # Examples
1138    ///
1139    /// Basic usage:
1140    ///
1141    /// ```
1142    /// let a = [-1i32, 0, 1];
1143    ///
1144    /// let mut iter = a.into_iter().take_while(|x| x.is_negative());
1145    ///
1146    /// assert_eq!(iter.next(), Some(-1));
1147    /// assert_eq!(iter.next(), None);
1148    /// ```
1149    ///
1150    /// Because the closure passed to `take_while()` takes a reference, and many
1151    /// iterators iterate over references, this leads to a possibly confusing
1152    /// situation, where the type of the closure is a double reference:
1153    ///
1154    /// ```
1155    /// let s = &[-1, 0, 1];
1156    ///
1157    /// let mut iter = s.iter().take_while(|x| **x < 0); // need two *s!
1158    ///
1159    /// assert_eq!(iter.next(), Some(&-1));
1160    /// assert_eq!(iter.next(), None);
1161    /// ```
1162    ///
1163    /// Stopping after an initial `false`:
1164    ///
1165    /// ```
1166    /// let a = [-1, 0, 1, -2];
1167    ///
1168    /// let mut iter = a.into_iter().take_while(|&x| x < 0);
1169    ///
1170    /// assert_eq!(iter.next(), Some(-1));
1171    ///
1172    /// // We have more elements that are less than zero, but since we already
1173    /// // got a false, take_while() ignores the remaining elements.
1174    /// assert_eq!(iter.next(), None);
1175    /// ```
1176    ///
1177    /// Because `take_while()` needs to look at the value in order to see if it
1178    /// should be included or not, consuming iterators will see that it is
1179    /// removed:
1180    ///
1181    /// ```
1182    /// let a = [1, 2, 3, 4];
1183    /// let mut iter = a.into_iter();
1184    ///
1185    /// let result: Vec<i32> = iter.by_ref().take_while(|&n| n != 3).collect();
1186    ///
1187    /// assert_eq!(result, [1, 2]);
1188    ///
1189    /// let result: Vec<i32> = iter.collect();
1190    ///
1191    /// assert_eq!(result, [4]);
1192    /// ```
1193    ///
1194    /// The `3` is no longer there, because it was consumed in order to see if
1195    /// the iteration should stop, but wasn't placed back into the iterator.
1196    #[inline]
1197    #[stable(feature = "rust1", since = "1.0.0")]
1198    fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1199    where
1200        Self: Sized,
1201        P: FnMut(&Self::Item) -> bool,
1202    {
1203        TakeWhile::new(self, predicate)
1204    }
1205
1206    /// Creates an iterator that both yields elements based on a predicate and maps.
1207    ///
1208    /// `map_while()` takes a closure as an argument. It will call this
1209    /// closure on each element of the iterator, and yield elements
1210    /// while it returns [`Some(_)`][`Some`].
1211    ///
1212    /// # Examples
1213    ///
1214    /// Basic usage:
1215    ///
1216    /// ```
1217    /// let a = [-1i32, 4, 0, 1];
1218    ///
1219    /// let mut iter = a.into_iter().map_while(|x| 16i32.checked_div(x));
1220    ///
1221    /// assert_eq!(iter.next(), Some(-16));
1222    /// assert_eq!(iter.next(), Some(4));
1223    /// assert_eq!(iter.next(), None);
1224    /// ```
1225    ///
1226    /// Here's the same example, but with [`take_while`] and [`map`]:
1227    ///
1228    /// [`take_while`]: Iterator::take_while
1229    /// [`map`]: Iterator::map
1230    ///
1231    /// ```
1232    /// let a = [-1i32, 4, 0, 1];
1233    ///
1234    /// let mut iter = a.into_iter()
1235    ///                 .map(|x| 16i32.checked_div(x))
1236    ///                 .take_while(|x| x.is_some())
1237    ///                 .map(|x| x.unwrap());
1238    ///
1239    /// assert_eq!(iter.next(), Some(-16));
1240    /// assert_eq!(iter.next(), Some(4));
1241    /// assert_eq!(iter.next(), None);
1242    /// ```
1243    ///
1244    /// Stopping after an initial [`None`]:
1245    ///
1246    /// ```
1247    /// let a = [0, 1, 2, -3, 4, 5, -6];
1248    ///
1249    /// let iter = a.into_iter().map_while(|x| u32::try_from(x).ok());
1250    /// let vec: Vec<_> = iter.collect();
1251    ///
1252    /// // We have more elements that could fit in u32 (such as 4, 5), but `map_while` returned `None` for `-3`
1253    /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1254    /// assert_eq!(vec, [0, 1, 2]);
1255    /// ```
1256    ///
1257    /// Because `map_while()` needs to look at the value in order to see if it
1258    /// should be included or not, consuming iterators will see that it is
1259    /// removed:
1260    ///
1261    /// ```
1262    /// let a = [1, 2, -3, 4];
1263    /// let mut iter = a.into_iter();
1264    ///
1265    /// let result: Vec<u32> = iter.by_ref()
1266    ///                            .map_while(|n| u32::try_from(n).ok())
1267    ///                            .collect();
1268    ///
1269    /// assert_eq!(result, [1, 2]);
1270    ///
1271    /// let result: Vec<i32> = iter.collect();
1272    ///
1273    /// assert_eq!(result, [4]);
1274    /// ```
1275    ///
1276    /// The `-3` is no longer there, because it was consumed in order to see if
1277    /// the iteration should stop, but wasn't placed back into the iterator.
1278    ///
1279    /// Note that unlike [`take_while`] this iterator is **not** fused.
1280    /// It is also not specified what this iterator returns after the first [`None`] is returned.
1281    /// If you need a fused iterator, use [`fuse`].
1282    ///
1283    /// [`fuse`]: Iterator::fuse
1284    #[inline]
1285    #[stable(feature = "iter_map_while", since = "1.57.0")]
1286    fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1287    where
1288        Self: Sized,
1289        P: FnMut(Self::Item) -> Option<B>,
1290    {
1291        MapWhile::new(self, predicate)
1292    }
1293
1294    /// Creates an iterator that skips the first `n` elements.
1295    ///
1296    /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1297    /// iterator is reached (whichever happens first). After that, all the remaining
1298    /// elements are yielded. In particular, if the original iterator is too short,
1299    /// then the returned iterator is empty.
1300    ///
1301    /// Rather than overriding this method directly, instead override the `nth` method.
1302    ///
1303    /// # Examples
1304    ///
1305    /// ```
1306    /// let a = [1, 2, 3];
1307    ///
1308    /// let mut iter = a.into_iter().skip(2);
1309    ///
1310    /// assert_eq!(iter.next(), Some(3));
1311    /// assert_eq!(iter.next(), None);
1312    /// ```
1313    #[inline]
1314    #[stable(feature = "rust1", since = "1.0.0")]
1315    fn skip(self, n: usize) -> Skip<Self>
1316    where
1317        Self: Sized,
1318    {
1319        Skip::new(self, n)
1320    }
1321
1322    /// Creates an iterator that yields the first `n` elements, or fewer
1323    /// if the underlying iterator ends sooner.
1324    ///
1325    /// `take(n)` yields elements until `n` elements are yielded or the end of
1326    /// the iterator is reached (whichever happens first).
1327    /// The returned iterator is a prefix of length `n` if the original iterator
1328    /// contains at least `n` elements, otherwise it contains all of the
1329    /// (fewer than `n`) elements of the original iterator.
1330    ///
1331    /// # Examples
1332    ///
1333    /// Basic usage:
1334    ///
1335    /// ```
1336    /// let a = [1, 2, 3];
1337    ///
1338    /// let mut iter = a.into_iter().take(2);
1339    ///
1340    /// assert_eq!(iter.next(), Some(1));
1341    /// assert_eq!(iter.next(), Some(2));
1342    /// assert_eq!(iter.next(), None);
1343    /// ```
1344    ///
1345    /// `take()` is often used with an infinite iterator, to make it finite:
1346    ///
1347    /// ```
1348    /// let mut iter = (0..).take(3);
1349    ///
1350    /// assert_eq!(iter.next(), Some(0));
1351    /// assert_eq!(iter.next(), Some(1));
1352    /// assert_eq!(iter.next(), Some(2));
1353    /// assert_eq!(iter.next(), None);
1354    /// ```
1355    ///
1356    /// If less than `n` elements are available,
1357    /// `take` will limit itself to the size of the underlying iterator:
1358    ///
1359    /// ```
1360    /// let v = [1, 2];
1361    /// let mut iter = v.into_iter().take(5);
1362    /// assert_eq!(iter.next(), Some(1));
1363    /// assert_eq!(iter.next(), Some(2));
1364    /// assert_eq!(iter.next(), None);
1365    /// ```
1366    ///
1367    /// Use [`by_ref`] to take from the iterator without consuming it, and then
1368    /// continue using the original iterator:
1369    ///
1370    /// ```
1371    /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
1372    ///
1373    /// // Take the first two words.
1374    /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1375    /// assert_eq!(hello_world, vec!["hello", "world"]);
1376    ///
1377    /// // Collect the rest of the words.
1378    /// // We can only do this because we used `by_ref` earlier.
1379    /// let of_rust: Vec<_> = words.collect();
1380    /// assert_eq!(of_rust, vec!["of", "Rust"]);
1381    /// ```
1382    ///
1383    /// [`by_ref`]: Iterator::by_ref
1384    #[doc(alias = "limit")]
1385    #[inline]
1386    #[stable(feature = "rust1", since = "1.0.0")]
1387    fn take(self, n: usize) -> Take<Self>
1388    where
1389        Self: Sized,
1390    {
1391        Take::new(self, n)
1392    }
1393
1394    /// An iterator adapter which, like [`fold`], holds internal state, but
1395    /// unlike [`fold`], produces a new iterator.
1396    ///
1397    /// [`fold`]: Iterator::fold
1398    ///
1399    /// `scan()` takes two arguments: an initial value which seeds the internal
1400    /// state, and a closure with two arguments, the first being a mutable
1401    /// reference to the internal state and the second an iterator element.
1402    /// The closure can assign to the internal state to share state between
1403    /// iterations.
1404    ///
1405    /// On iteration, the closure will be applied to each element of the
1406    /// iterator and the return value from the closure, an [`Option`], is
1407    /// returned by the `next` method. Thus the closure can return
1408    /// `Some(value)` to yield `value`, or `None` to end the iteration.
1409    ///
1410    /// # Examples
1411    ///
1412    /// ```
1413    /// let a = [1, 2, 3, 4];
1414    ///
1415    /// let mut iter = a.into_iter().scan(1, |state, x| {
1416    ///     // each iteration, we'll multiply the state by the element ...
1417    ///     *state = *state * x;
1418    ///
1419    ///     // ... and terminate if the state exceeds 6
1420    ///     if *state > 6 {
1421    ///         return None;
1422    ///     }
1423    ///     // ... else yield the negation of the state
1424    ///     Some(-*state)
1425    /// });
1426    ///
1427    /// assert_eq!(iter.next(), Some(-1));
1428    /// assert_eq!(iter.next(), Some(-2));
1429    /// assert_eq!(iter.next(), Some(-6));
1430    /// assert_eq!(iter.next(), None);
1431    /// ```
1432    #[inline]
1433    #[stable(feature = "rust1", since = "1.0.0")]
1434    fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1435    where
1436        Self: Sized,
1437        F: FnMut(&mut St, Self::Item) -> Option<B>,
1438    {
1439        Scan::new(self, initial_state, f)
1440    }
1441
1442    /// Creates an iterator that works like map, but flattens nested structure.
1443    ///
1444    /// The [`map`] adapter is very useful, but only when the closure
1445    /// argument produces values. If it produces an iterator instead, there's
1446    /// an extra layer of indirection. `flat_map()` will remove this extra layer
1447    /// on its own.
1448    ///
1449    /// You can think of `flat_map(f)` as the semantic equivalent
1450    /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1451    ///
1452    /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1453    /// one item for each element, and `flat_map()`'s closure returns an
1454    /// iterator for each element.
1455    ///
1456    /// [`map`]: Iterator::map
1457    /// [`flatten`]: Iterator::flatten
1458    ///
1459    /// # Examples
1460    ///
1461    /// ```
1462    /// let words = ["alpha", "beta", "gamma"];
1463    ///
1464    /// // chars() returns an iterator
1465    /// let merged: String = words.iter()
1466    ///                           .flat_map(|s| s.chars())
1467    ///                           .collect();
1468    /// assert_eq!(merged, "alphabetagamma");
1469    /// ```
1470    #[inline]
1471    #[stable(feature = "rust1", since = "1.0.0")]
1472    fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1473    where
1474        Self: Sized,
1475        U: IntoIterator,
1476        F: FnMut(Self::Item) -> U,
1477    {
1478        FlatMap::new(self, f)
1479    }
1480
1481    /// Creates an iterator that flattens nested structure.
1482    ///
1483    /// This is useful when you have an iterator of iterators or an iterator of
1484    /// things that can be turned into iterators and you want to remove one
1485    /// level of indirection.
1486    ///
1487    /// # Examples
1488    ///
1489    /// Basic usage:
1490    ///
1491    /// ```
1492    /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1493    /// let flattened: Vec<_> = data.into_iter().flatten().collect();
1494    /// assert_eq!(flattened, [1, 2, 3, 4, 5, 6]);
1495    /// ```
1496    ///
1497    /// Mapping and then flattening:
1498    ///
1499    /// ```
1500    /// let words = ["alpha", "beta", "gamma"];
1501    ///
1502    /// // chars() returns an iterator
1503    /// let merged: String = words.iter()
1504    ///                           .map(|s| s.chars())
1505    ///                           .flatten()
1506    ///                           .collect();
1507    /// assert_eq!(merged, "alphabetagamma");
1508    /// ```
1509    ///
1510    /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1511    /// in this case since it conveys intent more clearly:
1512    ///
1513    /// ```
1514    /// let words = ["alpha", "beta", "gamma"];
1515    ///
1516    /// // chars() returns an iterator
1517    /// let merged: String = words.iter()
1518    ///                           .flat_map(|s| s.chars())
1519    ///                           .collect();
1520    /// assert_eq!(merged, "alphabetagamma");
1521    /// ```
1522    ///
1523    /// Flattening works on any `IntoIterator` type, including `Option` and `Result`:
1524    ///
1525    /// ```
1526    /// let options = vec![Some(123), Some(321), None, Some(231)];
1527    /// let flattened_options: Vec<_> = options.into_iter().flatten().collect();
1528    /// assert_eq!(flattened_options, [123, 321, 231]);
1529    ///
1530    /// let results = vec![Ok(123), Ok(321), Err(456), Ok(231)];
1531    /// let flattened_results: Vec<_> = results.into_iter().flatten().collect();
1532    /// assert_eq!(flattened_results, [123, 321, 231]);
1533    /// ```
1534    ///
1535    /// Flattening only removes one level of nesting at a time:
1536    ///
1537    /// ```
1538    /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1539    ///
1540    /// let d2: Vec<_> = d3.into_iter().flatten().collect();
1541    /// assert_eq!(d2, [[1, 2], [3, 4], [5, 6], [7, 8]]);
1542    ///
1543    /// let d1: Vec<_> = d3.into_iter().flatten().flatten().collect();
1544    /// assert_eq!(d1, [1, 2, 3, 4, 5, 6, 7, 8]);
1545    /// ```
1546    ///
1547    /// Here we see that `flatten()` does not perform a "deep" flatten.
1548    /// Instead, only one level of nesting is removed. That is, if you
1549    /// `flatten()` a three-dimensional array, the result will be
1550    /// two-dimensional and not one-dimensional. To get a one-dimensional
1551    /// structure, you have to `flatten()` again.
1552    ///
1553    /// [`flat_map()`]: Iterator::flat_map
1554    #[inline]
1555    #[stable(feature = "iterator_flatten", since = "1.29.0")]
1556    fn flatten(self) -> Flatten<Self>
1557    where
1558        Self: Sized,
1559        Self::Item: IntoIterator,
1560    {
1561        Flatten::new(self)
1562    }
1563
1564    /// Calls the given function `f` for each contiguous window of size `N` over
1565    /// `self` and returns an iterator over the outputs of `f`. Like [`slice::windows()`],
1566    /// the windows during mapping overlap as well.
1567    ///
1568    /// In the following example, the closure is called three times with the
1569    /// arguments `&['a', 'b']`, `&['b', 'c']` and `&['c', 'd']` respectively.
1570    ///
1571    /// ```
1572    /// #![feature(iter_map_windows)]
1573    ///
1574    /// let strings = "abcd".chars()
1575    ///     .map_windows(|[x, y]| format!("{}+{}", x, y))
1576    ///     .collect::<Vec<String>>();
1577    ///
1578    /// assert_eq!(strings, vec!["a+b", "b+c", "c+d"]);
1579    /// ```
1580    ///
1581    /// Note that the const parameter `N` is usually inferred by the
1582    /// destructured argument in the closure.
1583    ///
1584    /// The returned iterator yields 𝑘 − `N` + 1 items (where 𝑘 is the number of
1585    /// items yielded by `self`). If 𝑘 is less than `N`, this method yields an
1586    /// empty iterator.
1587    ///
1588    /// The returned iterator implements [`FusedIterator`], because once `self`
1589    /// returns `None`, even if it returns a `Some(T)` again in the next iterations,
1590    /// we cannot put it into a contiguous array buffer, and thus the returned iterator
1591    /// should be fused.
1592    ///
1593    /// [`slice::windows()`]: slice::windows
1594    /// [`FusedIterator`]: crate::iter::FusedIterator
1595    ///
1596    /// # Panics
1597    ///
1598    /// Panics if `N` is zero. This check will most probably get changed to a
1599    /// compile time error before this method gets stabilized.
1600    ///
1601    /// ```should_panic
1602    /// #![feature(iter_map_windows)]
1603    ///
1604    /// let iter = std::iter::repeat(0).map_windows(|&[]| ());
1605    /// ```
1606    ///
1607    /// # Examples
1608    ///
1609    /// Building the sums of neighboring numbers.
1610    ///
1611    /// ```
1612    /// #![feature(iter_map_windows)]
1613    ///
1614    /// let mut it = [1, 3, 8, 1].iter().map_windows(|&[a, b]| a + b);
1615    /// assert_eq!(it.next(), Some(4));  // 1 + 3
1616    /// assert_eq!(it.next(), Some(11)); // 3 + 8
1617    /// assert_eq!(it.next(), Some(9));  // 8 + 1
1618    /// assert_eq!(it.next(), None);
1619    /// ```
1620    ///
1621    /// Since the elements in the following example implement `Copy`, we can
1622    /// just copy the array and get an iterator over the windows.
1623    ///
1624    /// ```
1625    /// #![feature(iter_map_windows)]
1626    ///
1627    /// let mut it = "ferris".chars().map_windows(|w: &[_; 3]| *w);
1628    /// assert_eq!(it.next(), Some(['f', 'e', 'r']));
1629    /// assert_eq!(it.next(), Some(['e', 'r', 'r']));
1630    /// assert_eq!(it.next(), Some(['r', 'r', 'i']));
1631    /// assert_eq!(it.next(), Some(['r', 'i', 's']));
1632    /// assert_eq!(it.next(), None);
1633    /// ```
1634    ///
1635    /// You can also use this function to check the sortedness of an iterator.
1636    /// For the simple case, rather use [`Iterator::is_sorted`].
1637    ///
1638    /// ```
1639    /// #![feature(iter_map_windows)]
1640    ///
1641    /// let mut it = [0.5, 1.0, 3.5, 3.0, 8.5, 8.5, f32::NAN].iter()
1642    ///     .map_windows(|[a, b]| a <= b);
1643    ///
1644    /// assert_eq!(it.next(), Some(true));  // 0.5 <= 1.0
1645    /// assert_eq!(it.next(), Some(true));  // 1.0 <= 3.5
1646    /// assert_eq!(it.next(), Some(false)); // 3.5 <= 3.0
1647    /// assert_eq!(it.next(), Some(true));  // 3.0 <= 8.5
1648    /// assert_eq!(it.next(), Some(true));  // 8.5 <= 8.5
1649    /// assert_eq!(it.next(), Some(false)); // 8.5 <= NAN
1650    /// assert_eq!(it.next(), None);
1651    /// ```
1652    ///
1653    /// For non-fused iterators, they are fused after `map_windows`.
1654    ///
1655    /// ```
1656    /// #![feature(iter_map_windows)]
1657    ///
1658    /// #[derive(Default)]
1659    /// struct NonFusedIterator {
1660    ///     state: i32,
1661    /// }
1662    ///
1663    /// impl Iterator for NonFusedIterator {
1664    ///     type Item = i32;
1665    ///
1666    ///     fn next(&mut self) -> Option<i32> {
1667    ///         let val = self.state;
1668    ///         self.state = self.state + 1;
1669    ///
1670    ///         // yields `0..5` first, then only even numbers since `6..`.
1671    ///         if val < 5 || val % 2 == 0 {
1672    ///             Some(val)
1673    ///         } else {
1674    ///             None
1675    ///         }
1676    ///     }
1677    /// }
1678    ///
1679    ///
1680    /// let mut iter = NonFusedIterator::default();
1681    ///
1682    /// // yields 0..5 first.
1683    /// assert_eq!(iter.next(), Some(0));
1684    /// assert_eq!(iter.next(), Some(1));
1685    /// assert_eq!(iter.next(), Some(2));
1686    /// assert_eq!(iter.next(), Some(3));
1687    /// assert_eq!(iter.next(), Some(4));
1688    /// // then we can see our iterator going back and forth
1689    /// assert_eq!(iter.next(), None);
1690    /// assert_eq!(iter.next(), Some(6));
1691    /// assert_eq!(iter.next(), None);
1692    /// assert_eq!(iter.next(), Some(8));
1693    /// assert_eq!(iter.next(), None);
1694    ///
1695    /// // however, with `.map_windows()`, it is fused.
1696    /// let mut iter = NonFusedIterator::default()
1697    ///     .map_windows(|arr: &[_; 2]| *arr);
1698    ///
1699    /// assert_eq!(iter.next(), Some([0, 1]));
1700    /// assert_eq!(iter.next(), Some([1, 2]));
1701    /// assert_eq!(iter.next(), Some([2, 3]));
1702    /// assert_eq!(iter.next(), Some([3, 4]));
1703    /// assert_eq!(iter.next(), None);
1704    ///
1705    /// // it will always return `None` after the first time.
1706    /// assert_eq!(iter.next(), None);
1707    /// assert_eq!(iter.next(), None);
1708    /// assert_eq!(iter.next(), None);
1709    /// ```
1710    #[inline]
1711    #[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
1712    fn map_windows<F, R, const N: usize>(self, f: F) -> MapWindows<Self, F, N>
1713    where
1714        Self: Sized,
1715        F: FnMut(&[Self::Item; N]) -> R,
1716    {
1717        MapWindows::new(self, f)
1718    }
1719
1720    /// Creates an iterator which ends after the first [`None`].
1721    ///
1722    /// After an iterator returns [`None`], future calls may or may not yield
1723    /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1724    /// [`None`] is given, it will always return [`None`] forever.
1725    ///
1726    /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1727    /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1728    /// if the [`FusedIterator`] trait is improperly implemented.
1729    ///
1730    /// [`Some(T)`]: Some
1731    /// [`FusedIterator`]: crate::iter::FusedIterator
1732    ///
1733    /// # Examples
1734    ///
1735    /// ```
1736    /// // an iterator which alternates between Some and None
1737    /// struct Alternate {
1738    ///     state: i32,
1739    /// }
1740    ///
1741    /// impl Iterator for Alternate {
1742    ///     type Item = i32;
1743    ///
1744    ///     fn next(&mut self) -> Option<i32> {
1745    ///         let val = self.state;
1746    ///         self.state = self.state + 1;
1747    ///
1748    ///         // if it's even, Some(i32), else None
1749    ///         (val % 2 == 0).then_some(val)
1750    ///     }
1751    /// }
1752    ///
1753    /// let mut iter = Alternate { state: 0 };
1754    ///
1755    /// // we can see our iterator going back and forth
1756    /// assert_eq!(iter.next(), Some(0));
1757    /// assert_eq!(iter.next(), None);
1758    /// assert_eq!(iter.next(), Some(2));
1759    /// assert_eq!(iter.next(), None);
1760    ///
1761    /// // however, once we fuse it...
1762    /// let mut iter = iter.fuse();
1763    ///
1764    /// assert_eq!(iter.next(), Some(4));
1765    /// assert_eq!(iter.next(), None);
1766    ///
1767    /// // it will always return `None` after the first time.
1768    /// assert_eq!(iter.next(), None);
1769    /// assert_eq!(iter.next(), None);
1770    /// assert_eq!(iter.next(), None);
1771    /// ```
1772    #[inline]
1773    #[stable(feature = "rust1", since = "1.0.0")]
1774    fn fuse(self) -> Fuse<Self>
1775    where
1776        Self: Sized,
1777    {
1778        Fuse::new(self)
1779    }
1780
1781    /// Does something with each element of an iterator, passing the value on.
1782    ///
1783    /// When using iterators, you'll often chain several of them together.
1784    /// While working on such code, you might want to check out what's
1785    /// happening at various parts in the pipeline. To do that, insert
1786    /// a call to `inspect()`.
1787    ///
1788    /// It's more common for `inspect()` to be used as a debugging tool than to
1789    /// exist in your final code, but applications may find it useful in certain
1790    /// situations when errors need to be logged before being discarded.
1791    ///
1792    /// # Examples
1793    ///
1794    /// Basic usage:
1795    ///
1796    /// ```
1797    /// let a = [1, 4, 2, 3];
1798    ///
1799    /// // this iterator sequence is complex.
1800    /// let sum = a.iter()
1801    ///     .cloned()
1802    ///     .filter(|x| x % 2 == 0)
1803    ///     .fold(0, |sum, i| sum + i);
1804    ///
1805    /// println!("{sum}");
1806    ///
1807    /// // let's add some inspect() calls to investigate what's happening
1808    /// let sum = a.iter()
1809    ///     .cloned()
1810    ///     .inspect(|x| println!("about to filter: {x}"))
1811    ///     .filter(|x| x % 2 == 0)
1812    ///     .inspect(|x| println!("made it through filter: {x}"))
1813    ///     .fold(0, |sum, i| sum + i);
1814    ///
1815    /// println!("{sum}");
1816    /// ```
1817    ///
1818    /// This will print:
1819    ///
1820    /// ```text
1821    /// 6
1822    /// about to filter: 1
1823    /// about to filter: 4
1824    /// made it through filter: 4
1825    /// about to filter: 2
1826    /// made it through filter: 2
1827    /// about to filter: 3
1828    /// 6
1829    /// ```
1830    ///
1831    /// Logging errors before discarding them:
1832    ///
1833    /// ```
1834    /// let lines = ["1", "2", "a"];
1835    ///
1836    /// let sum: i32 = lines
1837    ///     .iter()
1838    ///     .map(|line| line.parse::<i32>())
1839    ///     .inspect(|num| {
1840    ///         if let Err(ref e) = *num {
1841    ///             println!("Parsing error: {e}");
1842    ///         }
1843    ///     })
1844    ///     .filter_map(Result::ok)
1845    ///     .sum();
1846    ///
1847    /// println!("Sum: {sum}");
1848    /// ```
1849    ///
1850    /// This will print:
1851    ///
1852    /// ```text
1853    /// Parsing error: invalid digit found in string
1854    /// Sum: 3
1855    /// ```
1856    #[inline]
1857    #[stable(feature = "rust1", since = "1.0.0")]
1858    fn inspect<F>(self, f: F) -> Inspect<Self, F>
1859    where
1860        Self: Sized,
1861        F: FnMut(&Self::Item),
1862    {
1863        Inspect::new(self, f)
1864    }
1865
1866    /// Creates a "by reference" adapter for this instance of `Iterator`.
1867    ///
1868    /// Consuming method calls (direct or indirect calls to `next`)
1869    /// on the "by reference" adapter will consume the original iterator,
1870    /// but ownership-taking methods (those with a `self` parameter)
1871    /// only take ownership of the "by reference" iterator.
1872    ///
1873    /// This is useful for applying ownership-taking methods
1874    /// (such as `take` in the example below)
1875    /// without giving up ownership of the original iterator,
1876    /// so you can use the original iterator afterwards.
1877    ///
1878    /// Uses [impl<I: Iterator + ?Sized> Iterator for &mut I { type Item = I::Item; ...}](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#impl-Iterator-for-%26mut+I).
1879    ///
1880    /// # Examples
1881    ///
1882    /// ```
1883    /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
1884    ///
1885    /// // Take the first two words.
1886    /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1887    /// assert_eq!(hello_world, vec!["hello", "world"]);
1888    ///
1889    /// // Collect the rest of the words.
1890    /// // We can only do this because we used `by_ref` earlier.
1891    /// let of_rust: Vec<_> = words.collect();
1892    /// assert_eq!(of_rust, vec!["of", "Rust"]);
1893    /// ```
1894    #[stable(feature = "rust1", since = "1.0.0")]
1895    fn by_ref(&mut self) -> &mut Self
1896    where
1897        Self: Sized,
1898    {
1899        self
1900    }
1901
1902    /// Transforms an iterator into a collection.
1903    ///
1904    /// `collect()` can take anything iterable, and turn it into a relevant
1905    /// collection. This is one of the more powerful methods in the standard
1906    /// library, used in a variety of contexts.
1907    ///
1908    /// The most basic pattern in which `collect()` is used is to turn one
1909    /// collection into another. You take a collection, call [`iter`] on it,
1910    /// do a bunch of transformations, and then `collect()` at the end.
1911    ///
1912    /// `collect()` can also create instances of types that are not typical
1913    /// collections. For example, a [`String`] can be built from [`char`]s,
1914    /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1915    /// into `Result<Collection<T>, E>`. See the examples below for more.
1916    ///
1917    /// Because `collect()` is so general, it can cause problems with type
1918    /// inference. As such, `collect()` is one of the few times you'll see
1919    /// the syntax affectionately known as the 'turbofish': `::<>`. This
1920    /// helps the inference algorithm understand specifically which collection
1921    /// you're trying to collect into.
1922    ///
1923    /// # Examples
1924    ///
1925    /// Basic usage:
1926    ///
1927    /// ```
1928    /// let a = [1, 2, 3];
1929    ///
1930    /// let doubled: Vec<i32> = a.iter()
1931    ///                          .map(|x| x * 2)
1932    ///                          .collect();
1933    ///
1934    /// assert_eq!(vec![2, 4, 6], doubled);
1935    /// ```
1936    ///
1937    /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1938    /// we could collect into, for example, a [`VecDeque<T>`] instead:
1939    ///
1940    /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1941    ///
1942    /// ```
1943    /// use std::collections::VecDeque;
1944    ///
1945    /// let a = [1, 2, 3];
1946    ///
1947    /// let doubled: VecDeque<i32> = a.iter().map(|x| x * 2).collect();
1948    ///
1949    /// assert_eq!(2, doubled[0]);
1950    /// assert_eq!(4, doubled[1]);
1951    /// assert_eq!(6, doubled[2]);
1952    /// ```
1953    ///
1954    /// Using the 'turbofish' instead of annotating `doubled`:
1955    ///
1956    /// ```
1957    /// let a = [1, 2, 3];
1958    ///
1959    /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1960    ///
1961    /// assert_eq!(vec![2, 4, 6], doubled);
1962    /// ```
1963    ///
1964    /// Because `collect()` only cares about what you're collecting into, you can
1965    /// still use a partial type hint, `_`, with the turbofish:
1966    ///
1967    /// ```
1968    /// let a = [1, 2, 3];
1969    ///
1970    /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1971    ///
1972    /// assert_eq!(vec![2, 4, 6], doubled);
1973    /// ```
1974    ///
1975    /// Using `collect()` to make a [`String`]:
1976    ///
1977    /// ```
1978    /// let chars = ['g', 'd', 'k', 'k', 'n'];
1979    ///
1980    /// let hello: String = chars.into_iter()
1981    ///     .map(|x| x as u8)
1982    ///     .map(|x| (x + 1) as char)
1983    ///     .collect();
1984    ///
1985    /// assert_eq!("hello", hello);
1986    /// ```
1987    ///
1988    /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1989    /// see if any of them failed:
1990    ///
1991    /// ```
1992    /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1993    ///
1994    /// let result: Result<Vec<_>, &str> = results.into_iter().collect();
1995    ///
1996    /// // gives us the first error
1997    /// assert_eq!(Err("nope"), result);
1998    ///
1999    /// let results = [Ok(1), Ok(3)];
2000    ///
2001    /// let result: Result<Vec<_>, &str> = results.into_iter().collect();
2002    ///
2003    /// // gives us the list of answers
2004    /// assert_eq!(Ok(vec![1, 3]), result);
2005    /// ```
2006    ///
2007    /// [`iter`]: Iterator::next
2008    /// [`String`]: ../../std/string/struct.String.html
2009    /// [`char`]: type@char
2010    #[inline]
2011    #[stable(feature = "rust1", since = "1.0.0")]
2012    #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
2013    #[rustc_diagnostic_item = "iterator_collect_fn"]
2014    fn collect<B: FromIterator<Self::Item>>(self) -> B
2015    where
2016        Self: Sized,
2017    {
2018        // This is too aggressive to turn on for everything all the time, but PR#137908
2019        // accidentally noticed that some rustc iterators had malformed `size_hint`s,
2020        // so this will help catch such things in debug-assertions-std runners,
2021        // even if users won't actually ever see it.
2022        if cfg!(debug_assertions) {
2023            let hint = self.size_hint();
2024            assert!(hint.1.is_none_or(|high| high >= hint.0), "Malformed size_hint {hint:?}");
2025        }
2026
2027        FromIterator::from_iter(self)
2028    }
2029
2030    /// Fallibly transforms an iterator into a collection, short circuiting if
2031    /// a failure is encountered.
2032    ///
2033    /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
2034    /// conversions during collection. Its main use case is simplifying conversions from
2035    /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
2036    /// types (e.g. [`Result`]).
2037    ///
2038    /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
2039    /// only the inner type produced on `Try::Output` must implement it. Concretely,
2040    /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
2041    /// [`FromIterator`], even though [`ControlFlow`] doesn't.
2042    ///
2043    /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
2044    /// may continue to be used, in which case it will continue iterating starting after the element that
2045    /// triggered the failure. See the last example below for an example of how this works.
2046    ///
2047    /// # Examples
2048    /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
2049    /// ```
2050    /// #![feature(iterator_try_collect)]
2051    ///
2052    /// let u = vec![Some(1), Some(2), Some(3)];
2053    /// let v = u.into_iter().try_collect::<Vec<i32>>();
2054    /// assert_eq!(v, Some(vec![1, 2, 3]));
2055    /// ```
2056    ///
2057    /// Failing to collect in the same way:
2058    /// ```
2059    /// #![feature(iterator_try_collect)]
2060    ///
2061    /// let u = vec![Some(1), Some(2), None, Some(3)];
2062    /// let v = u.into_iter().try_collect::<Vec<i32>>();
2063    /// assert_eq!(v, None);
2064    /// ```
2065    ///
2066    /// A similar example, but with `Result`:
2067    /// ```
2068    /// #![feature(iterator_try_collect)]
2069    ///
2070    /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
2071    /// let v = u.into_iter().try_collect::<Vec<i32>>();
2072    /// assert_eq!(v, Ok(vec![1, 2, 3]));
2073    ///
2074    /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
2075    /// let v = u.into_iter().try_collect::<Vec<i32>>();
2076    /// assert_eq!(v, Err(()));
2077    /// ```
2078    ///
2079    /// Finally, even [`ControlFlow`] works, despite the fact that it
2080    /// doesn't implement [`FromIterator`]. Note also that the iterator can
2081    /// continue to be used, even if a failure is encountered:
2082    ///
2083    /// ```
2084    /// #![feature(iterator_try_collect)]
2085    ///
2086    /// use core::ops::ControlFlow::{Break, Continue};
2087    ///
2088    /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
2089    /// let mut it = u.into_iter();
2090    ///
2091    /// let v = it.try_collect::<Vec<_>>();
2092    /// assert_eq!(v, Break(3));
2093    ///
2094    /// let v = it.try_collect::<Vec<_>>();
2095    /// assert_eq!(v, Continue(vec![4, 5]));
2096    /// ```
2097    ///
2098    /// [`collect`]: Iterator::collect
2099    #[inline]
2100    #[unstable(feature = "iterator_try_collect", issue = "94047")]
2101    fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
2102    where
2103        Self: Sized,
2104        Self::Item: Try<Residual: Residual<B>>,
2105        B: FromIterator<<Self::Item as Try>::Output>,
2106    {
2107        try_process(ByRefSized(self), |i| i.collect())
2108    }
2109
2110    /// Collects all the items from an iterator into a collection.
2111    ///
2112    /// This method consumes the iterator and adds all its items to the
2113    /// passed collection. The collection is then returned, so the call chain
2114    /// can be continued.
2115    ///
2116    /// This is useful when you already have a collection and want to add
2117    /// the iterator items to it.
2118    ///
2119    /// This method is a convenience method to call [Extend::extend](trait.Extend.html),
2120    /// but instead of being called on a collection, it's called on an iterator.
2121    ///
2122    /// # Examples
2123    ///
2124    /// Basic usage:
2125    ///
2126    /// ```
2127    /// #![feature(iter_collect_into)]
2128    ///
2129    /// let a = [1, 2, 3];
2130    /// let mut vec: Vec::<i32> = vec![0, 1];
2131    ///
2132    /// a.iter().map(|x| x * 2).collect_into(&mut vec);
2133    /// a.iter().map(|x| x * 10).collect_into(&mut vec);
2134    ///
2135    /// assert_eq!(vec, vec![0, 1, 2, 4, 6, 10, 20, 30]);
2136    /// ```
2137    ///
2138    /// `Vec` can have a manual set capacity to avoid reallocating it:
2139    ///
2140    /// ```
2141    /// #![feature(iter_collect_into)]
2142    ///
2143    /// let a = [1, 2, 3];
2144    /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
2145    ///
2146    /// a.iter().map(|x| x * 2).collect_into(&mut vec);
2147    /// a.iter().map(|x| x * 10).collect_into(&mut vec);
2148    ///
2149    /// assert_eq!(6, vec.capacity());
2150    /// assert_eq!(vec, vec![2, 4, 6, 10, 20, 30]);
2151    /// ```
2152    ///
2153    /// The returned mutable reference can be used to continue the call chain:
2154    ///
2155    /// ```
2156    /// #![feature(iter_collect_into)]
2157    ///
2158    /// let a = [1, 2, 3];
2159    /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
2160    ///
2161    /// let count = a.iter().collect_into(&mut vec).iter().count();
2162    ///
2163    /// assert_eq!(count, vec.len());
2164    /// assert_eq!(vec, vec![1, 2, 3]);
2165    ///
2166    /// let count = a.iter().collect_into(&mut vec).iter().count();
2167    ///
2168    /// assert_eq!(count, vec.len());
2169    /// assert_eq!(vec, vec![1, 2, 3, 1, 2, 3]);
2170    /// ```
2171    #[inline]
2172    #[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")]
2173    fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
2174    where
2175        Self: Sized,
2176    {
2177        collection.extend(self);
2178        collection
2179    }
2180
2181    /// Consumes an iterator, creating two collections from it.
2182    ///
2183    /// The predicate passed to `partition()` can return `true`, or `false`.
2184    /// `partition()` returns a pair, all of the elements for which it returned
2185    /// `true`, and all of the elements for which it returned `false`.
2186    ///
2187    /// See also [`is_partitioned()`] and [`partition_in_place()`].
2188    ///
2189    /// [`is_partitioned()`]: Iterator::is_partitioned
2190    /// [`partition_in_place()`]: Iterator::partition_in_place
2191    ///
2192    /// # Examples
2193    ///
2194    /// ```
2195    /// let a = [1, 2, 3];
2196    ///
2197    /// let (even, odd): (Vec<_>, Vec<_>) = a
2198    ///     .into_iter()
2199    ///     .partition(|n| n % 2 == 0);
2200    ///
2201    /// assert_eq!(even, [2]);
2202    /// assert_eq!(odd, [1, 3]);
2203    /// ```
2204    #[stable(feature = "rust1", since = "1.0.0")]
2205    fn partition<B, F>(self, f: F) -> (B, B)
2206    where
2207        Self: Sized,
2208        B: Default + Extend<Self::Item>,
2209        F: FnMut(&Self::Item) -> bool,
2210    {
2211        #[inline]
2212        fn extend<'a, T, B: Extend<T>>(
2213            mut f: impl FnMut(&T) -> bool + 'a,
2214            left: &'a mut B,
2215            right: &'a mut B,
2216        ) -> impl FnMut((), T) + 'a {
2217            move |(), x| {
2218                if f(&x) {
2219                    left.extend_one(x);
2220                } else {
2221                    right.extend_one(x);
2222                }
2223            }
2224        }
2225
2226        let mut left: B = Default::default();
2227        let mut right: B = Default::default();
2228
2229        self.fold((), extend(f, &mut left, &mut right));
2230
2231        (left, right)
2232    }
2233
2234    /// Reorders the elements of this iterator *in-place* according to the given predicate,
2235    /// such that all those that return `true` precede all those that return `false`.
2236    /// Returns the number of `true` elements found.
2237    ///
2238    /// The relative order of partitioned items is not maintained.
2239    ///
2240    /// # Current implementation
2241    ///
2242    /// The current algorithm tries to find the first element for which the predicate evaluates
2243    /// to false and the last element for which it evaluates to true, and repeatedly swaps them.
2244    ///
2245    /// Time complexity: *O*(*n*)
2246    ///
2247    /// See also [`is_partitioned()`] and [`partition()`].
2248    ///
2249    /// [`is_partitioned()`]: Iterator::is_partitioned
2250    /// [`partition()`]: Iterator::partition
2251    ///
2252    /// # Examples
2253    ///
2254    /// ```
2255    /// #![feature(iter_partition_in_place)]
2256    ///
2257    /// let mut a = [1, 2, 3, 4, 5, 6, 7];
2258    ///
2259    /// // Partition in-place between evens and odds
2260    /// let i = a.iter_mut().partition_in_place(|n| n % 2 == 0);
2261    ///
2262    /// assert_eq!(i, 3);
2263    /// assert!(a[..i].iter().all(|n| n % 2 == 0)); // evens
2264    /// assert!(a[i..].iter().all(|n| n % 2 == 1)); // odds
2265    /// ```
2266    #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
2267    fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
2268    where
2269        Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
2270        P: FnMut(&T) -> bool,
2271    {
2272        // FIXME: should we worry about the count overflowing? The only way to have more than
2273        // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
2274
2275        // These closure "factory" functions exist to avoid genericity in `Self`.
2276
2277        #[inline]
2278        fn is_false<'a, T>(
2279            predicate: &'a mut impl FnMut(&T) -> bool,
2280            true_count: &'a mut usize,
2281        ) -> impl FnMut(&&mut T) -> bool + 'a {
2282            move |x| {
2283                let p = predicate(&**x);
2284                *true_count += p as usize;
2285                !p
2286            }
2287        }
2288
2289        #[inline]
2290        fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
2291            move |x| predicate(&**x)
2292        }
2293
2294        // Repeatedly find the first `false` and swap it with the last `true`.
2295        let mut true_count = 0;
2296        while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
2297            if let Some(tail) = self.rfind(is_true(predicate)) {
2298                crate::mem::swap(head, tail);
2299                true_count += 1;
2300            } else {
2301                break;
2302            }
2303        }
2304        true_count
2305    }
2306
2307    /// Checks if the elements of this iterator are partitioned according to the given predicate,
2308    /// such that all those that return `true` precede all those that return `false`.
2309    ///
2310    /// See also [`partition()`] and [`partition_in_place()`].
2311    ///
2312    /// [`partition()`]: Iterator::partition
2313    /// [`partition_in_place()`]: Iterator::partition_in_place
2314    ///
2315    /// # Examples
2316    ///
2317    /// ```
2318    /// #![feature(iter_is_partitioned)]
2319    ///
2320    /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
2321    /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
2322    /// ```
2323    #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
2324    fn is_partitioned<P>(mut self, mut predicate: P) -> bool
2325    where
2326        Self: Sized,
2327        P: FnMut(Self::Item) -> bool,
2328    {
2329        // Either all items test `true`, or the first clause stops at `false`
2330        // and we check that there are no more `true` items after that.
2331        self.all(&mut predicate) || !self.any(predicate)
2332    }
2333
2334    /// An iterator method that applies a function as long as it returns
2335    /// successfully, producing a single, final value.
2336    ///
2337    /// `try_fold()` takes two arguments: an initial value, and a closure with
2338    /// two arguments: an 'accumulator', and an element. The closure either
2339    /// returns successfully, with the value that the accumulator should have
2340    /// for the next iteration, or it returns failure, with an error value that
2341    /// is propagated back to the caller immediately (short-circuiting).
2342    ///
2343    /// The initial value is the value the accumulator will have on the first
2344    /// call. If applying the closure succeeded against every element of the
2345    /// iterator, `try_fold()` returns the final accumulator as success.
2346    ///
2347    /// Folding is useful whenever you have a collection of something, and want
2348    /// to produce a single value from it.
2349    ///
2350    /// # Note to Implementors
2351    ///
2352    /// Several of the other (forward) methods have default implementations in
2353    /// terms of this one, so try to implement this explicitly if it can
2354    /// do something better than the default `for` loop implementation.
2355    ///
2356    /// In particular, try to have this call `try_fold()` on the internal parts
2357    /// from which this iterator is composed. If multiple calls are needed,
2358    /// the `?` operator may be convenient for chaining the accumulator value
2359    /// along, but beware any invariants that need to be upheld before those
2360    /// early returns. This is a `&mut self` method, so iteration needs to be
2361    /// resumable after hitting an error here.
2362    ///
2363    /// # Examples
2364    ///
2365    /// Basic usage:
2366    ///
2367    /// ```
2368    /// let a = [1, 2, 3];
2369    ///
2370    /// // the checked sum of all of the elements of the array
2371    /// let sum = a.into_iter().try_fold(0i8, |acc, x| acc.checked_add(x));
2372    ///
2373    /// assert_eq!(sum, Some(6));
2374    /// ```
2375    ///
2376    /// Short-circuiting:
2377    ///
2378    /// ```
2379    /// let a = [10, 20, 30, 100, 40, 50];
2380    /// let mut iter = a.into_iter();
2381    ///
2382    /// // This sum overflows when adding the 100 element
2383    /// let sum = iter.try_fold(0i8, |acc, x| acc.checked_add(x));
2384    /// assert_eq!(sum, None);
2385    ///
2386    /// // Because it short-circuited, the remaining elements are still
2387    /// // available through the iterator.
2388    /// assert_eq!(iter.len(), 2);
2389    /// assert_eq!(iter.next(), Some(40));
2390    /// ```
2391    ///
2392    /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
2393    /// a similar idea:
2394    ///
2395    /// ```
2396    /// use std::ops::ControlFlow;
2397    ///
2398    /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
2399    ///     if let Some(next) = prev.checked_add(x) {
2400    ///         ControlFlow::Continue(next)
2401    ///     } else {
2402    ///         ControlFlow::Break(prev)
2403    ///     }
2404    /// });
2405    /// assert_eq!(triangular, ControlFlow::Break(120));
2406    ///
2407    /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
2408    ///     if let Some(next) = prev.checked_add(x) {
2409    ///         ControlFlow::Continue(next)
2410    ///     } else {
2411    ///         ControlFlow::Break(prev)
2412    ///     }
2413    /// });
2414    /// assert_eq!(triangular, ControlFlow::Continue(435));
2415    /// ```
2416    #[inline]
2417    #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2418    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
2419    where
2420        Self: Sized,
2421        F: FnMut(B, Self::Item) -> R,
2422        R: Try<Output = B>,
2423    {
2424        let mut accum = init;
2425        while let Some(x) = self.next() {
2426            accum = f(accum, x)?;
2427        }
2428        try { accum }
2429    }
2430
2431    /// An iterator method that applies a fallible function to each item in the
2432    /// iterator, stopping at the first error and returning that error.
2433    ///
2434    /// This can also be thought of as the fallible form of [`for_each()`]
2435    /// or as the stateless version of [`try_fold()`].
2436    ///
2437    /// [`for_each()`]: Iterator::for_each
2438    /// [`try_fold()`]: Iterator::try_fold
2439    ///
2440    /// # Examples
2441    ///
2442    /// ```
2443    /// use std::fs::rename;
2444    /// use std::io::{stdout, Write};
2445    /// use std::path::Path;
2446    ///
2447    /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2448    ///
2449    /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}"));
2450    /// assert!(res.is_ok());
2451    ///
2452    /// let mut it = data.iter().cloned();
2453    /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2454    /// assert!(res.is_err());
2455    /// // It short-circuited, so the remaining items are still in the iterator:
2456    /// assert_eq!(it.next(), Some("stale_bread.json"));
2457    /// ```
2458    ///
2459    /// The [`ControlFlow`] type can be used with this method for the situations
2460    /// in which you'd use `break` and `continue` in a normal loop:
2461    ///
2462    /// ```
2463    /// use std::ops::ControlFlow;
2464    ///
2465    /// let r = (2..100).try_for_each(|x| {
2466    ///     if 323 % x == 0 {
2467    ///         return ControlFlow::Break(x)
2468    ///     }
2469    ///
2470    ///     ControlFlow::Continue(())
2471    /// });
2472    /// assert_eq!(r, ControlFlow::Break(17));
2473    /// ```
2474    #[inline]
2475    #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2476    fn try_for_each<F, R>(&mut self, f: F) -> R
2477    where
2478        Self: Sized,
2479        F: FnMut(Self::Item) -> R,
2480        R: Try<Output = ()>,
2481    {
2482        #[inline]
2483        fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2484            move |(), x| f(x)
2485        }
2486
2487        self.try_fold((), call(f))
2488    }
2489
2490    /// Folds every element into an accumulator by applying an operation,
2491    /// returning the final result.
2492    ///
2493    /// `fold()` takes two arguments: an initial value, and a closure with two
2494    /// arguments: an 'accumulator', and an element. The closure returns the value that
2495    /// the accumulator should have for the next iteration.
2496    ///
2497    /// The initial value is the value the accumulator will have on the first
2498    /// call.
2499    ///
2500    /// After applying this closure to every element of the iterator, `fold()`
2501    /// returns the accumulator.
2502    ///
2503    /// This operation is sometimes called 'reduce' or 'inject'.
2504    ///
2505    /// Folding is useful whenever you have a collection of something, and want
2506    /// to produce a single value from it.
2507    ///
2508    /// Note: `fold()`, and similar methods that traverse the entire iterator,
2509    /// might not terminate for infinite iterators, even on traits for which a
2510    /// result is determinable in finite time.
2511    ///
2512    /// Note: [`reduce()`] can be used to use the first element as the initial
2513    /// value, if the accumulator type and item type is the same.
2514    ///
2515    /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2516    /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2517    /// operators like `-` the order will affect the final result.
2518    /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2519    ///
2520    /// # Note to Implementors
2521    ///
2522    /// Several of the other (forward) methods have default implementations in
2523    /// terms of this one, so try to implement this explicitly if it can
2524    /// do something better than the default `for` loop implementation.
2525    ///
2526    /// In particular, try to have this call `fold()` on the internal parts
2527    /// from which this iterator is composed.
2528    ///
2529    /// # Examples
2530    ///
2531    /// Basic usage:
2532    ///
2533    /// ```
2534    /// let a = [1, 2, 3];
2535    ///
2536    /// // the sum of all of the elements of the array
2537    /// let sum = a.iter().fold(0, |acc, x| acc + x);
2538    ///
2539    /// assert_eq!(sum, 6);
2540    /// ```
2541    ///
2542    /// Let's walk through each step of the iteration here:
2543    ///
2544    /// | element | acc | x | result |
2545    /// |---------|-----|---|--------|
2546    /// |         | 0   |   |        |
2547    /// | 1       | 0   | 1 | 1      |
2548    /// | 2       | 1   | 2 | 3      |
2549    /// | 3       | 3   | 3 | 6      |
2550    ///
2551    /// And so, our final result, `6`.
2552    ///
2553    /// This example demonstrates the left-associative nature of `fold()`:
2554    /// it builds a string, starting with an initial value
2555    /// and continuing with each element from the front until the back:
2556    ///
2557    /// ```
2558    /// let numbers = [1, 2, 3, 4, 5];
2559    ///
2560    /// let zero = "0".to_string();
2561    ///
2562    /// let result = numbers.iter().fold(zero, |acc, &x| {
2563    ///     format!("({acc} + {x})")
2564    /// });
2565    ///
2566    /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2567    /// ```
2568    /// It's common for people who haven't used iterators a lot to
2569    /// use a `for` loop with a list of things to build up a result. Those
2570    /// can be turned into `fold()`s:
2571    ///
2572    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2573    ///
2574    /// ```
2575    /// let numbers = [1, 2, 3, 4, 5];
2576    ///
2577    /// let mut result = 0;
2578    ///
2579    /// // for loop:
2580    /// for i in &numbers {
2581    ///     result = result + i;
2582    /// }
2583    ///
2584    /// // fold:
2585    /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2586    ///
2587    /// // they're the same
2588    /// assert_eq!(result, result2);
2589    /// ```
2590    ///
2591    /// [`reduce()`]: Iterator::reduce
2592    #[doc(alias = "inject", alias = "foldl")]
2593    #[inline]
2594    #[stable(feature = "rust1", since = "1.0.0")]
2595    fn fold<B, F>(mut self, init: B, mut f: F) -> B
2596    where
2597        Self: Sized,
2598        F: FnMut(B, Self::Item) -> B,
2599    {
2600        let mut accum = init;
2601        while let Some(x) = self.next() {
2602            accum = f(accum, x);
2603        }
2604        accum
2605    }
2606
2607    /// Reduces the elements to a single one, by repeatedly applying a reducing
2608    /// operation.
2609    ///
2610    /// If the iterator is empty, returns [`None`]; otherwise, returns the
2611    /// result of the reduction.
2612    ///
2613    /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2614    /// For iterators with at least one element, this is the same as [`fold()`]
2615    /// with the first element of the iterator as the initial accumulator value, folding
2616    /// every subsequent element into it.
2617    ///
2618    /// [`fold()`]: Iterator::fold
2619    ///
2620    /// # Example
2621    ///
2622    /// ```
2623    /// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap_or(0);
2624    /// assert_eq!(reduced, 45);
2625    ///
2626    /// // Which is equivalent to doing it with `fold`:
2627    /// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
2628    /// assert_eq!(reduced, folded);
2629    /// ```
2630    #[inline]
2631    #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2632    fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2633    where
2634        Self: Sized,
2635        F: FnMut(Self::Item, Self::Item) -> Self::Item,
2636    {
2637        let first = self.next()?;
2638        Some(self.fold(first, f))
2639    }
2640
2641    /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
2642    /// closure returns a failure, the failure is propagated back to the caller immediately.
2643    ///
2644    /// The return type of this method depends on the return type of the closure. If the closure
2645    /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
2646    /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
2647    /// `Option<Option<Self::Item>>`.
2648    ///
2649    /// When called on an empty iterator, this function will return either `Some(None)` or
2650    /// `Ok(None)` depending on the type of the provided closure.
2651    ///
2652    /// For iterators with at least one element, this is essentially the same as calling
2653    /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
2654    ///
2655    /// [`try_fold()`]: Iterator::try_fold
2656    ///
2657    /// # Examples
2658    ///
2659    /// Safely calculate the sum of a series of numbers:
2660    ///
2661    /// ```
2662    /// #![feature(iterator_try_reduce)]
2663    ///
2664    /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
2665    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2666    /// assert_eq!(sum, Some(Some(58)));
2667    /// ```
2668    ///
2669    /// Determine when a reduction short circuited:
2670    ///
2671    /// ```
2672    /// #![feature(iterator_try_reduce)]
2673    ///
2674    /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
2675    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2676    /// assert_eq!(sum, None);
2677    /// ```
2678    ///
2679    /// Determine when a reduction was not performed because there are no elements:
2680    ///
2681    /// ```
2682    /// #![feature(iterator_try_reduce)]
2683    ///
2684    /// let numbers: Vec<usize> = Vec::new();
2685    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2686    /// assert_eq!(sum, Some(None));
2687    /// ```
2688    ///
2689    /// Use a [`Result`] instead of an [`Option`]:
2690    ///
2691    /// ```
2692    /// #![feature(iterator_try_reduce)]
2693    ///
2694    /// let numbers = vec!["1", "2", "3", "4", "5"];
2695    /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
2696    ///     numbers.into_iter().try_reduce(|x, y| {
2697    ///         if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
2698    ///     });
2699    /// assert_eq!(max, Ok(Some("5")));
2700    /// ```
2701    #[inline]
2702    #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
2703    fn try_reduce<R>(
2704        &mut self,
2705        f: impl FnMut(Self::Item, Self::Item) -> R,
2706    ) -> ChangeOutputType<R, Option<R::Output>>
2707    where
2708        Self: Sized,
2709        R: Try<Output = Self::Item, Residual: Residual<Option<Self::Item>>>,
2710    {
2711        let first = match self.next() {
2712            Some(i) => i,
2713            None => return Try::from_output(None),
2714        };
2715
2716        match self.try_fold(first, f).branch() {
2717            ControlFlow::Break(r) => FromResidual::from_residual(r),
2718            ControlFlow::Continue(i) => Try::from_output(Some(i)),
2719        }
2720    }
2721
2722    /// Tests if every element of the iterator matches a predicate.
2723    ///
2724    /// `all()` takes a closure that returns `true` or `false`. It applies
2725    /// this closure to each element of the iterator, and if they all return
2726    /// `true`, then so does `all()`. If any of them return `false`, it
2727    /// returns `false`.
2728    ///
2729    /// `all()` is short-circuiting; in other words, it will stop processing
2730    /// as soon as it finds a `false`, given that no matter what else happens,
2731    /// the result will also be `false`.
2732    ///
2733    /// An empty iterator returns `true`.
2734    ///
2735    /// # Examples
2736    ///
2737    /// Basic usage:
2738    ///
2739    /// ```
2740    /// let a = [1, 2, 3];
2741    ///
2742    /// assert!(a.into_iter().all(|x| x > 0));
2743    ///
2744    /// assert!(!a.into_iter().all(|x| x > 2));
2745    /// ```
2746    ///
2747    /// Stopping at the first `false`:
2748    ///
2749    /// ```
2750    /// let a = [1, 2, 3];
2751    ///
2752    /// let mut iter = a.into_iter();
2753    ///
2754    /// assert!(!iter.all(|x| x != 2));
2755    ///
2756    /// // we can still use `iter`, as there are more elements.
2757    /// assert_eq!(iter.next(), Some(3));
2758    /// ```
2759    #[inline]
2760    #[stable(feature = "rust1", since = "1.0.0")]
2761    fn all<F>(&mut self, f: F) -> bool
2762    where
2763        Self: Sized,
2764        F: FnMut(Self::Item) -> bool,
2765    {
2766        #[inline]
2767        fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2768            move |(), x| {
2769                if f(x) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
2770            }
2771        }
2772        self.try_fold((), check(f)) == ControlFlow::Continue(())
2773    }
2774
2775    /// Tests if any element of the iterator matches a predicate.
2776    ///
2777    /// `any()` takes a closure that returns `true` or `false`. It applies
2778    /// this closure to each element of the iterator, and if any of them return
2779    /// `true`, then so does `any()`. If they all return `false`, it
2780    /// returns `false`.
2781    ///
2782    /// `any()` is short-circuiting; in other words, it will stop processing
2783    /// as soon as it finds a `true`, given that no matter what else happens,
2784    /// the result will also be `true`.
2785    ///
2786    /// An empty iterator returns `false`.
2787    ///
2788    /// # Examples
2789    ///
2790    /// Basic usage:
2791    ///
2792    /// ```
2793    /// let a = [1, 2, 3];
2794    ///
2795    /// assert!(a.into_iter().any(|x| x > 0));
2796    ///
2797    /// assert!(!a.into_iter().any(|x| x > 5));
2798    /// ```
2799    ///
2800    /// Stopping at the first `true`:
2801    ///
2802    /// ```
2803    /// let a = [1, 2, 3];
2804    ///
2805    /// let mut iter = a.into_iter();
2806    ///
2807    /// assert!(iter.any(|x| x != 2));
2808    ///
2809    /// // we can still use `iter`, as there are more elements.
2810    /// assert_eq!(iter.next(), Some(2));
2811    /// ```
2812    #[inline]
2813    #[stable(feature = "rust1", since = "1.0.0")]
2814    fn any<F>(&mut self, f: F) -> bool
2815    where
2816        Self: Sized,
2817        F: FnMut(Self::Item) -> bool,
2818    {
2819        #[inline]
2820        fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2821            move |(), x| {
2822                if f(x) { ControlFlow::Break(()) } else { ControlFlow::Continue(()) }
2823            }
2824        }
2825
2826        self.try_fold((), check(f)) == ControlFlow::Break(())
2827    }
2828
2829    /// Searches for an element of an iterator that satisfies a predicate.
2830    ///
2831    /// `find()` takes a closure that returns `true` or `false`. It applies
2832    /// this closure to each element of the iterator, and if any of them return
2833    /// `true`, then `find()` returns [`Some(element)`]. If they all return
2834    /// `false`, it returns [`None`].
2835    ///
2836    /// `find()` is short-circuiting; in other words, it will stop processing
2837    /// as soon as the closure returns `true`.
2838    ///
2839    /// Because `find()` takes a reference, and many iterators iterate over
2840    /// references, this leads to a possibly confusing situation where the
2841    /// argument is a double reference. You can see this effect in the
2842    /// examples below, with `&&x`.
2843    ///
2844    /// If you need the index of the element, see [`position()`].
2845    ///
2846    /// [`Some(element)`]: Some
2847    /// [`position()`]: Iterator::position
2848    ///
2849    /// # Examples
2850    ///
2851    /// Basic usage:
2852    ///
2853    /// ```
2854    /// let a = [1, 2, 3];
2855    ///
2856    /// assert_eq!(a.into_iter().find(|&x| x == 2), Some(2));
2857    /// assert_eq!(a.into_iter().find(|&x| x == 5), None);
2858    /// ```
2859    ///
2860    /// Stopping at the first `true`:
2861    ///
2862    /// ```
2863    /// let a = [1, 2, 3];
2864    ///
2865    /// let mut iter = a.into_iter();
2866    ///
2867    /// assert_eq!(iter.find(|&x| x == 2), Some(2));
2868    ///
2869    /// // we can still use `iter`, as there are more elements.
2870    /// assert_eq!(iter.next(), Some(3));
2871    /// ```
2872    ///
2873    /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2874    #[inline]
2875    #[stable(feature = "rust1", since = "1.0.0")]
2876    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2877    where
2878        Self: Sized,
2879        P: FnMut(&Self::Item) -> bool,
2880    {
2881        #[inline]
2882        fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2883            move |(), x| {
2884                if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::Continue(()) }
2885            }
2886        }
2887
2888        self.try_fold((), check(predicate)).break_value()
2889    }
2890
2891    /// Applies function to the elements of iterator and returns
2892    /// the first non-none result.
2893    ///
2894    /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2895    ///
2896    /// # Examples
2897    ///
2898    /// ```
2899    /// let a = ["lol", "NaN", "2", "5"];
2900    ///
2901    /// let first_number = a.iter().find_map(|s| s.parse().ok());
2902    ///
2903    /// assert_eq!(first_number, Some(2));
2904    /// ```
2905    #[inline]
2906    #[stable(feature = "iterator_find_map", since = "1.30.0")]
2907    fn find_map<B, F>(&mut self, f: F) -> Option<B>
2908    where
2909        Self: Sized,
2910        F: FnMut(Self::Item) -> Option<B>,
2911    {
2912        #[inline]
2913        fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2914            move |(), x| match f(x) {
2915                Some(x) => ControlFlow::Break(x),
2916                None => ControlFlow::Continue(()),
2917            }
2918        }
2919
2920        self.try_fold((), check(f)).break_value()
2921    }
2922
2923    /// Applies function to the elements of iterator and returns
2924    /// the first true result or the first error.
2925    ///
2926    /// The return type of this method depends on the return type of the closure.
2927    /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>, E>`.
2928    /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
2929    ///
2930    /// # Examples
2931    ///
2932    /// ```
2933    /// #![feature(try_find)]
2934    ///
2935    /// let a = ["1", "2", "lol", "NaN", "5"];
2936    ///
2937    /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2938    ///     Ok(s.parse::<i32>()? == search)
2939    /// };
2940    ///
2941    /// let result = a.into_iter().try_find(|&s| is_my_num(s, 2));
2942    /// assert_eq!(result, Ok(Some("2")));
2943    ///
2944    /// let result = a.into_iter().try_find(|&s| is_my_num(s, 5));
2945    /// assert!(result.is_err());
2946    /// ```
2947    ///
2948    /// This also supports other types which implement [`Try`], not just [`Result`].
2949    ///
2950    /// ```
2951    /// #![feature(try_find)]
2952    ///
2953    /// use std::num::NonZero;
2954    ///
2955    /// let a = [3, 5, 7, 4, 9, 0, 11u32];
2956    /// let result = a.into_iter().try_find(|&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2957    /// assert_eq!(result, Some(Some(4)));
2958    /// let result = a.into_iter().take(3).try_find(|&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2959    /// assert_eq!(result, Some(None));
2960    /// let result = a.into_iter().rev().try_find(|&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2961    /// assert_eq!(result, None);
2962    /// ```
2963    #[inline]
2964    #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2965    fn try_find<R>(
2966        &mut self,
2967        f: impl FnMut(&Self::Item) -> R,
2968    ) -> ChangeOutputType<R, Option<Self::Item>>
2969    where
2970        Self: Sized,
2971        R: Try<Output = bool, Residual: Residual<Option<Self::Item>>>,
2972    {
2973        #[inline]
2974        fn check<I, V, R>(
2975            mut f: impl FnMut(&I) -> V,
2976        ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
2977        where
2978            V: Try<Output = bool, Residual = R>,
2979            R: Residual<Option<I>>,
2980        {
2981            move |(), x| match f(&x).branch() {
2982                ControlFlow::Continue(false) => ControlFlow::Continue(()),
2983                ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
2984                ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
2985            }
2986        }
2987
2988        match self.try_fold((), check(f)) {
2989            ControlFlow::Break(x) => x,
2990            ControlFlow::Continue(()) => Try::from_output(None),
2991        }
2992    }
2993
2994    /// Searches for an element in an iterator, returning its index.
2995    ///
2996    /// `position()` takes a closure that returns `true` or `false`. It applies
2997    /// this closure to each element of the iterator, and if one of them
2998    /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2999    /// them return `false`, it returns [`None`].
3000    ///
3001    /// `position()` is short-circuiting; in other words, it will stop
3002    /// processing as soon as it finds a `true`.
3003    ///
3004    /// # Overflow Behavior
3005    ///
3006    /// The method does no guarding against overflows, so if there are more
3007    /// than [`usize::MAX`] non-matching elements, it either produces the wrong
3008    /// result or panics. If overflow checks are enabled, a panic is
3009    /// guaranteed.
3010    ///
3011    /// # Panics
3012    ///
3013    /// This function might panic if the iterator has more than `usize::MAX`
3014    /// non-matching elements.
3015    ///
3016    /// [`Some(index)`]: Some
3017    ///
3018    /// # Examples
3019    ///
3020    /// Basic usage:
3021    ///
3022    /// ```
3023    /// let a = [1, 2, 3];
3024    ///
3025    /// assert_eq!(a.into_iter().position(|x| x == 2), Some(1));
3026    ///
3027    /// assert_eq!(a.into_iter().position(|x| x == 5), None);
3028    /// ```
3029    ///
3030    /// Stopping at the first `true`:
3031    ///
3032    /// ```
3033    /// let a = [1, 2, 3, 4];
3034    ///
3035    /// let mut iter = a.into_iter();
3036    ///
3037    /// assert_eq!(iter.position(|x| x >= 2), Some(1));
3038    ///
3039    /// // we can still use `iter`, as there are more elements.
3040    /// assert_eq!(iter.next(), Some(3));
3041    ///
3042    /// // The returned index depends on iterator state
3043    /// assert_eq!(iter.position(|x| x == 4), Some(0));
3044    ///
3045    /// ```
3046    #[inline]
3047    #[stable(feature = "rust1", since = "1.0.0")]
3048    fn position<P>(&mut self, predicate: P) -> Option<usize>
3049    where
3050        Self: Sized,
3051        P: FnMut(Self::Item) -> bool,
3052    {
3053        #[inline]
3054        fn check<'a, T>(
3055            mut predicate: impl FnMut(T) -> bool + 'a,
3056            acc: &'a mut usize,
3057        ) -> impl FnMut((), T) -> ControlFlow<usize, ()> + 'a {
3058            #[rustc_inherit_overflow_checks]
3059            move |_, x| {
3060                if predicate(x) {
3061                    ControlFlow::Break(*acc)
3062                } else {
3063                    *acc += 1;
3064                    ControlFlow::Continue(())
3065                }
3066            }
3067        }
3068
3069        let mut acc = 0;
3070        self.try_fold((), check(predicate, &mut acc)).break_value()
3071    }
3072
3073    /// Searches for an element in an iterator from the right, returning its
3074    /// index.
3075    ///
3076    /// `rposition()` takes a closure that returns `true` or `false`. It applies
3077    /// this closure to each element of the iterator, starting from the end,
3078    /// and if one of them returns `true`, then `rposition()` returns
3079    /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
3080    ///
3081    /// `rposition()` is short-circuiting; in other words, it will stop
3082    /// processing as soon as it finds a `true`.
3083    ///
3084    /// [`Some(index)`]: Some
3085    ///
3086    /// # Examples
3087    ///
3088    /// Basic usage:
3089    ///
3090    /// ```
3091    /// let a = [1, 2, 3];
3092    ///
3093    /// assert_eq!(a.into_iter().rposition(|x| x == 3), Some(2));
3094    ///
3095    /// assert_eq!(a.into_iter().rposition(|x| x == 5), None);
3096    /// ```
3097    ///
3098    /// Stopping at the first `true`:
3099    ///
3100    /// ```
3101    /// let a = [-1, 2, 3, 4];
3102    ///
3103    /// let mut iter = a.into_iter();
3104    ///
3105    /// assert_eq!(iter.rposition(|x| x >= 2), Some(3));
3106    ///
3107    /// // we can still use `iter`, as there are more elements.
3108    /// assert_eq!(iter.next(), Some(-1));
3109    /// assert_eq!(iter.next_back(), Some(3));
3110    /// ```
3111    #[inline]
3112    #[stable(feature = "rust1", since = "1.0.0")]
3113    fn rposition<P>(&mut self, predicate: P) -> Option<usize>
3114    where
3115        P: FnMut(Self::Item) -> bool,
3116        Self: Sized + ExactSizeIterator + DoubleEndedIterator,
3117    {
3118        // No need for an overflow check here, because `ExactSizeIterator`
3119        // implies that the number of elements fits into a `usize`.
3120        #[inline]
3121        fn check<T>(
3122            mut predicate: impl FnMut(T) -> bool,
3123        ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
3124            move |i, x| {
3125                let i = i - 1;
3126                if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
3127            }
3128        }
3129
3130        let n = self.len();
3131        self.try_rfold(n, check(predicate)).break_value()
3132    }
3133
3134    /// Returns the maximum element of an iterator.
3135    ///
3136    /// If several elements are equally maximum, the last element is
3137    /// returned. If the iterator is empty, [`None`] is returned.
3138    ///
3139    /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
3140    /// incomparable. You can work around this by using [`Iterator::reduce`]:
3141    /// ```
3142    /// assert_eq!(
3143    ///     [2.4, f32::NAN, 1.3]
3144    ///         .into_iter()
3145    ///         .reduce(f32::max)
3146    ///         .unwrap_or(0.),
3147    ///     2.4
3148    /// );
3149    /// ```
3150    ///
3151    /// # Examples
3152    ///
3153    /// ```
3154    /// let a = [1, 2, 3];
3155    /// let b: [u32; 0] = [];
3156    ///
3157    /// assert_eq!(a.into_iter().max(), Some(3));
3158    /// assert_eq!(b.into_iter().max(), None);
3159    /// ```
3160    #[inline]
3161    #[stable(feature = "rust1", since = "1.0.0")]
3162    fn max(self) -> Option<Self::Item>
3163    where
3164        Self: Sized,
3165        Self::Item: Ord,
3166    {
3167        self.max_by(Ord::cmp)
3168    }
3169
3170    /// Returns the minimum element of an iterator.
3171    ///
3172    /// If several elements are equally minimum, the first element is returned.
3173    /// If the iterator is empty, [`None`] is returned.
3174    ///
3175    /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
3176    /// incomparable. You can work around this by using [`Iterator::reduce`]:
3177    /// ```
3178    /// assert_eq!(
3179    ///     [2.4, f32::NAN, 1.3]
3180    ///         .into_iter()
3181    ///         .reduce(f32::min)
3182    ///         .unwrap_or(0.),
3183    ///     1.3
3184    /// );
3185    /// ```
3186    ///
3187    /// # Examples
3188    ///
3189    /// ```
3190    /// let a = [1, 2, 3];
3191    /// let b: [u32; 0] = [];
3192    ///
3193    /// assert_eq!(a.into_iter().min(), Some(1));
3194    /// assert_eq!(b.into_iter().min(), None);
3195    /// ```
3196    #[inline]
3197    #[stable(feature = "rust1", since = "1.0.0")]
3198    fn min(self) -> Option<Self::Item>
3199    where
3200        Self: Sized,
3201        Self::Item: Ord,
3202    {
3203        self.min_by(Ord::cmp)
3204    }
3205
3206    /// Returns the element that gives the maximum value from the
3207    /// specified function.
3208    ///
3209    /// If several elements are equally maximum, the last element is
3210    /// returned. If the iterator is empty, [`None`] is returned.
3211    ///
3212    /// # Examples
3213    ///
3214    /// ```
3215    /// let a = [-3_i32, 0, 1, 5, -10];
3216    /// assert_eq!(a.into_iter().max_by_key(|x| x.abs()).unwrap(), -10);
3217    /// ```
3218    #[inline]
3219    #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3220    fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3221    where
3222        Self: Sized,
3223        F: FnMut(&Self::Item) -> B,
3224    {
3225        #[inline]
3226        fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3227            move |x| (f(&x), x)
3228        }
3229
3230        #[inline]
3231        fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3232            x_p.cmp(y_p)
3233        }
3234
3235        let (_, x) = self.map(key(f)).max_by(compare)?;
3236        Some(x)
3237    }
3238
3239    /// Returns the element that gives the maximum value with respect to the
3240    /// specified comparison function.
3241    ///
3242    /// If several elements are equally maximum, the last element is
3243    /// returned. If the iterator is empty, [`None`] is returned.
3244    ///
3245    /// # Examples
3246    ///
3247    /// ```
3248    /// let a = [-3_i32, 0, 1, 5, -10];
3249    /// assert_eq!(a.into_iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
3250    /// ```
3251    #[inline]
3252    #[stable(feature = "iter_max_by", since = "1.15.0")]
3253    fn max_by<F>(self, compare: F) -> Option<Self::Item>
3254    where
3255        Self: Sized,
3256        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3257    {
3258        #[inline]
3259        fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3260            move |x, y| cmp::max_by(x, y, &mut compare)
3261        }
3262
3263        self.reduce(fold(compare))
3264    }
3265
3266    /// Returns the element that gives the minimum value from the
3267    /// specified function.
3268    ///
3269    /// If several elements are equally minimum, the first element is
3270    /// returned. If the iterator is empty, [`None`] is returned.
3271    ///
3272    /// # Examples
3273    ///
3274    /// ```
3275    /// let a = [-3_i32, 0, 1, 5, -10];
3276    /// assert_eq!(a.into_iter().min_by_key(|x| x.abs()).unwrap(), 0);
3277    /// ```
3278    #[inline]
3279    #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3280    fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3281    where
3282        Self: Sized,
3283        F: FnMut(&Self::Item) -> B,
3284    {
3285        #[inline]
3286        fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3287            move |x| (f(&x), x)
3288        }
3289
3290        #[inline]
3291        fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3292            x_p.cmp(y_p)
3293        }
3294
3295        let (_, x) = self.map(key(f)).min_by(compare)?;
3296        Some(x)
3297    }
3298
3299    /// Returns the element that gives the minimum value with respect to the
3300    /// specified comparison function.
3301    ///
3302    /// If several elements are equally minimum, the first element is
3303    /// returned. If the iterator is empty, [`None`] is returned.
3304    ///
3305    /// # Examples
3306    ///
3307    /// ```
3308    /// let a = [-3_i32, 0, 1, 5, -10];
3309    /// assert_eq!(a.into_iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
3310    /// ```
3311    #[inline]
3312    #[stable(feature = "iter_min_by", since = "1.15.0")]
3313    fn min_by<F>(self, compare: F) -> Option<Self::Item>
3314    where
3315        Self: Sized,
3316        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3317    {
3318        #[inline]
3319        fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3320            move |x, y| cmp::min_by(x, y, &mut compare)
3321        }
3322
3323        self.reduce(fold(compare))
3324    }
3325
3326    /// Reverses an iterator's direction.
3327    ///
3328    /// Usually, iterators iterate from left to right. After using `rev()`,
3329    /// an iterator will instead iterate from right to left.
3330    ///
3331    /// This is only possible if the iterator has an end, so `rev()` only
3332    /// works on [`DoubleEndedIterator`]s.
3333    ///
3334    /// # Examples
3335    ///
3336    /// ```
3337    /// let a = [1, 2, 3];
3338    ///
3339    /// let mut iter = a.into_iter().rev();
3340    ///
3341    /// assert_eq!(iter.next(), Some(3));
3342    /// assert_eq!(iter.next(), Some(2));
3343    /// assert_eq!(iter.next(), Some(1));
3344    ///
3345    /// assert_eq!(iter.next(), None);
3346    /// ```
3347    #[inline]
3348    #[doc(alias = "reverse")]
3349    #[stable(feature = "rust1", since = "1.0.0")]
3350    fn rev(self) -> Rev<Self>
3351    where
3352        Self: Sized + DoubleEndedIterator,
3353    {
3354        Rev::new(self)
3355    }
3356
3357    /// Converts an iterator of pairs into a pair of containers.
3358    ///
3359    /// `unzip()` consumes an entire iterator of pairs, producing two
3360    /// collections: one from the left elements of the pairs, and one
3361    /// from the right elements.
3362    ///
3363    /// This function is, in some sense, the opposite of [`zip`].
3364    ///
3365    /// [`zip`]: Iterator::zip
3366    ///
3367    /// # Examples
3368    ///
3369    /// ```
3370    /// let a = [(1, 2), (3, 4), (5, 6)];
3371    ///
3372    /// let (left, right): (Vec<_>, Vec<_>) = a.into_iter().unzip();
3373    ///
3374    /// assert_eq!(left, [1, 3, 5]);
3375    /// assert_eq!(right, [2, 4, 6]);
3376    ///
3377    /// // you can also unzip multiple nested tuples at once
3378    /// let a = [(1, (2, 3)), (4, (5, 6))];
3379    ///
3380    /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.into_iter().unzip();
3381    /// assert_eq!(x, [1, 4]);
3382    /// assert_eq!(y, [2, 5]);
3383    /// assert_eq!(z, [3, 6]);
3384    /// ```
3385    #[stable(feature = "rust1", since = "1.0.0")]
3386    fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
3387    where
3388        FromA: Default + Extend<A>,
3389        FromB: Default + Extend<B>,
3390        Self: Sized + Iterator<Item = (A, B)>,
3391    {
3392        let mut unzipped: (FromA, FromB) = Default::default();
3393        unzipped.extend(self);
3394        unzipped
3395    }
3396
3397    /// Creates an iterator which copies all of its elements.
3398    ///
3399    /// This is useful when you have an iterator over `&T`, but you need an
3400    /// iterator over `T`.
3401    ///
3402    /// # Examples
3403    ///
3404    /// ```
3405    /// let a = [1, 2, 3];
3406    ///
3407    /// let v_copied: Vec<_> = a.iter().copied().collect();
3408    ///
3409    /// // copied is the same as .map(|&x| x)
3410    /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3411    ///
3412    /// assert_eq!(v_copied, [1, 2, 3]);
3413    /// assert_eq!(v_map, [1, 2, 3]);
3414    /// ```
3415    #[stable(feature = "iter_copied", since = "1.36.0")]
3416    #[rustc_diagnostic_item = "iter_copied"]
3417    fn copied<'a, T: 'a>(self) -> Copied<Self>
3418    where
3419        Self: Sized + Iterator<Item = &'a T>,
3420        T: Copy,
3421    {
3422        Copied::new(self)
3423    }
3424
3425    /// Creates an iterator which [`clone`]s all of its elements.
3426    ///
3427    /// This is useful when you have an iterator over `&T`, but you need an
3428    /// iterator over `T`.
3429    ///
3430    /// There is no guarantee whatsoever about the `clone` method actually
3431    /// being called *or* optimized away. So code should not depend on
3432    /// either.
3433    ///
3434    /// [`clone`]: Clone::clone
3435    ///
3436    /// # Examples
3437    ///
3438    /// Basic usage:
3439    ///
3440    /// ```
3441    /// let a = [1, 2, 3];
3442    ///
3443    /// let v_cloned: Vec<_> = a.iter().cloned().collect();
3444    ///
3445    /// // cloned is the same as .map(|&x| x), for integers
3446    /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3447    ///
3448    /// assert_eq!(v_cloned, [1, 2, 3]);
3449    /// assert_eq!(v_map, [1, 2, 3]);
3450    /// ```
3451    ///
3452    /// To get the best performance, try to clone late:
3453    ///
3454    /// ```
3455    /// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]];
3456    /// // don't do this:
3457    /// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect();
3458    /// assert_eq!(&[vec![23]], &slower[..]);
3459    /// // instead call `cloned` late
3460    /// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect();
3461    /// assert_eq!(&[vec![23]], &faster[..]);
3462    /// ```
3463    #[stable(feature = "rust1", since = "1.0.0")]
3464    #[rustc_diagnostic_item = "iter_cloned"]
3465    fn cloned<'a, T: 'a>(self) -> Cloned<Self>
3466    where
3467        Self: Sized + Iterator<Item = &'a T>,
3468        T: Clone,
3469    {
3470        Cloned::new(self)
3471    }
3472
3473    /// Repeats an iterator endlessly.
3474    ///
3475    /// Instead of stopping at [`None`], the iterator will instead start again,
3476    /// from the beginning. After iterating again, it will start at the
3477    /// beginning again. And again. And again. Forever. Note that in case the
3478    /// original iterator is empty, the resulting iterator will also be empty.
3479    ///
3480    /// # Examples
3481    ///
3482    /// ```
3483    /// let a = [1, 2, 3];
3484    ///
3485    /// let mut iter = a.into_iter().cycle();
3486    ///
3487    /// loop {
3488    ///     assert_eq!(iter.next(), Some(1));
3489    ///     assert_eq!(iter.next(), Some(2));
3490    ///     assert_eq!(iter.next(), Some(3));
3491    /// #   break;
3492    /// }
3493    /// ```
3494    #[stable(feature = "rust1", since = "1.0.0")]
3495    #[inline]
3496    fn cycle(self) -> Cycle<Self>
3497    where
3498        Self: Sized + Clone,
3499    {
3500        Cycle::new(self)
3501    }
3502
3503    /// Returns an iterator over `N` elements of the iterator at a time.
3504    ///
3505    /// The chunks do not overlap. If `N` does not divide the length of the
3506    /// iterator, then the last up to `N-1` elements will be omitted and can be
3507    /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
3508    /// function of the iterator.
3509    ///
3510    /// # Panics
3511    ///
3512    /// Panics if `N` is zero.
3513    ///
3514    /// # Examples
3515    ///
3516    /// Basic usage:
3517    ///
3518    /// ```
3519    /// #![feature(iter_array_chunks)]
3520    ///
3521    /// let mut iter = "lorem".chars().array_chunks();
3522    /// assert_eq!(iter.next(), Some(['l', 'o']));
3523    /// assert_eq!(iter.next(), Some(['r', 'e']));
3524    /// assert_eq!(iter.next(), None);
3525    /// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
3526    /// ```
3527    ///
3528    /// ```
3529    /// #![feature(iter_array_chunks)]
3530    ///
3531    /// let data = [1, 1, 2, -2, 6, 0, 3, 1];
3532    /// //          ^-----^  ^------^
3533    /// for [x, y, z] in data.iter().array_chunks() {
3534    ///     assert_eq!(x + y + z, 4);
3535    /// }
3536    /// ```
3537    #[track_caller]
3538    #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
3539    fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
3540    where
3541        Self: Sized,
3542    {
3543        ArrayChunks::new(self)
3544    }
3545
3546    /// Sums the elements of an iterator.
3547    ///
3548    /// Takes each element, adds them together, and returns the result.
3549    ///
3550    /// An empty iterator returns the *additive identity* ("zero") of the type,
3551    /// which is `0` for integers and `-0.0` for floats.
3552    ///
3553    /// `sum()` can be used to sum any type implementing [`Sum`][`core::iter::Sum`],
3554    /// including [`Option`][`Option::sum`] and [`Result`][`Result::sum`].
3555    ///
3556    /// # Panics
3557    ///
3558    /// When calling `sum()` and a primitive integer type is being returned, this
3559    /// method will panic if the computation overflows and overflow checks are
3560    /// enabled.
3561    ///
3562    /// # Examples
3563    ///
3564    /// ```
3565    /// let a = [1, 2, 3];
3566    /// let sum: i32 = a.iter().sum();
3567    ///
3568    /// assert_eq!(sum, 6);
3569    ///
3570    /// let b: Vec<f32> = vec![];
3571    /// let sum: f32 = b.iter().sum();
3572    /// assert_eq!(sum, -0.0_f32);
3573    /// ```
3574    #[stable(feature = "iter_arith", since = "1.11.0")]
3575    fn sum<S>(self) -> S
3576    where
3577        Self: Sized,
3578        S: Sum<Self::Item>,
3579    {
3580        Sum::sum(self)
3581    }
3582
3583    /// Iterates over the entire iterator, multiplying all the elements
3584    ///
3585    /// An empty iterator returns the one value of the type.
3586    ///
3587    /// `product()` can be used to multiply any type implementing [`Product`][`core::iter::Product`],
3588    /// including [`Option`][`Option::product`] and [`Result`][`Result::product`].
3589    ///
3590    /// # Panics
3591    ///
3592    /// When calling `product()` and a primitive integer type is being returned,
3593    /// method will panic if the computation overflows and overflow checks are
3594    /// enabled.
3595    ///
3596    /// # Examples
3597    ///
3598    /// ```
3599    /// fn factorial(n: u32) -> u32 {
3600    ///     (1..=n).product()
3601    /// }
3602    /// assert_eq!(factorial(0), 1);
3603    /// assert_eq!(factorial(1), 1);
3604    /// assert_eq!(factorial(5), 120);
3605    /// ```
3606    #[stable(feature = "iter_arith", since = "1.11.0")]
3607    fn product<P>(self) -> P
3608    where
3609        Self: Sized,
3610        P: Product<Self::Item>,
3611    {
3612        Product::product(self)
3613    }
3614
3615    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3616    /// of another.
3617    ///
3618    /// # Examples
3619    ///
3620    /// ```
3621    /// use std::cmp::Ordering;
3622    ///
3623    /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3624    /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3625    /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3626    /// ```
3627    #[stable(feature = "iter_order", since = "1.5.0")]
3628    fn cmp<I>(self, other: I) -> Ordering
3629    where
3630        I: IntoIterator<Item = Self::Item>,
3631        Self::Item: Ord,
3632        Self: Sized,
3633    {
3634        self.cmp_by(other, |x, y| x.cmp(&y))
3635    }
3636
3637    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3638    /// of another with respect to the specified comparison function.
3639    ///
3640    /// # Examples
3641    ///
3642    /// ```
3643    /// #![feature(iter_order_by)]
3644    ///
3645    /// use std::cmp::Ordering;
3646    ///
3647    /// let xs = [1, 2, 3, 4];
3648    /// let ys = [1, 4, 9, 16];
3649    ///
3650    /// assert_eq!(xs.into_iter().cmp_by(ys, |x, y| x.cmp(&y)), Ordering::Less);
3651    /// assert_eq!(xs.into_iter().cmp_by(ys, |x, y| (x * x).cmp(&y)), Ordering::Equal);
3652    /// assert_eq!(xs.into_iter().cmp_by(ys, |x, y| (2 * x).cmp(&y)), Ordering::Greater);
3653    /// ```
3654    #[unstable(feature = "iter_order_by", issue = "64295")]
3655    fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
3656    where
3657        Self: Sized,
3658        I: IntoIterator,
3659        F: FnMut(Self::Item, I::Item) -> Ordering,
3660    {
3661        #[inline]
3662        fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
3663        where
3664            F: FnMut(X, Y) -> Ordering,
3665        {
3666            move |x, y| match cmp(x, y) {
3667                Ordering::Equal => ControlFlow::Continue(()),
3668                non_eq => ControlFlow::Break(non_eq),
3669            }
3670        }
3671
3672        match iter_compare(self, other.into_iter(), compare(cmp)) {
3673            ControlFlow::Continue(ord) => ord,
3674            ControlFlow::Break(ord) => ord,
3675        }
3676    }
3677
3678    /// [Lexicographically](Ord#lexicographical-comparison) compares the [`PartialOrd`] elements of
3679    /// this [`Iterator`] with those of another. The comparison works like short-circuit
3680    /// evaluation, returning a result without comparing the remaining elements.
3681    /// As soon as an order can be determined, the evaluation stops and a result is returned.
3682    ///
3683    /// # Examples
3684    ///
3685    /// ```
3686    /// use std::cmp::Ordering;
3687    ///
3688    /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3689    /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3690    /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3691    /// ```
3692    ///
3693    /// For floating-point numbers, NaN does not have a total order and will result
3694    /// in `None` when compared:
3695    ///
3696    /// ```
3697    /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3698    /// ```
3699    ///
3700    /// The results are determined by the order of evaluation.
3701    ///
3702    /// ```
3703    /// use std::cmp::Ordering;
3704    ///
3705    /// assert_eq!([1.0, f64::NAN].iter().partial_cmp([2.0, f64::NAN].iter()), Some(Ordering::Less));
3706    /// assert_eq!([2.0, f64::NAN].iter().partial_cmp([1.0, f64::NAN].iter()), Some(Ordering::Greater));
3707    /// assert_eq!([f64::NAN, 1.0].iter().partial_cmp([f64::NAN, 2.0].iter()), None);
3708    /// ```
3709    ///
3710    #[stable(feature = "iter_order", since = "1.5.0")]
3711    fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3712    where
3713        I: IntoIterator,
3714        Self::Item: PartialOrd<I::Item>,
3715        Self: Sized,
3716    {
3717        self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3718    }
3719
3720    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3721    /// of another with respect to the specified comparison function.
3722    ///
3723    /// # Examples
3724    ///
3725    /// ```
3726    /// #![feature(iter_order_by)]
3727    ///
3728    /// use std::cmp::Ordering;
3729    ///
3730    /// let xs = [1.0, 2.0, 3.0, 4.0];
3731    /// let ys = [1.0, 4.0, 9.0, 16.0];
3732    ///
3733    /// assert_eq!(
3734    ///     xs.iter().partial_cmp_by(ys, |x, y| x.partial_cmp(&y)),
3735    ///     Some(Ordering::Less)
3736    /// );
3737    /// assert_eq!(
3738    ///     xs.iter().partial_cmp_by(ys, |x, y| (x * x).partial_cmp(&y)),
3739    ///     Some(Ordering::Equal)
3740    /// );
3741    /// assert_eq!(
3742    ///     xs.iter().partial_cmp_by(ys, |x, y| (2.0 * x).partial_cmp(&y)),
3743    ///     Some(Ordering::Greater)
3744    /// );
3745    /// ```
3746    #[unstable(feature = "iter_order_by", issue = "64295")]
3747    fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
3748    where
3749        Self: Sized,
3750        I: IntoIterator,
3751        F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3752    {
3753        #[inline]
3754        fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
3755        where
3756            F: FnMut(X, Y) -> Option<Ordering>,
3757        {
3758            move |x, y| match partial_cmp(x, y) {
3759                Some(Ordering::Equal) => ControlFlow::Continue(()),
3760                non_eq => ControlFlow::Break(non_eq),
3761            }
3762        }
3763
3764        match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
3765            ControlFlow::Continue(ord) => Some(ord),
3766            ControlFlow::Break(ord) => ord,
3767        }
3768    }
3769
3770    /// Determines if the elements of this [`Iterator`] are equal to those of
3771    /// another.
3772    ///
3773    /// # Examples
3774    ///
3775    /// ```
3776    /// assert_eq!([1].iter().eq([1].iter()), true);
3777    /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3778    /// ```
3779    #[stable(feature = "iter_order", since = "1.5.0")]
3780    fn eq<I>(self, other: I) -> bool
3781    where
3782        I: IntoIterator,
3783        Self::Item: PartialEq<I::Item>,
3784        Self: Sized,
3785    {
3786        self.eq_by(other, |x, y| x == y)
3787    }
3788
3789    /// Determines if the elements of this [`Iterator`] are equal to those of
3790    /// another with respect to the specified equality function.
3791    ///
3792    /// # Examples
3793    ///
3794    /// ```
3795    /// #![feature(iter_order_by)]
3796    ///
3797    /// let xs = [1, 2, 3, 4];
3798    /// let ys = [1, 4, 9, 16];
3799    ///
3800    /// assert!(xs.iter().eq_by(ys, |x, y| x * x == y));
3801    /// ```
3802    #[unstable(feature = "iter_order_by", issue = "64295")]
3803    fn eq_by<I, F>(self, other: I, eq: F) -> bool
3804    where
3805        Self: Sized,
3806        I: IntoIterator,
3807        F: FnMut(Self::Item, I::Item) -> bool,
3808    {
3809        #[inline]
3810        fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
3811        where
3812            F: FnMut(X, Y) -> bool,
3813        {
3814            move |x, y| {
3815                if eq(x, y) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
3816            }
3817        }
3818
3819        match iter_compare(self, other.into_iter(), compare(eq)) {
3820            ControlFlow::Continue(ord) => ord == Ordering::Equal,
3821            ControlFlow::Break(()) => false,
3822        }
3823    }
3824
3825    /// Determines if the elements of this [`Iterator`] are not equal to those of
3826    /// another.
3827    ///
3828    /// # Examples
3829    ///
3830    /// ```
3831    /// assert_eq!([1].iter().ne([1].iter()), false);
3832    /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3833    /// ```
3834    #[stable(feature = "iter_order", since = "1.5.0")]
3835    fn ne<I>(self, other: I) -> bool
3836    where
3837        I: IntoIterator,
3838        Self::Item: PartialEq<I::Item>,
3839        Self: Sized,
3840    {
3841        !self.eq(other)
3842    }
3843
3844    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3845    /// less than those of another.
3846    ///
3847    /// # Examples
3848    ///
3849    /// ```
3850    /// assert_eq!([1].iter().lt([1].iter()), false);
3851    /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3852    /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3853    /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3854    /// ```
3855    #[stable(feature = "iter_order", since = "1.5.0")]
3856    fn lt<I>(self, other: I) -> bool
3857    where
3858        I: IntoIterator,
3859        Self::Item: PartialOrd<I::Item>,
3860        Self: Sized,
3861    {
3862        self.partial_cmp(other) == Some(Ordering::Less)
3863    }
3864
3865    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3866    /// less or equal to those of another.
3867    ///
3868    /// # Examples
3869    ///
3870    /// ```
3871    /// assert_eq!([1].iter().le([1].iter()), true);
3872    /// assert_eq!([1].iter().le([1, 2].iter()), true);
3873    /// assert_eq!([1, 2].iter().le([1].iter()), false);
3874    /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3875    /// ```
3876    #[stable(feature = "iter_order", since = "1.5.0")]
3877    fn le<I>(self, other: I) -> bool
3878    where
3879        I: IntoIterator,
3880        Self::Item: PartialOrd<I::Item>,
3881        Self: Sized,
3882    {
3883        matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3884    }
3885
3886    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3887    /// greater than those of another.
3888    ///
3889    /// # Examples
3890    ///
3891    /// ```
3892    /// assert_eq!([1].iter().gt([1].iter()), false);
3893    /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3894    /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3895    /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3896    /// ```
3897    #[stable(feature = "iter_order", since = "1.5.0")]
3898    fn gt<I>(self, other: I) -> bool
3899    where
3900        I: IntoIterator,
3901        Self::Item: PartialOrd<I::Item>,
3902        Self: Sized,
3903    {
3904        self.partial_cmp(other) == Some(Ordering::Greater)
3905    }
3906
3907    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3908    /// greater than or equal to those of another.
3909    ///
3910    /// # Examples
3911    ///
3912    /// ```
3913    /// assert_eq!([1].iter().ge([1].iter()), true);
3914    /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3915    /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3916    /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3917    /// ```
3918    #[stable(feature = "iter_order", since = "1.5.0")]
3919    fn ge<I>(self, other: I) -> bool
3920    where
3921        I: IntoIterator,
3922        Self::Item: PartialOrd<I::Item>,
3923        Self: Sized,
3924    {
3925        matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3926    }
3927
3928    /// Checks if the elements of this iterator are sorted.
3929    ///
3930    /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3931    /// iterator yields exactly zero or one element, `true` is returned.
3932    ///
3933    /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3934    /// implies that this function returns `false` if any two consecutive items are not
3935    /// comparable.
3936    ///
3937    /// # Examples
3938    ///
3939    /// ```
3940    /// assert!([1, 2, 2, 9].iter().is_sorted());
3941    /// assert!(![1, 3, 2, 4].iter().is_sorted());
3942    /// assert!([0].iter().is_sorted());
3943    /// assert!(std::iter::empty::<i32>().is_sorted());
3944    /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3945    /// ```
3946    #[inline]
3947    #[stable(feature = "is_sorted", since = "1.82.0")]
3948    fn is_sorted(self) -> bool
3949    where
3950        Self: Sized,
3951        Self::Item: PartialOrd,
3952    {
3953        self.is_sorted_by(|a, b| a <= b)
3954    }
3955
3956    /// Checks if the elements of this iterator are sorted using the given comparator function.
3957    ///
3958    /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3959    /// function to determine whether two elements are to be considered in sorted order.
3960    ///
3961    /// # Examples
3962    ///
3963    /// ```
3964    /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a <= b));
3965    /// assert!(![1, 2, 2, 9].iter().is_sorted_by(|a, b| a < b));
3966    ///
3967    /// assert!([0].iter().is_sorted_by(|a, b| true));
3968    /// assert!([0].iter().is_sorted_by(|a, b| false));
3969    ///
3970    /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| false));
3971    /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| true));
3972    /// ```
3973    #[stable(feature = "is_sorted", since = "1.82.0")]
3974    fn is_sorted_by<F>(mut self, compare: F) -> bool
3975    where
3976        Self: Sized,
3977        F: FnMut(&Self::Item, &Self::Item) -> bool,
3978    {
3979        #[inline]
3980        fn check<'a, T>(
3981            last: &'a mut T,
3982            mut compare: impl FnMut(&T, &T) -> bool + 'a,
3983        ) -> impl FnMut(T) -> bool + 'a {
3984            move |curr| {
3985                if !compare(&last, &curr) {
3986                    return false;
3987                }
3988                *last = curr;
3989                true
3990            }
3991        }
3992
3993        let mut last = match self.next() {
3994            Some(e) => e,
3995            None => return true,
3996        };
3997
3998        self.all(check(&mut last, compare))
3999    }
4000
4001    /// Checks if the elements of this iterator are sorted using the given key extraction
4002    /// function.
4003    ///
4004    /// Instead of comparing the iterator's elements directly, this function compares the keys of
4005    /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
4006    /// its documentation for more information.
4007    ///
4008    /// [`is_sorted`]: Iterator::is_sorted
4009    ///
4010    /// # Examples
4011    ///
4012    /// ```
4013    /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
4014    /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
4015    /// ```
4016    #[inline]
4017    #[stable(feature = "is_sorted", since = "1.82.0")]
4018    fn is_sorted_by_key<F, K>(self, f: F) -> bool
4019    where
4020        Self: Sized,
4021        F: FnMut(Self::Item) -> K,
4022        K: PartialOrd,
4023    {
4024        self.map(f).is_sorted()
4025    }
4026
4027    /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
4028    // The unusual name is to avoid name collisions in method resolution
4029    // see #76479.
4030    #[inline]
4031    #[doc(hidden)]
4032    #[unstable(feature = "trusted_random_access", issue = "none")]
4033    unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
4034    where
4035        Self: TrustedRandomAccessNoCoerce,
4036    {
4037        unreachable!("Always specialized");
4038    }
4039}
4040
4041/// Compares two iterators element-wise using the given function.
4042///
4043/// If `ControlFlow::Continue(())` is returned from the function, the comparison moves on to the next
4044/// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
4045/// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
4046/// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
4047/// the iterators.
4048///
4049/// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
4050/// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
4051#[inline]
4052fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
4053where
4054    A: Iterator,
4055    B: Iterator,
4056    F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
4057{
4058    #[inline]
4059    fn compare<'a, B, X, T>(
4060        b: &'a mut B,
4061        mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
4062    ) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
4063    where
4064        B: Iterator,
4065    {
4066        move |x| match b.next() {
4067            None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
4068            Some(y) => f(x, y).map_break(ControlFlow::Break),
4069        }
4070    }
4071
4072    match a.try_for_each(compare(&mut b, f)) {
4073        ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
4074            None => Ordering::Equal,
4075            Some(_) => Ordering::Less,
4076        }),
4077        ControlFlow::Break(x) => x,
4078    }
4079}
4080
4081/// Implements `Iterator` for mutable references to iterators, such as those produced by [`Iterator::by_ref`].
4082///
4083/// This implementation passes all method calls on to the original iterator.
4084#[stable(feature = "rust1", since = "1.0.0")]
4085impl<I: Iterator + ?Sized> Iterator for &mut I {
4086    type Item = I::Item;
4087    #[inline]
4088    fn next(&mut self) -> Option<I::Item> {
4089        (**self).next()
4090    }
4091    fn size_hint(&self) -> (usize, Option<usize>) {
4092        (**self).size_hint()
4093    }
4094    fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
4095        (**self).advance_by(n)
4096    }
4097    fn nth(&mut self, n: usize) -> Option<Self::Item> {
4098        (**self).nth(n)
4099    }
4100    fn fold<B, F>(self, init: B, f: F) -> B
4101    where
4102        F: FnMut(B, Self::Item) -> B,
4103    {
4104        self.spec_fold(init, f)
4105    }
4106    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4107    where
4108        F: FnMut(B, Self::Item) -> R,
4109        R: Try<Output = B>,
4110    {
4111        self.spec_try_fold(init, f)
4112    }
4113}
4114
4115/// Helper trait to specialize `fold` and `try_fold` for `&mut I where I: Sized`
4116trait IteratorRefSpec: Iterator {
4117    fn spec_fold<B, F>(self, init: B, f: F) -> B
4118    where
4119        F: FnMut(B, Self::Item) -> B;
4120
4121    fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4122    where
4123        F: FnMut(B, Self::Item) -> R,
4124        R: Try<Output = B>;
4125}
4126
4127impl<I: Iterator + ?Sized> IteratorRefSpec for &mut I {
4128    default fn spec_fold<B, F>(self, init: B, mut f: F) -> B
4129    where
4130        F: FnMut(B, Self::Item) -> B,
4131    {
4132        let mut accum = init;
4133        while let Some(x) = self.next() {
4134            accum = f(accum, x);
4135        }
4136        accum
4137    }
4138
4139    default fn spec_try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
4140    where
4141        F: FnMut(B, Self::Item) -> R,
4142        R: Try<Output = B>,
4143    {
4144        let mut accum = init;
4145        while let Some(x) = self.next() {
4146            accum = f(accum, x)?;
4147        }
4148        try { accum }
4149    }
4150}
4151
4152impl<I: Iterator> IteratorRefSpec for &mut I {
4153    impl_fold_via_try_fold! { spec_fold -> spec_try_fold }
4154
4155    fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4156    where
4157        F: FnMut(B, Self::Item) -> R,
4158        R: Try<Output = B>,
4159    {
4160        (**self).try_fold(init, f)
4161    }
4162}