iterator.rs - source
Description: Source of the Rust file `library/core/src/iter/traits/iterator.rs`.

#
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 super::TrustedLen;
    8use crate::array;
    9use crate::cmp::{self, Ordering};
    10use crate::num::NonZero;
    11use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
    12
    13fn _assert_is_dyn_compatible(_: &dyn Iterator<Item = ()>) {}
    14
    15/// A trait for dealing with iterators.
    16///
    17/// This is the main iterator trait. For more about the concept of iterators
    18/// generally, please see the [module-level documentation]. In particular, you
    19/// may want to know how to [implement `Iterator`][impl].
    20///
    21/// [module-level documentation]: crate::iter
    22/// [impl]: crate::iter#implementing-iterator
    23#[stable(feature = "rust1", since = "1.0.0")]
    24#[rustc_on_unimplemented(
    25    on(
    26        Self = "core::ops::range::RangeTo<Idx>",
    27        note = "you might have meant to use a bounded `Range`"
    28    ),
    29    on(
    30        Self = "core::ops::range::RangeToInclusive<Idx>",
    31        note = "you might have meant to use a bounded `RangeInclusive`"
    32    ),
    33    label = "`{Self}` is not an iterator",
    34    message = "`{Self}` is not an iterator"
    35)]
    36#[doc(notable_trait)]
    37#[lang = "iterator"]
    38#[rustc_diagnostic_item = "Iterator"]
    39#[must_use = "iterators are lazy and do nothing unless consumed"]
    40pub trait Iterator {
    41    /// The type of the elements being iterated over.
    42    #[rustc_diagnostic_item = "IteratorItem"]
    43    #[stable(feature = "rust1", since = "1.0.0")]
    44    type Item;
    45
    46    /// Advances the iterator and returns the next value.
    47    ///
    48    /// Returns [`None`] when iteration is finished. Individual iterator
    49    /// implementations may choose to resume iteration, and so calling `next()`
    50    /// again may or may not eventually start returning [`Some(Item)`] again at some
    51    /// point.
    52    ///
    53    /// [`Some(Item)`]: Some
    54    ///
    55    /// # Examples
    56    ///
    57    /// ```
    58    /// let a = [1, 2, 3];
    59    ///
    60    /// let mut iter = a.into_iter();
    61    ///
    62    /// // A call to next() returns the next value...
    63    /// assert_eq!(Some(1), iter.next());
    64    /// assert_eq!(Some(2), iter.next());
    65    /// assert_eq!(Some(3), iter.next());
    66    ///
    67    /// // ... and then None once it's over.
    68    /// assert_eq!(None, iter.next());
    69    ///
    70    /// // More calls may or may not return `None`. Here, they always will.
    71    /// assert_eq!(None, iter.next());
    72    /// assert_eq!(None, iter.next());
    73    /// ```
    74    #[lang = "next"]
    75    #[stable(feature = "rust1", since = "1.0.0")]
    76    fn next(&mut self) -> Option<Self::Item>;
    77
    78    /// Advances the iterator and returns an array containing the next `N` values.
    79    ///
    80    /// If there are not enough elements to fill the array then `Err` is returned
    81    /// containing an iterator over the remaining elements.
    82    ///
    83    /// # Examples
    84    ///
    85    /// Basic usage:
    86    ///
    87    /// ```
    88    /// #![feature(iter_next_chunk)]
    89    ///
    90    /// let mut iter = "lorem".chars();
    91    ///
    92    /// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']);              // N is inferred as 2
    93    /// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']);         // N is inferred as 3
    94    /// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4
    95    /// ```
    96    ///
    97    /// Split a string and get the first three items.
    98    ///
    99    /// ```
    100    /// #![feature(iter_next_chunk)]
    101    ///
    102    /// let quote = "not all those who wander are lost";
    103    /// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap();
    104    /// assert_eq!(first, "not");
    105    /// assert_eq!(second, "all");
    106    /// assert_eq!(third, "those");
    107    /// ```
    108    #[inline]
    109    #[unstable(feature = "iter_next_chunk", issue = "98326")]
    110    fn next_chunk<const N: usize>(
    111        &mut self,
    112    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
    113    where
    114        Self: Sized,
    115    {
    116        array::iter_next_chunk(self)
    117    }
    118
    119    /// Returns the bounds on the remaining length of the iterator.
    120    ///
    121    /// Specifically, `size_hint()` returns a tuple where the first element
    122    /// is the lower bound, and the second element is the upper bound.
    123    ///
    124    /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
    125    /// A [`None`] here means that either there is no known upper bound, or the
    126    /// upper bound is larger than [`usize`].
    127    ///
    128    /// # Implementation notes
    129    ///
    130    /// It is not enforced that an iterator implementation yields the declared
    131    /// number of elements. A buggy iterator may yield less than the lower bound
    132    /// or more than the upper bound of elements.
    133    ///
    134    /// `size_hint()` is primarily intended to be used for optimizations such as
    135    /// reserving space for the elements of the iterator, but must not be
    136    /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
    137    /// implementation of `size_hint()` should not lead to memory safety
    138    /// violations.
    139    ///
    140    /// That said, the implementation should provide a correct estimation,
    141    /// because otherwise it would be a violation of the trait's protocol.
    142    ///
    143    /// The default implementation returns <code>(0, [None])</code> which is correct for any
    144    /// iterator.
    145    ///
    146    /// # Examples
    147    ///
    148    /// Basic usage:
    149    ///
    150    /// ```
    151    /// let a = [1, 2, 3];
    152    /// let mut iter = a.iter();
    153    ///
    154    /// assert_eq!((3, Some(3)), iter.size_hint());
    155    /// let _ = iter.next();
    156    /// assert_eq!((2, Some(2)), iter.size_hint());
    157    /// ```
    158    ///
    159    /// A more complex example:
    160    ///
    161    /// ```
    162    /// // The even numbers in the range of zero to nine.
    163    /// let iter = (0..10).filter(|x| x % 2 == 0);
    164    ///
    165    /// // We might iterate from zero to ten times. Knowing that it's five
    166    /// // exactly wouldn't be possible without executing filter().
    167    /// assert_eq!((0, Some(10)), iter.size_hint());
    168    ///
    169    /// // Let's add five more numbers with chain()
    170    /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
    171    ///
    172    /// // now both bounds are increased by five
    173    /// assert_eq!((5, Some(15)), iter.size_hint());
    174    /// ```
    175    ///
    176    /// Returning `None` for an upper bound:
    177    ///
    178    /// ```
    179    /// // an infinite iterator has no upper bound
    180    /// // and the maximum possible lower bound
    181    /// let iter = 0..;
    182    ///
    183    /// assert_eq!((usize::MAX, None), iter.size_hint());
    184    /// ```
    185    #[inline]
    186    #[stable(feature = "rust1", since = "1.0.0")]
    187    fn size_hint(&self) -> (usize, Option<usize>) {
    188        (0, None)
    189    }
    190
    191    /// Consumes the iterator, counting the number of iterations and returning it.
    192    ///
    193    /// This method will call [`next`] repeatedly until [`None`] is encountered,
    194    /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
    195    /// called at least once even if the iterator does not have any elements.
    196    ///
    197    /// [`next`]: Iterator::next
    198    ///
    199    /// # Overflow Behavior
    200    ///
    201    /// The method does no guarding against overflows, so counting elements of
    202    /// an iterator with more than [`usize::MAX`] elements either produces the
    203    /// wrong result or panics. If overflow checks are enabled, a panic is
    204    /// guaranteed.
    205    ///
    206    /// # Panics
    207    ///
    208    /// This function might panic if the iterator has more than [`usize::MAX`]
    209    /// elements.
    210    ///
    211    /// # Examples
    212    ///
    213    /// ```
    214    /// let a = [1, 2, 3];
    215    /// assert_eq!(a.iter().count(), 3);
    216    ///
    217    /// let a = [1, 2, 3, 4, 5];
    218    /// assert_eq!(a.iter().count(), 5);
    219    /// ```
    220    #[inline]
    221    #[stable(feature = "rust1", since = "1.0.0")]
    222    fn count(self) -> usize
    223    where
    224        Self: Sized,
    225    {
    226        self.fold(
    227            0,
    228            #[rustc_inherit_overflow_checks]
    229            |count, _| count + 1,
    230        )
    231    }
    232
    233    /// Consumes the iterator, returning the last element.
    234    ///
    235    /// This method will evaluate the iterator until it returns [`None`]. While
    236    /// doing so, it keeps track of the current element. After [`None`] is
    237    /// returned, `last()` will then return the last element it saw.
    238    ///
    239    /// # Panics
    240    ///
    241    /// This function might panic if the iterator is infinite.
    242    ///
    243    /// # Examples
    244    ///
    245    /// ```
    246    /// let a = [1, 2, 3];
    247    /// assert_eq!(a.into_iter().last(), Some(3));
    248    ///
    249    /// let a = [1, 2, 3, 4, 5];
    250    /// assert_eq!(a.into_iter().last(), Some(5));
    251    /// ```
    252    #[inline]
    253    #[stable(feature = "rust1", since = "1.0.0")]
    254    fn last(self) -> Option<Self::Item>
    255    where
    256        Self: Sized,
    257    {
    258        #[inline]
    259        fn some<T>(_: Option<T>, x: T) -> Option<T> {
    260            Some(x)
    261        }
    262
    263        self.fold(None, some)
    264    }
    265
    266    /// Advances the iterator by `n` elements.
    267    ///
    268    /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
    269    /// times until [`None`] is encountered.
    270    ///
    271    /// `advance_by(n)` will return `Ok(())` if the iterator successfully advances by
    272    /// `n` elements, or a `Err(NonZero<usize>)` with value `k` if [`None`] is encountered,
    273    /// where `k` is remaining number of steps that could not be advanced because the iterator ran out.
    274    /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
    275    /// Otherwise, `k` is always less than `n`.
    276    ///
    277    /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
    278    /// can advance its outer iterator until it finds an inner iterator that is not empty, which
    279    /// then often allows it to return a more accurate `size_hint()` than in its initial state.
    280    ///
    281    /// [`Flatten`]: crate::iter::Flatten
    282    /// [`next`]: Iterator::next
    283    ///
    284    /// # Examples
    285    ///
    286    /// ```
    287    /// #![feature(iter_advance_by)]
    288    ///
    289    /// use std::num::NonZero;
    290    ///
    291    /// let a = [1, 2, 3, 4];
    292    /// let mut iter = a.into_iter();
    293    ///
    294    /// assert_eq!(iter.advance_by(2), Ok(()));
    295    /// assert_eq!(iter.next(), Some(3));
    296    /// assert_eq!(iter.advance_by(0), Ok(()));
    297    /// assert_eq!(iter.advance_by(100), Err(NonZero::new(99).unwrap())); // only `4` was skipped
    298    /// ```
    299    #[inline]
    300    #[unstable(feature = "iter_advance_by", issue = "77404")]
    301    fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
    302        /// Helper trait to specialize `advance_by` via `try_fold` for `Sized` iterators.
    303        trait SpecAdvanceBy {
    304            fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>>;
    305        }
    306
    307        impl<I: Iterator + ?Sized> SpecAdvanceBy for I {
    308            default fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
    309                for i in 0..n {
    310                    if self.next().is_none() {
    311                        // SAFETY: `i` is always less than `n`.
    312                        return Err(unsafe { NonZero::new_unchecked(n - i) });
    313                    }
    314                }
    315                Ok(())
    316            }
    317        }
    318
    319        impl<I: Iterator> SpecAdvanceBy for I {
    320            fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
    321                let Some(n) = NonZero::new(n) else {
    322                    return Ok(());
    323                };
    324
    325                let res = self.try_fold(n, |n, _| NonZero::new(n.get() - 1));
    326
    327                match res {
    328                    None => Ok(()),
    329                    Some(n) => Err(n),
    330                }
    331            }
    332        }
    333
    334        self.spec_advance_by(n)
    335    }
    336
    337    /// Returns the `n`th element of the iterator.
    338    ///
    339    /// Like most indexing operations, the count starts from zero, so `nth(0)`
    340    /// returns the first value, `nth(1)` the second, and so on.
    341    ///
    342    /// Note that all preceding elements, as well as the returned element, will be
    343    /// consumed from the iterator. That means that the preceding elements will be
    344    /// discarded, and also that calling `nth(0)` multiple times on the same iterator
    345    /// will return different elements.
    346    ///
    347    /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
    348    /// iterator.
    349    ///
    350    /// # Examples
    351    ///
    352    /// Basic usage:
    353    ///
    354    /// ```
    355    /// let a = [1, 2, 3];
    356    /// assert_eq!(a.into_iter().nth(1), Some(2));
    357    /// ```
    358    ///
    359    /// Calling `nth()` multiple times doesn't rewind the iterator:
    360    ///
    361    /// ```
    362    /// let a = [1, 2, 3];
    363    ///
    364    /// let mut iter = a.into_iter();
    365    ///
    366    /// assert_eq!(iter.nth(1), Some(2));
    367    /// assert_eq!(iter.nth(1), None);
    368    /// ```
    369    ///
    370    /// Returning `None` if there are less than `n + 1` elements:
    371    ///
    372    /// ```
    373    /// let a = [1, 2, 3];
    374    /// assert_eq!(a.into_iter().nth(10), None);
    375    /// ```
    376    #[inline]
    377    #[stable(feature = "rust1", since = "1.0.0")]
    378    fn nth(&mut self, n: usize) -> Option<Self::Item> {
    379        self.advance_by(n).ok()?;
    380        self.next()
    381    }
    382
    383    /// Creates an iterator starting at the same point, but stepping by
    384    /// the given amount at each iteration.
    385    ///
    386    /// Note 1: The first element of the iterator will always be returned,
    387    /// regardless of the step given.
    388    ///
    389    /// Note 2: The time at which ignored elements are pulled is not fixed.
    390    /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
    391    /// `self.nth(step-1)`, …, but is also free to behave like the sequence
    392    /// `advance_n_and_return_first(&mut self, step)`,
    393    /// `advance_n_and_return_first(&mut self, step)`, …
    394    /// Which way is used may change for some iterators for performance reasons.
    395    /// The second way will advance the iterator earlier and may consume more items.
    396    ///
    397    /// `advance_n_and_return_first` is the equivalent of:
    398    /// ```
    399    /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
    400    /// where
    401    ///     I: Iterator,
    402    /// {
    403    ///     let next = iter.next();
    404    ///     if n > 1 {
    405    ///         iter.nth(n - 2);
    406    ///     }
    407    ///     next
    408    /// }
    409    /// ```
    410    ///
    411    /// # Panics
    412    ///
    413    /// The method will panic if the given step is `0`.
    414    ///
    415    /// # Examples
    416    ///
    417    /// ```
    418    /// let a = [0, 1, 2, 3, 4, 5];
    419    /// let mut iter = a.into_iter().step_by(2);
    420    ///
    421    /// assert_eq!(iter.next(), Some(0));
    422    /// assert_eq!(iter.next(), Some(2));
    423    /// assert_eq!(iter.next(), Some(4));
    424    /// assert_eq!(iter.next(), None);
    425    /// ```
    426    #[inline]
    427    #[stable(feature = "iterator_step_by", since = "1.28.0")]
    428    fn step_by(self, step: usize) -> StepBy<Self>
    429    where
    430        Self: Sized,
    431    {
    432        StepBy::new(self, step)
    433    }
    434
    435    /// Takes two iterators and creates a new iterator over both in sequence.
    436    ///
    437    /// `chain()` will return a new iterator which will first iterate over
    438    /// values from the first iterator and then over values from the second
    439    /// iterator.
    440    ///
    441    /// In other words, it links two iterators together, in a chain. 🔗
    442    ///
    443    /// [`once`] is commonly used to adapt a single value into a chain of
    444    /// other kinds of iteration.
    445    ///
    446    /// # Examples
    447    ///
    448    /// Basic usage:
    449    ///
    450    /// ```
    451    /// let s1 = "abc".chars();
    452    /// let s2 = "def".chars();
    453    ///
    454    /// let mut iter = s1.chain(s2);
    455    ///
    456    /// assert_eq!(iter.next(), Some('a'));
    457    /// assert_eq!(iter.next(), Some('b'));
    458    /// assert_eq!(iter.next(), Some('c'));
    459    /// assert_eq!(iter.next(), Some('d'));
    460    /// assert_eq!(iter.next(), Some('e'));
    461    /// assert_eq!(iter.next(), Some('f'));
    462    /// assert_eq!(iter.next(), None);
    463    /// ```
    464    ///
    465    /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
    466    /// anything that can be converted into an [`Iterator`], not just an
    467    /// [`Iterator`] itself. For example, arrays (`[T]`) implement
    468    /// [`IntoIterator`], and so can be passed to `chain()` directly:
    469    ///
    470    /// ```
    471    /// let a1 = [1, 2, 3];
    472    /// let a2 = [4, 5, 6];
    473    ///
    474    /// let mut iter = a1.into_iter().chain(a2);
    475    ///
    476    /// assert_eq!(iter.next(), Some(1));
    477    /// assert_eq!(iter.next(), Some(2));
    478    /// assert_eq!(iter.next(), Some(3));
    479    /// assert_eq!(iter.next(), Some(4));
    480    /// assert_eq!(iter.next(), Some(5));
    481    /// assert_eq!(iter.next(), Some(6));
    482    /// assert_eq!(iter.next(), None);
    483    /// ```
    484    ///
    485    /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
    486    ///
    487    /// ```
    488    /// #[cfg(windows)]
    489    /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
    490    ///     use std::os::windows::ffi::OsStrExt;
    491    ///     s.encode_wide().chain(std::iter::once(0)).collect()
    492    /// }
    493    /// ```
    494    ///
    495    /// [`once`]: crate::iter::once
    496    /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
    497    #[inline]
    498    #[stable(feature = "rust1", since = "1.0.0")]
    499    fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
    500    where
    501        Self: Sized,
    502        U: IntoIterator<Item = Self::Item>,
    503    {
    504        Chain::new(self, other.into_iter())
    505    }
    506
    507    /// 'Zips up' two iterators into a single iterator of pairs.
    508    ///
    509    /// `zip()` returns a new iterator that will iterate over two other
    510    /// iterators, returning a tuple where the first element comes from the
    511    /// first iterator, and the second element comes from the second iterator.
    512    ///
    513    /// In other words, it zips two iterators together, into a single one.
    514    ///
    515    /// If either iterator returns [`None`], [`next`] from the zipped iterator
    516    /// will return [`None`].
    517    /// If the zipped iterator has no more elements to return then each further attempt to advance
    518    /// it will first try to advance the first iterator at most one time and if it still yielded an item
    519    /// try to advance the second iterator at most one time.
    520    ///
    521    /// To 'undo' the result of zipping up two iterators, see [`unzip`].
    522    ///
    523    /// [`unzip`]: Iterator::unzip
    524    ///
    525    /// # Examples
    526    ///
    527    /// Basic usage:
    528    ///
    529    /// ```
    530    /// let s1 = "abc".chars();
    531    /// let s2 = "def".chars();
    532    ///
    533    /// let mut iter = s1.zip(s2);
    534    ///
    535    /// assert_eq!(iter.next(), Some(('a', 'd')));
    536    /// assert_eq!(iter.next(), Some(('b', 'e')));
    537    /// assert_eq!(iter.next(), Some(('c', 'f')));
    538    /// assert_eq!(iter.next(), None);
    539    /// ```
    540    ///
    541    /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
    542    /// anything that can be converted into an [`Iterator`], not just an
    543    /// [`Iterator`] itself. For example, arrays (`[T]`) implement
    544    /// [`IntoIterator`], and so can be passed to `zip()` directly:
    545    ///
    546    /// ```
    547    /// let a1 = [1, 2, 3];
    548    /// let a2 = [4, 5, 6];
    549    ///
    550    /// let mut iter = a1.into_iter().zip(a2);
    551    ///
    552    /// assert_eq!(iter.next(), Some((1, 4)));
    553    /// assert_eq!(iter.next(), Some((2, 5)));
    554    /// assert_eq!(iter.next(), Some((3, 6)));
    555    /// assert_eq!(iter.next(), None);
    556    /// ```
    557    ///
    558    /// `zip()` is often used to zip an infinite iterator to a finite one.
    559    /// This works because the finite iterator will eventually return [`None`],
    560    /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
    561    ///
    562    /// ```
    563    /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
    564    ///
    565    /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
    566    ///
    567    /// assert_eq!((0, 'f'), enumerate[0]);
    568    /// assert_eq!((0, 'f'), zipper[0]);
    569    ///
    570    /// assert_eq!((1, 'o'), enumerate[1]);
    571    /// assert_eq!((1, 'o'), zipper[1]);
    572    ///
    573    /// assert_eq!((2, 'o'), enumerate[2]);
    574    /// assert_eq!((2, 'o'), zipper[2]);
    575    /// ```
    576    ///
    577    /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
    578    ///
    579    /// ```
    580    /// use std::iter::zip;
    581    ///
    582    /// let a = [1, 2, 3];
    583    /// let b = [2, 3, 4];
    584    ///
    585    /// let mut zipped = zip(
    586    ///     a.into_iter().map(|x| x * 2).skip(1),
    587    ///     b.into_iter().map(|x| x * 2).skip(1),
    588    /// );
    589    ///
    590    /// assert_eq!(zipped.next(), Some((4, 6)));
    591    /// assert_eq!(zipped.next(), Some((6, 8)));
    592    /// assert_eq!(zipped.next(), None);
    593    /// ```
    594    ///
    595    /// compared to:
    596    ///
    597    /// ```
    598    /// # let a = [1, 2, 3];
    599    /// # let b = [2, 3, 4];
    600    /// #
    601    /// let mut zipped = a
    602    ///     .into_iter()
    603    ///     .map(|x| x * 2)
    604    ///     .skip(1)
    605    ///     .zip(b.into_iter().map(|x| x * 2).skip(1));
    606    /// #
    607    /// # assert_eq!(zipped.next(), Some((4, 6)));
    608    /// # assert_eq!(zipped.next(), Some((6, 8)));
    609    /// # assert_eq!(zipped.next(), None);
    610    /// ```
    611    ///
    612    /// [`enumerate`]: Iterator::enumerate
    613    /// [`next`]: Iterator::next
    614    /// [`zip`]: crate::iter::zip
    615    #[inline]
    616    #[stable(feature = "rust1", since = "1.0.0")]
    617    fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
    618    where
    619        Self: Sized,
    620        U: IntoIterator,
    621    {
    622        Zip::new(self, other.into_iter())
    623    }
    624
    625    /// Creates a new iterator which places a copy of `separator` between adjacent
    626    /// items of the original iterator.
    627    ///
    628    /// In case `separator` does not implement [`Clone`] or needs to be
    629    /// computed every time, use [`intersperse_with`].
    630    ///
    631    /// # Examples
    632    ///
    633    /// Basic usage:
    634    ///
    635    /// ```
    636    /// #![feature(iter_intersperse)]
    637    ///
    638    /// let mut a = [0, 1, 2].into_iter().intersperse(100);
    639    /// assert_eq!(a.next(), Some(0));   // The first element from `a`.
    640    /// assert_eq!(a.next(), Some(100)); // The separator.
    641    /// assert_eq!(a.next(), Some(1));   // The next element from `a`.
    642    /// assert_eq!(a.next(), Some(100)); // The separator.
    643    /// assert_eq!(a.next(), Some(2));   // The last element from `a`.
    644    /// assert_eq!(a.next(), None);       // The iterator is finished.
    645    /// ```
    646    ///
    647    /// `intersperse` can be very useful to join an iterator's items using a common element:
    648    /// ```
    649    /// #![feature(iter_intersperse)]
    650    ///
    651    /// let words = ["Hello", "World", "!"];
    652    /// let hello: String = words.into_iter().intersperse(" ").collect();
    653    /// assert_eq!(hello, "Hello World !");
    654    /// ```
    655    ///
    656    /// [`Clone`]: crate::clone::Clone
    657    /// [`intersperse_with`]: Iterator::intersperse_with
    658    #[inline]
    659    #[unstable(feature = "iter_intersperse", issue = "79524")]
    660    fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
    661    where
    662        Self: Sized,
    663        Self::Item: Clone,
    664    {
    665        Intersperse::new(self, separator)
    666    }
    667
    668    /// Creates a new iterator which places an item generated by `separator`
    669    /// between adjacent items of the original iterator.
    670    ///
    671    /// The closure will be called exactly once each time an item is placed
    672    /// between two adjacent items from the underlying iterator; specifically,
    673    /// the closure is not called if the underlying iterator yields less than
    674    /// two items and after the last item is yielded.
    675    ///
    676    /// If the iterator's item implements [`Clone`], it may be easier to use
    677    /// [`intersperse`].
    678    ///
    679    /// # Examples
    680    ///
    681    /// Basic usage:
    682    ///
    683    /// ```
    684    /// #![feature(iter_intersperse)]
    685    ///
    686    /// #[derive(PartialEq, Debug)]
    687    /// struct NotClone(usize);
    688    ///
    689    /// let v = [NotClone(0), NotClone(1), NotClone(2)];
    690    /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
    691    ///
    692    /// assert_eq!(it.next(), Some(NotClone(0)));  // The first element from `v`.
    693    /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
    694    /// assert_eq!(it.next(), Some(NotClone(1)));  // The next element from `v`.
    695    /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
    696    /// assert_eq!(it.next(), Some(NotClone(2)));  // The last element from `v`.
    697    /// assert_eq!(it.next(), None);               // The iterator is finished.
    698    /// ```
    699    ///
    700    /// `intersperse_with` can be used in situations where the separator needs
    701    /// to be computed:
    702    /// ```
    703    /// #![feature(iter_intersperse)]
    704    ///
    705    /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
    706    ///
    707    /// // The closure mutably borrows its context to generate an item.
    708    /// let mut happy_emojis = [" ❤️ ", " 😀 "].into_iter();
    709    /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
    710    ///
    711    /// let result = src.intersperse_with(separator).collect::<String>();
    712    /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
    713    /// ```
    714    /// [`Clone`]: crate::clone::Clone
    715    /// [`intersperse`]: Iterator::intersperse
    716    #[inline]
    717    #[unstable(feature = "iter_intersperse", issue = "79524")]
    718    fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
    719    where
    720        Self: Sized,
    721        G: FnMut() -> Self::Item,
    722    {
    723        IntersperseWith::new(self, separator)
    724    }
    725
    726    /// Takes a closure and creates an iterator which calls that closure on each
    727    /// element.
    728    ///
    729    /// `map()` transforms one iterator into another, by means of its argument:
    730    /// something that implements [`FnMut`]. It produces a new iterator which
    731    /// calls this closure on each element of the original iterator.
    732    ///
    733    /// If you are good at thinking in types, you can think of `map()` like this:
    734    /// If you have an iterator that gives you elements of some type `A`, and
    735    /// you want an iterator of some other type `B`, you can use `map()`,
    736    /// passing a closure that takes an `A` and returns a `B`.
    737    ///
    738    /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
    739    /// lazy, it is best used when you're already working with other iterators.
    740    /// If you're doing some sort of looping for a side effect, it's considered
    741    /// more idiomatic to use [`for`] than `map()`.
    742    ///
    743    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    744    ///
    745    /// # Examples
    746    ///
    747    /// Basic usage:
    748    ///
    749    /// ```
    750    /// let a = [1, 2, 3];
    751    ///
    752    /// let mut iter = a.iter().map(|x| 2 * x);
    753    ///
    754    /// assert_eq!(iter.next(), Some(2));
    755    /// assert_eq!(iter.next(), Some(4));
    756    /// assert_eq!(iter.next(), Some(6));
    757    /// assert_eq!(iter.next(), None);
    758    /// ```
    759    ///
    760    /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
    761    ///
    762    /// ```
    763    /// # #![allow(unused_must_use)]
    764    /// // don't do this:
    765    /// (0..5).map(|x| println!("{x}"));
    766    ///
    767    /// // it won't even execute, as it is lazy. Rust will warn you about this.
    768    ///
    769    /// // Instead, use a for-loop:
    770    /// for x in 0..5 {
    771    ///     println!("{x}");
    772    /// }
    773    /// ```
    774    #[rustc_diagnostic_item = "IteratorMap"]
    775    #[inline]
    776    #[stable(feature = "rust1", since = "1.0.0")]
    777    fn map<B, F>(self, f: F) -> Map<Self, F>
    778    where
    779        Self: Sized,
    780        F: FnMut(Self::Item) -> B,
    781    {
    782        Map::new(self, f)
    783    }
    784
    785    /// Calls a closure on each element of an iterator.
    786    ///
    787    /// This is equivalent to using a [`for`] loop on the iterator, although
    788    /// `break` and `continue` are not possible from a closure. It's generally
    789    /// more idiomatic to use a `for` loop, but `for_each` may be more legible
    790    /// when processing items at the end of longer iterator chains. In some
    791    /// cases `for_each` may also be faster than a loop, because it will use
    792    /// internal iteration on adapters like `Chain`.
    793    ///
    794    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    795    ///
    796    /// # Examples
    797    ///
    798    /// Basic usage:
    799    ///
    800    /// ```
    801    /// use std::sync::mpsc::channel;
    802    ///
    803    /// let (tx, rx) = channel();
    804    /// (0..5).map(|x| x * 2 + 1)
    805    ///       .for_each(move |x| tx.send(x).unwrap());
    806    ///
    807    /// let v: Vec<_> = rx.iter().collect();
    808    /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
    809    /// ```
    810    ///
    811    /// For such a small example, a `for` loop may be cleaner, but `for_each`
    812    /// might be preferable to keep a functional style with longer iterators:
    813    ///
    814    /// ```
    815    /// (0..5).flat_map(|x| (x * 100)..(x * 110))
    816    ///       .enumerate()
    817    ///       .filter(|&(i, x)| (i + x) % 3 == 0)
    818    ///       .for_each(|(i, x)| println!("{i}:{x}"));
    819    /// ```
    820    #[inline]
    821    #[stable(feature = "iterator_for_each", since = "1.21.0")]
    822    fn for_each<F>(self, f: F)
    823    where
    824        Self: Sized,
    825        F: FnMut(Self::Item),
    826    {
    827        #[inline]
    828        fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
    829            move |(), item| f(item)
    830        }
    831
    832        self.fold((), call(f));
    833    }
    834
    835    /// Creates an iterator which uses a closure to determine if an element
    836    /// should be yielded.
    837    ///
    838    /// Given an element the closure must return `true` or `false`. The returned
    839    /// iterator will yield only the elements for which the closure returns
    840    /// `true`.
    841    ///
    842    /// # Examples
    843    ///
    844    /// Basic usage:
    845    ///
    846    /// ```
    847    /// let a = [0i32, 1, 2];
    848    ///
    849    /// let mut iter = a.into_iter().filter(|x| x.is_positive());
    850    ///
    851    /// assert_eq!(iter.next(), Some(1));
    852    /// assert_eq!(iter.next(), Some(2));
    853    /// assert_eq!(iter.next(), None);
    854    /// ```
    855    ///
    856    /// Because the closure passed to `filter()` takes a reference, and many
    857    /// iterators iterate over references, this leads to a possibly confusing
    858    /// situation, where the type of the closure is a double reference:
    859    ///
    860    /// ```
    861    /// let s = &[0, 1, 2];
    862    ///
    863    /// let mut iter = s.iter().filter(|x| **x > 1); // needs two *s!
    864    ///
    865    /// assert_eq!(iter.next(), Some(&2));
    866    /// assert_eq!(iter.next(), None);
    867    /// ```
    868    ///
    869    /// It's common to instead use destructuring on the argument to strip away one:
    870    ///
    871    /// ```
    872    /// let s = &[0, 1, 2];
    873    ///
    874    /// let mut iter = s.iter().filter(|&x| *x > 1); // both & and *
    875    ///
    876    /// assert_eq!(iter.next(), Some(&2));
    877    /// assert_eq!(iter.next(), None);
    878    /// ```
    879    ///
    880    /// or both:
    881    ///
    882    /// ```
    883    /// let s = &[0, 1, 2];
    884    ///
    885    /// let mut iter = s.iter().filter(|&&x| x > 1); // two &s
    886    ///
    887    /// assert_eq!(iter.next(), Some(&2));
    888    /// assert_eq!(iter.next(), None);
    889    /// ```
    890    ///
    891    /// of these layers.
    892    ///
    893    /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
    894    #[inline]
    895    #[stable(feature = "rust1", since = "1.0.0")]
    896    #[rustc_diagnostic_item = "iter_filter"]
    897    fn filter<P>(self, predicate: P) -> Filter<Self, P>
    898    where
    899        Self: Sized,
    900        P: FnMut(&Self::Item) -> bool,
    901    {
    902        Filter::new(self, predicate)
    903    }
    904
    905    /// Creates an iterator that both filters and maps.
    906    ///
    907    /// The returned iterator yields only the `value`s for which the supplied
    908    /// closure returns `Some(value)`.
    909    ///
    910    /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
    911    /// concise. The example below shows how a `map().filter().map()` can be
    912    /// shortened to a single call to `filter_map`.
    913    ///
    914    /// [`filter`]: Iterator::filter
    915    /// [`map`]: Iterator::map
    916    ///
    917    /// # Examples
    918    ///
    919    /// Basic usage:
    920    ///
    921    /// ```
    922    /// let a = ["1", "two", "NaN", "four", "5"];
    923    ///
    924    /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
    925    ///
    926    /// assert_eq!(iter.next(), Some(1));
    927    /// assert_eq!(iter.next(), Some(5));
    928    /// assert_eq!(iter.next(), None);
    929    /// ```
    930    ///
    931    /// Here's the same example, but with [`filter`] and [`map`]:
    932    ///
    933    /// ```
    934    /// let a = ["1", "two", "NaN", "four", "5"];
    935    /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
    936    /// assert_eq!(iter.next(), Some(1));
    937    /// assert_eq!(iter.next(), Some(5));
    938    /// assert_eq!(iter.next(), None);
    939    /// ```
    940    #[inline]
    941    #[stable(feature = "rust1", since = "1.0.0")]
    942    fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
    943    where
    944        Self: Sized,
    945        F: FnMut(Self::Item) -> Option<B>,
    946    {
    947        FilterMap::new(self, f)
    948    }
    949
    950    /// Creates an iterator which gives the current iteration count as well as
    951    /// the next value.
    952    ///
    953    /// The iterator returned yields pairs `(i, val)`, where `i` is the
    954    /// current index of iteration and `val` is the value returned by the
    955    /// iterator.
    956    ///
    957    /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
    958    /// different sized integer, the [`zip`] function provides similar
    959    /// functionality.
    960    ///
    961    /// # Overflow Behavior
    962    ///
    963    /// The method does no guarding against overflows, so enumerating more than
    964    /// [`usize::MAX`] elements either produces the wrong result or panics. If
    965    /// overflow checks are enabled, a panic is guaranteed.
    966    ///
    967    /// # Panics
    968    ///
    969    /// The returned iterator might panic if the to-be-returned index would
    970    /// overflow a [`usize`].
    971    ///
    972    /// [`zip`]: Iterator::zip
    973    ///
    974    /// # Examples
    975    ///
    976    /// ```
    977    /// let a = ['a', 'b', 'c'];
    978    ///
    979    /// let mut iter = a.into_iter().enumerate();
    980    ///
    981    /// assert_eq!(iter.next(), Some((0, 'a')));
    982    /// assert_eq!(iter.next(), Some((1, 'b')));
    983    /// assert_eq!(iter.next(), Some((2, 'c')));
    984    /// assert_eq!(iter.next(), None);
    985    /// ```
    986    #[inline]
    987    #[stable(feature = "rust1", since = "1.0.0")]
    988    #[rustc_diagnostic_item = "enumerate_method"]
    989    fn enumerate(self) -> Enumerate<Self>
    990    where
    991        Self: Sized,
    992    {
    993        Enumerate::new(self)
    994    }
    995
    996    /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
    997    /// to look at the next element of the iterator without consuming it. See
    998    /// their documentation for more information.
    999    ///
    1000    /// Note that the underlying iterator is still advanced when [`peek`] or
    1001    /// [`peek_mut`] are called for the first time: In order to retrieve the
    1002    /// next element, [`next`] is called on the underlying iterator, hence any
    1003    /// side effects (i.e. anything other than fetching the next value) of
    1004    /// the [`next`] method will occur.
    1005    ///
    1006    ///
    1007    /// # Examples
    1008    ///
    1009    /// Basic usage:
    1010    ///
    1011    /// ```
    1012    /// let xs = [1, 2, 3];
    1013    ///
    1014    /// let mut iter = xs.into_iter().peekable();
    1015    ///
    1016    /// // peek() lets us see into the future
    1017    /// assert_eq!(iter.peek(), Some(&1));
    1018    /// assert_eq!(iter.next(), Some(1));
    1019    ///
    1020    /// assert_eq!(iter.next(), Some(2));
    1021    ///
    1022    /// // we can peek() multiple times, the iterator won't advance
    1023    /// assert_eq!(iter.peek(), Some(&3));
    1024    /// assert_eq!(iter.peek(), Some(&3));
    1025    ///
    1026    /// assert_eq!(iter.next(), Some(3));
    1027    ///
    1028    /// // after the iterator is finished, so is peek()
    1029    /// assert_eq!(iter.peek(), None);
    1030    /// assert_eq!(iter.next(), None);
    1031    /// ```
    1032    ///
    1033    /// Using [`peek_mut`] to mutate the next item without advancing the
    1034    /// iterator:
    1035    ///
    1036    /// ```
    1037    /// let xs = [1, 2, 3];
    1038    ///
    1039    /// let mut iter = xs.into_iter().peekable();
    1040    ///
    1041    /// // `peek_mut()` lets us see into the future
    1042    /// assert_eq!(iter.peek_mut(), Some(&mut 1));
    1043    /// assert_eq!(iter.peek_mut(), Some(&mut 1));
    1044    /// assert_eq!(iter.next(), Some(1));
    1045    ///
    1046    /// if let Some(p) = iter.peek_mut() {
    1047    ///     assert_eq!(*p, 2);
    1048    ///     // put a value into the iterator
    1049    ///     *p = 1000;
    1050    /// }
    1051    ///
    1052    /// // The value reappears as the iterator continues
    1053    /// assert_eq!(iter.collect::<Vec<_>>(), vec![1000, 3]);
    1054    /// ```
    1055    /// [`peek`]: Peekable::peek
    1056    /// [`peek_mut`]: Peekable::peek_mut
    1057    /// [`next`]: Iterator::next
    1058    #[inline]
    1059    #[stable(feature = "rust1", since = "1.0.0")]
    1060    fn peekable(self) -> Peekable<Self>
    1061    where
    1062        Self: Sized,
    1063    {
    1064        Peekable::new(self)
    1065    }
    1066
    1067    /// Creates an iterator that [`skip`]s elements based on a predicate.
    1068    ///
    1069    /// [`skip`]: Iterator::skip
    1070    ///
    1071    /// `skip_while()` takes a closure as an argument. It will call this
    1072    /// closure on each element of the iterator, and ignore elements
    1073    /// until it returns `false`.
    1074    ///
    1075    /// After `false` is returned, `skip_while()`'s job is over, and the
    1076    /// rest of the elements are yielded.
    1077    ///
    1078    /// # Examples
    1079    ///
    1080    /// Basic usage:
    1081    ///
    1082    /// ```
    1083    /// let a = [-1i32, 0, 1];
    1084    ///
    1085    /// let mut iter = a.into_iter().skip_while(|x| x.is_negative());
    1086    ///
    1087    /// assert_eq!(iter.next(), Some(0));
    1088    /// assert_eq!(iter.next(), Some(1));
    1089    /// assert_eq!(iter.next(), None);
    1090    /// ```
    1091    ///
    1092    /// Because the closure passed to `skip_while()` takes a reference, and many
    1093    /// iterators iterate over references, this leads to a possibly confusing
    1094    /// situation, where the type of the closure argument is a double reference:
    1095    ///
    1096    /// ```
    1097    /// let s = &[-1, 0, 1];
    1098    ///
    1099    /// let mut iter = s.iter().skip_while(|x| **x < 0); // need two *s!
    1100    ///
    1101    /// assert_eq!(iter.next(), Some(&0));
    1102    /// assert_eq!(iter.next(), Some(&1));
    1103    /// assert_eq!(iter.next(), None);
    1104    /// ```
    1105    ///
    1106    /// Stopping after an initial `false`:
    1107    ///
    1108    /// ```
    1109    /// let a = [-1, 0, 1, -2];
    1110    ///
    1111    /// let mut iter = a.into_iter().skip_while(|&x| x < 0);
    1112    ///
    1113    /// assert_eq!(iter.next(), Some(0));
    1114    /// assert_eq!(iter.next(), Some(1));
    1115    ///
    1116    /// // while this would have been false, since we already got a false,
    1117    /// // skip_while() isn't used any more
    1118    /// assert_eq!(iter.next(), Some(-2));
    1119    ///
    1120    /// assert_eq!(iter.next(), None);
    1121    /// ```
    1122    #[inline]
    1123    #[doc(alias = "drop_while")]
    1124    #[stable(feature = "rust1", since = "1.0.0")]
    1125    fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
    1126    where
    1127        Self: Sized,
    1128        P: FnMut(&Self::Item) -> bool,
    1129    {
    1130        SkipWhile::new(self, predicate)
    1131    }
    1132
    1133    /// Creates an iterator that yields elements based on a predicate.
    1134    ///
    1135    /// `take_while()` takes a closure as an argument. It will call this
    1136    /// closure on each element of the iterator, and yield elements
    1137    /// while it returns `true`.
    1138    ///
    1139    /// After `false` is returned, `take_while()`'s job is over, and the
    1140    /// rest of the elements are ignored.
    1141    ///
    1142    /// # Examples
    1143    ///
    1144    /// Basic usage:
    1145    ///
    1146    /// ```
    1147    /// let a = [-1i32, 0, 1];
    1148    ///
    1149    /// let mut iter = a.into_iter().take_while(|x| x.is_negative());
    1150    ///
    1151    /// assert_eq!(iter.next(), Some(-1));
    1152    /// assert_eq!(iter.next(), None);
    1153    /// ```
    1154    ///
    1155    /// Because the closure passed to `take_while()` takes a reference, and many
    1156    /// iterators iterate over references, this leads to a possibly confusing
    1157    /// situation, where the type of the closure is a double reference:
    1158    ///
    1159    /// ```
    1160    /// let s = &[-1, 0, 1];
    1161    ///
    1162    /// let mut iter = s.iter().take_while(|x| **x < 0); // need two *s!
    1163    ///
    1164    /// assert_eq!(iter.next(), Some(&-1));
    1165    /// assert_eq!(iter.next(), None);
    1166    /// ```
    1167    ///
    1168    /// Stopping after an initial `false`:
    1169    ///
    1170    /// ```
    1171    /// let a = [-1, 0, 1, -2];
    1172    ///
    1173    /// let mut iter = a.into_iter().take_while(|&x| x < 0);
    1174    ///
    1175    /// assert_eq!(iter.next(), Some(-1));
    1176    ///
    1177    /// // We have more elements that are less than zero, but since we already
    1178    /// // got a false, take_while() ignores the remaining elements.
    1179    /// assert_eq!(iter.next(), None);
    1180    /// ```
    1181    ///
    1182    /// Because `take_while()` needs to look at the value in order to see if it
    1183    /// should be included or not, consuming iterators will see that it is
    1184    /// removed:
    1185    ///
    1186    /// ```
    1187    /// let a = [1, 2, 3, 4];
    1188    /// let mut iter = a.into_iter();
    1189    ///
    1190    /// let result: Vec<i32> = iter.by_ref().take_while(|&n| n != 3).collect();
    1191    ///
    1192    /// assert_eq!(result, [1, 2]);
    1193    ///
    1194    /// let result: Vec<i32> = iter.collect();
    1195    ///
    1196    /// assert_eq!(result, [4]);
    1197    /// ```
    1198    ///
    1199    /// The `3` is no longer there, because it was consumed in order to see if
    1200    /// the iteration should stop, but wasn't placed back into the iterator.
    1201    #[inline]
    1202    #[stable(feature = "rust1", since = "1.0.0")]
    1203    fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
    1204    where
    1205        Self: Sized,
    1206        P: FnMut(&Self::Item) -> bool,
    1207    {
    1208        TakeWhile::new(self, predicate)
    1209    }
    1210
    1211    /// Creates an iterator that both yields elements based on a predicate and maps.
    1212    ///
    1213    /// `map_while()` takes a closure as an argument. It will call this
    1214    /// closure on each element of the iterator, and yield elements
    1215    /// while it returns [`Some(_)`][`Some`].
    1216    ///
    1217    /// # Examples
    1218    ///
    1219    /// Basic usage:
    1220    ///
    1221    /// ```
    1222    /// let a = [-1i32, 4, 0, 1];
    1223    ///
    1224    /// let mut iter = a.into_iter().map_while(|x| 16i32.checked_div(x));
    1225    ///
    1226    /// assert_eq!(iter.next(), Some(-16));
    1227    /// assert_eq!(iter.next(), Some(4));
    1228    /// assert_eq!(iter.next(), None);
    1229    /// ```
    1230    ///
    1231    /// Here's the same example, but with [`take_while`] and [`map`]:
    1232    ///
    1233    /// [`take_while`]: Iterator::take_while
    1234    /// [`map`]: Iterator::map
    1235    ///
    1236    /// ```
    1237    /// let a = [-1i32, 4, 0, 1];
    1238    ///
    1239    /// let mut iter = a.into_iter()
    1240    ///                 .map(|x| 16i32.checked_div(x))
    1241    ///                 .take_while(|x| x.is_some())
    1242    ///                 .map(|x| x.unwrap());
    1243    ///
    1244    /// assert_eq!(iter.next(), Some(-16));
    1245    /// assert_eq!(iter.next(), Some(4));
    1246    /// assert_eq!(iter.next(), None);
    1247    /// ```
    1248    ///
    1249    /// Stopping after an initial [`None`]:
    1250    ///
    1251    /// ```
    1252    /// let a = [0, 1, 2, -3, 4, 5, -6];
    1253    ///
    1254    /// let iter = a.into_iter().map_while(|x| u32::try_from(x).ok());
    1255    /// let vec: Vec<_> = iter.collect();
    1256    ///
    1257    /// // We have more elements that could fit in u32 (such as 4, 5), but `map_while` returned `None` for `-3`
    1258    /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
    1259    /// assert_eq!(vec, [0, 1, 2]);
    1260    /// ```
    1261    ///
    1262    /// Because `map_while()` needs to look at the value in order to see if it
    1263    /// should be included or not, consuming iterators will see that it is
    1264    /// removed:
    1265    ///
    1266    /// ```
    1267    /// let a = [1, 2, -3, 4];
    1268    /// let mut iter = a.into_iter();
    1269    ///
    1270    /// let result: Vec<u32> = iter.by_ref()
    1271    ///                            .map_while(|n| u32::try_from(n).ok())
    1272    ///                            .collect();
    1273    ///
    1274    /// assert_eq!(result, [1, 2]);
    1275    ///
    1276    /// let result: Vec<i32> = iter.collect();
    1277    ///
    1278    /// assert_eq!(result, [4]);
    1279    /// ```
    1280    ///
    1281    /// The `-3` is no longer there, because it was consumed in order to see if
    1282    /// the iteration should stop, but wasn't placed back into the iterator.
    1283    ///
    1284    /// Note that unlike [`take_while`] this iterator is **not** fused.
    1285    /// It is also not specified what this iterator returns after the first [`None`] is returned.
    1286    /// If you need a fused iterator, use [`fuse`].
    1287    ///
    1288    /// [`fuse`]: Iterator::fuse
    1289    #[inline]
    1290    #[stable(feature = "iter_map_while", since = "1.57.0")]
    1291    fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
    1292    where
    1293        Self: Sized,
    1294        P: FnMut(Self::Item) -> Option<B>,
    1295    {
    1296        MapWhile::new(self, predicate)
    1297    }
    1298
    1299    /// Creates an iterator that skips the first `n` elements.
    1300    ///
    1301    /// `skip(n)` skips elements until `n` elements are skipped or the end of the
    1302    /// iterator is reached (whichever happens first). After that, all the remaining
    1303    /// elements are yielded. In particular, if the original iterator is too short,
    1304    /// then the returned iterator is empty.
    1305    ///
    1306    /// Rather than overriding this method directly, instead override the `nth` method.
    1307    ///
    1308    /// # Examples
    1309    ///
    1310    /// ```
    1311    /// let a = [1, 2, 3];
    1312    ///
    1313    /// let mut iter = a.into_iter().skip(2);
    1314    ///
    1315    /// assert_eq!(iter.next(), Some(3));
    1316    /// assert_eq!(iter.next(), None);
    1317    /// ```
    1318    #[inline]
    1319    #[stable(feature = "rust1", since = "1.0.0")]
    1320    fn skip(self, n: usize) -> Skip<Self>
    1321    where
    1322        Self: Sized,
    1323    {
    1324        Skip::new(self, n)
    1325    }
    1326
    1327    /// Creates an iterator that yields the first `n` elements, or fewer
    1328    /// if the underlying iterator ends sooner.
    1329    ///
    1330    /// `take(n)` yields elements until `n` elements are yielded or the end of
    1331    /// the iterator is reached (whichever happens first).
    1332    /// The returned iterator is a prefix of length `n` if the original iterator
    1333    /// contains at least `n` elements, otherwise it contains all of the
    1334    /// (fewer than `n`) elements of the original iterator.
    1335    ///
    1336    /// # Examples
    1337    ///
    1338    /// Basic usage:
    1339    ///
    1340    /// ```
    1341    /// let a = [1, 2, 3];
    1342    ///
    1343    /// let mut iter = a.into_iter().take(2);
    1344    ///
    1345    /// assert_eq!(iter.next(), Some(1));
    1346    /// assert_eq!(iter.next(), Some(2));
    1347    /// assert_eq!(iter.next(), None);
    1348    /// ```
    1349    ///
    1350    /// `take()` is often used with an infinite iterator, to make it finite:
    1351    ///
    1352    /// ```
    1353    /// let mut iter = (0..).take(3);
    1354    ///
    1355    /// assert_eq!(iter.next(), Some(0));
    1356    /// assert_eq!(iter.next(), Some(1));
    1357    /// assert_eq!(iter.next(), Some(2));
    1358    /// assert_eq!(iter.next(), None);
    1359    /// ```
    1360    ///
    1361    /// If less than `n` elements are available,
    1362    /// `take` will limit itself to the size of the underlying iterator:
    1363    ///
    1364    /// ```
    1365    /// let v = [1, 2];
    1366    /// let mut iter = v.into_iter().take(5);
    1367    /// assert_eq!(iter.next(), Some(1));
    1368    /// assert_eq!(iter.next(), Some(2));
    1369    /// assert_eq!(iter.next(), None);
    1370    /// ```
    1371    ///
    1372    /// Use [`by_ref`] to take from the iterator without consuming it, and then
    1373    /// continue using the original iterator:
    1374    ///
    1375    /// ```
    1376    /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
    1377    ///
    1378    /// // Take the first two words.
    1379    /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
    1380    /// assert_eq!(hello_world, vec!["hello", "world"]);
    1381    ///
    1382    /// // Collect the rest of the words.
    1383    /// // We can only do this because we used `by_ref` earlier.
    1384    /// let of_rust: Vec<_> = words.collect();
    1385    /// assert_eq!(of_rust, vec!["of", "Rust"]);
    1386    /// ```
    1387    ///
    1388    /// [`by_ref`]: Iterator::by_ref
    1389    #[doc(alias = "limit")]
    1390    #[inline]
    1391    #[stable(feature = "rust1", since = "1.0.0")]
    1392    fn take(self, n: usize) -> Take<Self>
    1393    where
    1394        Self: Sized,
    1395    {
    1396        Take::new(self, n)
    1397    }
    1398
    1399    /// An iterator adapter which, like [`fold`], holds internal state, but
    1400    /// unlike [`fold`], produces a new iterator.
    1401    ///
    1402    /// [`fold`]: Iterator::fold
    1403    ///
    1404    /// `scan()` takes two arguments: an initial value which seeds the internal
    1405    /// state, and a closure with two arguments, the first being a mutable
    1406    /// reference to the internal state and the second an iterator element.
    1407    /// The closure can assign to the internal state to share state between
    1408    /// iterations.
    1409    ///
    1410    /// On iteration, the closure will be applied to each element of the
    1411    /// iterator and the return value from the closure, an [`Option`], is
    1412    /// returned by the `next` method. Thus the closure can return
    1413    /// `Some(value)` to yield `value`, or `None` to end the iteration.
    1414    ///
    1415    /// # Examples
    1416    ///
    1417    /// ```
    1418    /// let a = [1, 2, 3, 4];
    1419    ///
    1420    /// let mut iter = a.into_iter().scan(1, |state, x| {
    1421    ///     // each iteration, we'll multiply the state by the element ...
    1422    ///     *state = *state * x;
    1423    ///
    1424    ///     // ... and terminate if the state exceeds 6
    1425    ///     if *state > 6 {
    1426    ///         return None;
    1427    ///     }
    1428    ///     // ... else yield the negation of the state
    1429    ///     Some(-*state)
    1430    /// });
    1431    ///
    1432    /// assert_eq!(iter.next(), Some(-1));
    1433    /// assert_eq!(iter.next(), Some(-2));
    1434    /// assert_eq!(iter.next(), Some(-6));
    1435    /// assert_eq!(iter.next(), None);
    1436    /// ```
    1437    #[inline]
    1438    #[stable(feature = "rust1", since = "1.0.0")]
    1439    fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
    1440    where
    1441        Self: Sized,
    1442        F: FnMut(&mut St, Self::Item) -> Option<B>,
    1443    {
    1444        Scan::new(self, initial_state, f)
    1445    }
    1446
    1447    /// Creates an iterator that works like map, but flattens nested structure.
    1448    ///
    1449    /// The [`map`] adapter is very useful, but only when the closure
    1450    /// argument produces values. If it produces an iterator instead, there's
    1451    /// an extra layer of indirection. `flat_map()` will remove this extra layer
    1452    /// on its own.
    1453    ///
    1454    /// You can think of `flat_map(f)` as the semantic equivalent
    1455    /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
    1456    ///
    1457    /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
    1458    /// one item for each element, and `flat_map()`'s closure returns an
    1459    /// iterator for each element.
    1460    ///
    1461    /// [`map`]: Iterator::map
    1462    /// [`flatten`]: Iterator::flatten
    1463    ///
    1464    /// # Examples
    1465    ///
    1466    /// ```
    1467    /// let words = ["alpha", "beta", "gamma"];
    1468    ///
    1469    /// // chars() returns an iterator
    1470    /// let merged: String = words.iter()
    1471    ///                           .flat_map(|s| s.chars())
    1472    ///                           .collect();
    1473    /// assert_eq!(merged, "alphabetagamma");
    1474    /// ```
    1475    #[inline]
    1476    #[stable(feature = "rust1", since = "1.0.0")]
    1477    fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
    1478    where
    1479        Self: Sized,
    1480        U: IntoIterator,
    1481        F: FnMut(Self::Item) -> U,
    1482    {
    1483        FlatMap::new(self, f)
    1484    }
    1485
    1486    /// Creates an iterator that flattens nested structure.
    1487    ///
    1488    /// This is useful when you have an iterator of iterators or an iterator of
    1489    /// things that can be turned into iterators and you want to remove one
    1490    /// level of indirection.
    1491    ///
    1492    /// # Examples
    1493    ///
    1494    /// Basic usage:
    1495    ///
    1496    /// ```
    1497    /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
    1498    /// let flattened: Vec<_> = data.into_iter().flatten().collect();
    1499    /// assert_eq!(flattened, [1, 2, 3, 4, 5, 6]);
    1500    /// ```
    1501    ///
    1502    /// Mapping and then flattening:
    1503    ///
    1504    /// ```
    1505    /// let words = ["alpha", "beta", "gamma"];
    1506    ///
    1507    /// // chars() returns an iterator
    1508    /// let merged: String = words.iter()
    1509    ///                           .map(|s| s.chars())
    1510    ///                           .flatten()
    1511    ///                           .collect();
    1512    /// assert_eq!(merged, "alphabetagamma");
    1513    /// ```
    1514    ///
    1515    /// You can also rewrite this in terms of [`flat_map()`], which is preferable
    1516    /// in this case since it conveys intent more clearly:
    1517    ///
    1518    /// ```
    1519    /// let words = ["alpha", "beta", "gamma"];
    1520    ///
    1521    /// // chars() returns an iterator
    1522    /// let merged: String = words.iter()
    1523    ///                           .flat_map(|s| s.chars())
    1524    ///                           .collect();
    1525    /// assert_eq!(merged, "alphabetagamma");
    1526    /// ```
    1527    ///
    1528    /// Flattening works on any `IntoIterator` type, including `Option` and `Result`:
    1529    ///
    1530    /// ```
    1531    /// let options = vec![Some(123), Some(321), None, Some(231)];
    1532    /// let flattened_options: Vec<_> = options.into_iter().flatten().collect();
    1533    /// assert_eq!(flattened_options, [123, 321, 231]);
    1534    ///
    1535    /// let results = vec![Ok(123), Ok(321), Err(456), Ok(231)];
    1536    /// let flattened_results: Vec<_> = results.into_iter().flatten().collect();
    1537    /// assert_eq!(flattened_results, [123, 321, 231]);
    1538    /// ```
    1539    ///
    1540    /// Flattening only removes one level of nesting at a time:
    1541    ///
    1542    /// ```
    1543    /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
    1544    ///
    1545    /// let d2: Vec<_> = d3.into_iter().flatten().collect();
    1546    /// assert_eq!(d2, [[1, 2], [3, 4], [5, 6], [7, 8]]);
    1547    ///
    1548    /// let d1: Vec<_> = d3.into_iter().flatten().flatten().collect();
    1549    /// assert_eq!(d1, [1, 2, 3, 4, 5, 6, 7, 8]);
    1550    /// ```
    1551    ///
    1552    /// Here we see that `flatten()` does not perform a "deep" flatten.
    1553    /// Instead, only one level of nesting is removed. That is, if you
    1554    /// `flatten()` a three-dimensional array, the result will be
    1555    /// two-dimensional and not one-dimensional. To get a one-dimensional
    1556    /// structure, you have to `flatten()` again.
    1557    ///
    1558    /// [`flat_map()`]: Iterator::flat_map
    1559    #[inline]
    1560    #[stable(feature = "iterator_flatten", since = "1.29.0")]
    1561    fn flatten(self) -> Flatten<Self>
    1562    where
    1563        Self: Sized,
    1564        Self::Item: IntoIterator,
    1565    {
    1566        Flatten::new(self)
    1567    }
    1568
    1569    /// Calls the given function `f` for each contiguous window of size `N` over
    1570    /// `self` and returns an iterator over the outputs of `f`. Like [`slice::windows()`],
    1571    /// the windows during mapping overlap as well.
    1572    ///
    1573    /// In the following example, the closure is called three times with the
    1574    /// arguments `&['a', 'b']`, `&['b', 'c']` and `&['c', 'd']` respectively.
    1575    ///
    1576    /// ```
    1577    /// #![feature(iter_map_windows)]
    1578    ///
    1579    /// let strings = "abcd".chars()
    1580    ///     .map_windows(|[x, y]| format!("{}+{}", x, y))
    1581    ///     .collect::<Vec<String>>();
    1582    ///
    1583    /// assert_eq!(strings, vec!["a+b", "b+c", "c+d"]);
    1584    /// ```
    1585    ///
    1586    /// Note that the const parameter `N` is usually inferred by the
    1587    /// destructured argument in the closure.
    1588    ///
    1589    /// The returned iterator yields 𝑘 − `N` + 1 items (where 𝑘 is the number of
    1590    /// items yielded by `self`). If 𝑘 is less than `N`, this method yields an
    1591    /// empty iterator.
    1592    ///
    1593    /// The returned iterator implements [`FusedIterator`], because once `self`
    1594    /// returns `None`, even if it returns a `Some(T)` again in the next iterations,
    1595    /// we cannot put it into a contiguous array buffer, and thus the returned iterator
    1596    /// should be fused.
    1597    ///
    1598    /// [`slice::windows()`]: slice::windows
    1599    /// [`FusedIterator`]: crate::iter::FusedIterator
    1600    ///
    1601    /// # Panics
    1602    ///
    1603    /// Panics if `N` is zero. This check will most probably get changed to a
    1604    /// compile time error before this method gets stabilized.
    1605    ///
    1606    /// ```should_panic
    1607    /// #![feature(iter_map_windows)]
    1608    ///
    1609    /// let iter = std::iter::repeat(0).map_windows(|&[]| ());
    1610    /// ```
    1611    ///
    1612    /// # Examples
    1613    ///
    1614    /// Building the sums of neighboring numbers.
    1615    ///
    1616    /// ```
    1617    /// #![feature(iter_map_windows)]
    1618    ///
    1619    /// let mut it = [1, 3, 8, 1].iter().map_windows(|&[a, b]| a + b);
    1620    /// assert_eq!(it.next(), Some(4));  // 1 + 3
    1621    /// assert_eq!(it.next(), Some(11)); // 3 + 8
    1622    /// assert_eq!(it.next(), Some(9));  // 8 + 1
    1623    /// assert_eq!(it.next(), None);
    1624    /// ```
    1625    ///
    1626    /// Since the elements in the following example implement `Copy`, we can
    1627    /// just copy the array and get an iterator over the windows.
    1628    ///
    1629    /// ```
    1630    /// #![feature(iter_map_windows)]
    1631    ///
    1632    /// let mut it = "ferris".chars().map_windows(|w: &[_; 3]| *w);
    1633    /// assert_eq!(it.next(), Some(['f', 'e', 'r']));
    1634    /// assert_eq!(it.next(), Some(['e', 'r', 'r']));
    1635    /// assert_eq!(it.next(), Some(['r', 'r', 'i']));
    1636    /// assert_eq!(it.next(), Some(['r', 'i', 's']));
    1637    /// assert_eq!(it.next(), None);
    1638    /// ```
    1639    ///
    1640    /// You can also use this function to check the sortedness of an iterator.
    1641    /// For the simple case, rather use [`Iterator::is_sorted`].
    1642    ///
    1643    /// ```
    1644    /// #![feature(iter_map_windows)]
    1645    ///
    1646    /// let mut it = [0.5, 1.0, 3.5, 3.0, 8.5, 8.5, f32::NAN].iter()
    1647    ///     .map_windows(|[a, b]| a <= b);
    1648    ///
    1649    /// assert_eq!(it.next(), Some(true));  // 0.5 <= 1.0
    1650    /// assert_eq!(it.next(), Some(true));  // 1.0 <= 3.5
    1651    /// assert_eq!(it.next(), Some(false)); // 3.5 <= 3.0
    1652    /// assert_eq!(it.next(), Some(true));  // 3.0 <= 8.5
    1653    /// assert_eq!(it.next(), Some(true));  // 8.5 <= 8.5
    1654    /// assert_eq!(it.next(), Some(false)); // 8.5 <= NAN
    1655    /// assert_eq!(it.next(), None);
    1656    /// ```
    1657    ///
    1658    /// For non-fused iterators, they are fused after `map_windows`.
    1659    ///
    1660    /// ```
    1661    /// #![feature(iter_map_windows)]
    1662    ///
    1663    /// #[derive(Default)]
    1664    /// struct NonFusedIterator {
    1665    ///     state: i32,
    1666    /// }
    1667    ///
    1668    /// impl Iterator for NonFusedIterator {
    1669    ///     type Item = i32;
    1670    ///
    1671    ///     fn next(&mut self) -> Option<i32> {
    1672    ///         let val = self.state;
    1673    ///         self.state = self.state + 1;
    1674    ///
    1675    ///         // yields `0..5` first, then only even numbers since `6..`.
    1676    ///         if val < 5 || val % 2 == 0 {
    1677    ///             Some(val)
    1678    ///         } else {
    1679    ///             None
    1680    ///         }
    1681    ///     }
    1682    /// }
    1683    ///
    1684    ///
    1685    /// let mut iter = NonFusedIterator::default();
    1686    ///
    1687    /// // yields 0..5 first.
    1688    /// assert_eq!(iter.next(), Some(0));
    1689    /// assert_eq!(iter.next(), Some(1));
    1690    /// assert_eq!(iter.next(), Some(2));
    1691    /// assert_eq!(iter.next(), Some(3));
    1692    /// assert_eq!(iter.next(), Some(4));
    1693    /// // then we can see our iterator going back and forth
    1694    /// assert_eq!(iter.next(), None);
    1695    /// assert_eq!(iter.next(), Some(6));
    1696    /// assert_eq!(iter.next(), None);
    1697    /// assert_eq!(iter.next(), Some(8));
    1698    /// assert_eq!(iter.next(), None);
    1699    ///
    1700    /// // however, with `.map_windows()`, it is fused.
    1701    /// let mut iter = NonFusedIterator::default()
    1702    ///     .map_windows(|arr: &[_; 2]| *arr);
    1703    ///
    1704    /// assert_eq!(iter.next(), Some([0, 1]));
    1705    /// assert_eq!(iter.next(), Some([1, 2]));
    1706    /// assert_eq!(iter.next(), Some([2, 3]));
    1707    /// assert_eq!(iter.next(), Some([3, 4]));
    1708    /// assert_eq!(iter.next(), None);
    1709    ///
    1710    /// // it will always return `None` after the first time.
    1711    /// assert_eq!(iter.next(), None);
    1712    /// assert_eq!(iter.next(), None);
    1713    /// assert_eq!(iter.next(), None);
    1714    /// ```
    1715    #[inline]
    1716    #[unstable(feature = "iter_map_windows", issue = "87155")]
    1717    fn map_windows<F, R, const N: usize>(self, f: F) -> MapWindows<Self, F, N>
    1718    where
    1719        Self: Sized,
    1720        F: FnMut(&[Self::Item; N]) -> R,
    1721    {
    1722        MapWindows::new(self, f)
    1723    }
    1724
    1725    /// Creates an iterator which ends after the first [`None`].
    1726    ///
    1727    /// After an iterator returns [`None`], future calls may or may not yield
    1728    /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
    1729    /// [`None`] is given, it will always return [`None`] forever.
    1730    ///
    1731    /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
    1732    /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
    1733    /// if the [`FusedIterator`] trait is improperly implemented.
    1734    ///
    1735    /// [`Some(T)`]: Some
    1736    /// [`FusedIterator`]: crate::iter::FusedIterator
    1737    ///
    1738    /// # Examples
    1739    ///
    1740    /// ```
    1741    /// // an iterator which alternates between Some and None
    1742    /// struct Alternate {
    1743    ///     state: i32,
    1744    /// }
    1745    ///
    1746    /// impl Iterator for Alternate {
    1747    ///     type Item = i32;
    1748    ///
    1749    ///     fn next(&mut self) -> Option<i32> {
    1750    ///         let val = self.state;
    1751    ///         self.state = self.state + 1;
    1752    ///
    1753    ///         // if it's even, Some(i32), else None
    1754    ///         (val % 2 == 0).then_some(val)
    1755    ///     }
    1756    /// }
    1757    ///
    1758    /// let mut iter = Alternate { state: 0 };
    1759    ///
    1760    /// // we can see our iterator going back and forth
    1761    /// assert_eq!(iter.next(), Some(0));
    1762    /// assert_eq!(iter.next(), None);
    1763    /// assert_eq!(iter.next(), Some(2));
    1764    /// assert_eq!(iter.next(), None);
    1765    ///
    1766    /// // however, once we fuse it...
    1767    /// let mut iter = iter.fuse();
    1768    ///
    1769    /// assert_eq!(iter.next(), Some(4));
    1770    /// assert_eq!(iter.next(), None);
    1771    ///
    1772    /// // it will always return `None` after the first time.
    1773    /// assert_eq!(iter.next(), None);
    1774    /// assert_eq!(iter.next(), None);
    1775    /// assert_eq!(iter.next(), None);
    1776    /// ```
    1777    #[inline]
    1778    #[stable(feature = "rust1", since = "1.0.0")]
    1779    fn fuse(self) -> Fuse<Self>
    1780    where
    1781        Self: Sized,
    1782    {
    1783        Fuse::new(self)
    1784    }
    1785
    1786    /// Does something with each element of an iterator, passing the value on.
    1787    ///
    1788    /// When using iterators, you'll often chain several of them together.
    1789    /// While working on such code, you might want to check out what's
    1790    /// happening at various parts in the pipeline. To do that, insert
    1791    /// a call to `inspect()`.
    1792    ///
    1793    /// It's more common for `inspect()` to be used as a debugging tool than to
    1794    /// exist in your final code, but applications may find it useful in certain
    1795    /// situations when errors need to be logged before being discarded.
    1796    ///
    1797    /// # Examples
    1798    ///
    1799    /// Basic usage:
    1800    ///
    1801    /// ```
    1802    /// let a = [1, 4, 2, 3];
    1803    ///
    1804    /// // this iterator sequence is complex.
    1805    /// let sum = a.iter()
    1806    ///     .cloned()
    1807    ///     .filter(|x| x % 2 == 0)
    1808    ///     .fold(0, |sum, i| sum + i);
    1809    ///
    1810    /// println!("{sum}");
    1811    ///
    1812    /// // let's add some inspect() calls to investigate what's happening
    1813    /// let sum = a.iter()
    1814    ///     .cloned()
    1815    ///     .inspect(|x| println!("about to filter: {x}"))
    1816    ///     .filter(|x| x % 2 == 0)
    1817    ///     .inspect(|x| println!("made it through filter: {x}"))
    1818    ///     .fold(0, |sum, i| sum + i);
    1819    ///
    1820    /// println!("{sum}");
    1821    /// ```
    1822    ///
    1823    /// This will print:
    1824    ///
    1825    /// ```text
    1826    /// 6
    1827    /// about to filter: 1
    1828    /// about to filter: 4
    1829    /// made it through filter: 4
    1830    /// about to filter: 2
    1831    /// made it through filter: 2
    1832    /// about to filter: 3
    1833    /// 6
    1834    /// ```
    1835    ///
    1836    /// Logging errors before discarding them:
    1837    ///
    1838    /// ```
    1839    /// let lines = ["1", "2", "a"];
    1840    ///
    1841    /// let sum: i32 = lines
    1842    ///     .iter()
    1843    ///     .map(|line| line.parse::<i32>())
    1844    ///     .inspect(|num| {
    1845    ///         if let Err(ref e) = *num {
    1846    ///             println!("Parsing error: {e}");
    1847    ///         }
    1848    ///     })
    1849    ///     .filter_map(Result::ok)
    1850    ///     .sum();
    1851    ///
    1852    /// println!("Sum: {sum}");
    1853    /// ```
    1854    ///
    1855    /// This will print:
    1856    ///
    1857    /// ```text
    1858    /// Parsing error: invalid digit found in string
    1859    /// Sum: 3
    1860    /// ```
    1861    #[inline]
    1862    #[stable(feature = "rust1", since = "1.0.0")]
    1863    fn inspect<F>(self, f: F) -> Inspect<Self, F>
    1864    where
    1865        Self: Sized,
    1866        F: FnMut(&Self::Item),
    1867    {
    1868        Inspect::new(self, f)
    1869    }
    1870
    1871    /// Creates a "by reference" adapter for this instance of `Iterator`.
    1872    ///
    1873    /// Consuming method calls (direct or indirect calls to `next`)
    1874    /// on the "by reference" adapter will consume the original iterator,
    1875    /// but ownership-taking methods (those with a `self` parameter)
    1876    /// only take ownership of the "by reference" iterator.
    1877    ///
    1878    /// This is useful for applying ownership-taking methods
    1879    /// (such as `take` in the example below)
    1880    /// without giving up ownership of the original iterator,
    1881    /// so you can use the original iterator afterwards.
    1882    ///
    1883    /// 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).
    1884    ///
    1885    /// # Examples
    1886    ///
    1887    /// ```
    1888    /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
    1889    ///
    1890    /// // Take the first two words.
    1891    /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
    1892    /// assert_eq!(hello_world, vec!["hello", "world"]);
    1893    ///
    1894    /// // Collect the rest of the words.
    1895    /// // We can only do this because we used `by_ref` earlier.
    1896    /// let of_rust: Vec<_> = words.collect();
    1897    /// assert_eq!(of_rust, vec!["of", "Rust"]);
    1898    /// ```
    1899    #[stable(feature = "rust1", since = "1.0.0")]
    1900    fn by_ref(&mut self) -> &mut Self
    1901    where
    1902        Self: Sized,
    1903    {
    1904        self
    1905    }
    1906
    1907    /// Transforms an iterator into a collection.
    1908    ///
    1909    /// `collect()` takes ownership of an iterator and produces whichever
    1910    /// collection type you request. The iterator itself carries no knowledge of
    1911    /// the eventual container; the target collection is chosen entirely by the
    1912    /// type you ask `collect()` to return. This makes `collect()` one of the
    1913    /// more powerful methods in the standard library, and it shows up in a wide
    1914    /// variety of contexts.
    1915    ///
    1916    /// The most basic pattern in which `collect()` is used is to turn one
    1917    /// collection into another. You take a collection, call [`iter`] on it,
    1918    /// do a bunch of transformations, and then `collect()` at the end.
    1919    ///
    1920    /// `collect()` can also create instances of types that are not typical
    1921    /// collections. For example, a [`String`] can be built from [`char`]s,
    1922    /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
    1923    /// into `Result<Collection<T>, E>`. See the examples below for more.
    1924    ///
    1925    /// Because `collect()` is so general, it can cause problems with type
    1926    /// inference. As such, `collect()` is one of the few times you'll see
    1927    /// the syntax affectionately known as the 'turbofish': `::<>`. This
    1928    /// helps the inference algorithm understand specifically which collection
    1929    /// you're trying to collect into.
    1930    ///
    1931    /// # Examples
    1932    ///
    1933    /// Basic usage:
    1934    ///
    1935    /// ```
    1936    /// let a = [1, 2, 3];
    1937    ///
    1938    /// let doubled: Vec<i32> = a.iter()
    1939    ///                          .map(|x| x * 2)
    1940    ///                          .collect();
    1941    ///
    1942    /// assert_eq!(vec![2, 4, 6], doubled);
    1943    /// ```
    1944    ///
    1945    /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
    1946    /// we could collect into, for example, a [`VecDeque<T>`] instead:
    1947    ///
    1948    /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
    1949    ///
    1950    /// ```
    1951    /// use std::collections::VecDeque;
    1952    ///
    1953    /// let a = [1, 2, 3];
    1954    ///
    1955    /// let doubled: VecDeque<i32> = a.iter().map(|x| x * 2).collect();
    1956    ///
    1957    /// assert_eq!(2, doubled[0]);
    1958    /// assert_eq!(4, doubled[1]);
    1959    /// assert_eq!(6, doubled[2]);
    1960    /// ```
    1961    ///
    1962    /// Using the 'turbofish' instead of annotating `doubled`:
    1963    ///
    1964    /// ```
    1965    /// let a = [1, 2, 3];
    1966    ///
    1967    /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
    1968    ///
    1969    /// assert_eq!(vec![2, 4, 6], doubled);
    1970    /// ```
    1971    ///
    1972    /// Because `collect()` only cares about what you're collecting into, you can
    1973    /// still use a partial type hint, `_`, with the turbofish:
    1974    ///
    1975    /// ```
    1976    /// let a = [1, 2, 3];
    1977    ///
    1978    /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
    1979    ///
    1980    /// assert_eq!(vec![2, 4, 6], doubled);
    1981    /// ```
    1982    ///
    1983    /// Using `collect()` to make a [`String`]:
    1984    ///
    1985    /// ```
    1986    /// let chars = ['g', 'd', 'k', 'k', 'n'];
    1987    ///
    1988    /// let hello: String = chars.into_iter()
    1989    ///     .map(|x| x as u8)
    1990    ///     .map(|x| (x + 1) as char)
    1991    ///     .collect();
    1992    ///
    1993    /// assert_eq!("hello", hello);
    1994    /// ```
    1995    ///
    1996    /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
    1997    /// see if any of them failed:
    1998    ///
    1999    /// ```
    2000    /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
    2001    ///
    2002    /// let result: Result<Vec<_>, &str> = results.into_iter().collect();
    2003    ///
    2004    /// // gives us the first error
    2005    /// assert_eq!(Err("nope"), result);
    2006    ///
    2007    /// let results = [Ok(1), Ok(3)];
    2008    ///
    2009    /// let result: Result<Vec<_>, &str> = results.into_iter().collect();
    2010    ///
    2011    /// // gives us the list of answers
    2012    /// assert_eq!(Ok(vec![1, 3]), result);
    2013    /// ```
    2014    ///
    2015    /// [`iter`]: Iterator::next
    2016    /// [`String`]: ../../std/string/struct.String.html
    2017    /// [`char`]: type@char
    2018    #[inline]
    2019    #[stable(feature = "rust1", since = "1.0.0")]
    2020    #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
    2021    #[rustc_diagnostic_item = "iterator_collect_fn"]
    2022    fn collect<B: FromIterator<Self::Item>>(self) -> B
    2023    where
    2024        Self: Sized,
    2025    {
    2026        // This is too aggressive to turn on for everything all the time, but PR#137908
    2027        // accidentally noticed that some rustc iterators had malformed `size_hint`s,
    2028        // so this will help catch such things in debug-assertions-std runners,
    2029        // even if users won't actually ever see it.
    2030        if cfg!(debug_assertions) {
    2031            let hint = self.size_hint();
    2032            assert!(hint.1.is_none_or(|high| high >= hint.0), "Malformed size_hint {hint:?}");
    2033        }
    2034
    2035        FromIterator::from_iter(self)
    2036    }
    2037
    2038    /// Fallibly transforms an iterator into a collection, short circuiting if
    2039    /// a failure is encountered.
    2040    ///
    2041    /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
    2042    /// conversions during collection. Its main use case is simplifying conversions from
    2043    /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
    2044    /// types (e.g. [`Result`]).
    2045    ///
    2046    /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
    2047    /// only the inner type produced on `Try::Output` must implement it. Concretely,
    2048    /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
    2049    /// [`FromIterator`], even though [`ControlFlow`] doesn't.
    2050    ///
    2051    /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
    2052    /// may continue to be used, in which case it will continue iterating starting after the element that
    2053    /// triggered the failure. See the last example below for an example of how this works.
    2054    ///
    2055    /// # Examples
    2056    /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
    2057    /// ```
    2058    /// #![feature(iterator_try_collect)]
    2059    ///
    2060    /// let u = vec![Some(1), Some(2), Some(3)];
    2061    /// let v = u.into_iter().try_collect::<Vec<i32>>();
    2062    /// assert_eq!(v, Some(vec![1, 2, 3]));
    2063    /// ```
    2064    ///
    2065    /// Failing to collect in the same way:
    2066    /// ```
    2067    /// #![feature(iterator_try_collect)]
    2068    ///
    2069    /// let u = vec![Some(1), Some(2), None, Some(3)];
    2070    /// let v = u.into_iter().try_collect::<Vec<i32>>();
    2071    /// assert_eq!(v, None);
    2072    /// ```
    2073    ///
    2074    /// A similar example, but with `Result`:
    2075    /// ```
    2076    /// #![feature(iterator_try_collect)]
    2077    ///
    2078    /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
    2079    /// let v = u.into_iter().try_collect::<Vec<i32>>();
    2080    /// assert_eq!(v, Ok(vec![1, 2, 3]));
    2081    ///
    2082    /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
    2083    /// let v = u.into_iter().try_collect::<Vec<i32>>();
    2084    /// assert_eq!(v, Err(()));
    2085    /// ```
    2086    ///
    2087    /// Finally, even [`ControlFlow`] works, despite the fact that it
    2088    /// doesn't implement [`FromIterator`]. Note also that the iterator can
    2089    /// continue to be used, even if a failure is encountered:
    2090    ///
    2091    /// ```
    2092    /// #![feature(iterator_try_collect)]
    2093    ///
    2094    /// use core::ops::ControlFlow::{Break, Continue};
    2095    ///
    2096    /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
    2097    /// let mut it = u.into_iter();
    2098    ///
    2099    /// let v = it.try_collect::<Vec<_>>();
    2100    /// assert_eq!(v, Break(3));
    2101    ///
    2102    /// let v = it.try_collect::<Vec<_>>();
    2103    /// assert_eq!(v, Continue(vec![4, 5]));
    2104    /// ```
    2105    ///
    2106    /// [`collect`]: Iterator::collect
    2107    #[inline]
    2108    #[unstable(feature = "iterator_try_collect", issue = "94047")]
    2109    fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
    2110    where
    2111        Self: Sized,
    2112        Self::Item: Try<Residual: Residual<B>>,
    2113        B: FromIterator<<Self::Item as Try>::Output>,
    2114    {
    2115        try_process(ByRefSized(self), |i| i.collect())
    2116    }
    2117
    2118    /// Collects all the items from an iterator into a collection.
    2119    ///
    2120    /// This method consumes the iterator and adds all its items to the
    2121    /// passed collection. The collection is then returned, so the call chain
    2122    /// can be continued.
    2123    ///
    2124    /// This is useful when you already have a collection and want to add
    2125    /// the iterator items to it.
    2126    ///
    2127    /// This method is a convenience method to call [Extend::extend](trait.Extend.html),
    2128    /// but instead of being called on a collection, it's called on an iterator.
    2129    ///
    2130    /// # Examples
    2131    ///
    2132    /// Basic usage:
    2133    ///
    2134    /// ```
    2135    /// #![feature(iter_collect_into)]
    2136    ///
    2137    /// let a = [1, 2, 3];
    2138    /// let mut vec: Vec::<i32> = vec![0, 1];
    2139    ///
    2140    /// a.iter().map(|x| x * 2).collect_into(&mut vec);
    2141    /// a.iter().map(|x| x * 10).collect_into(&mut vec);
    2142    ///
    2143    /// assert_eq!(vec, vec![0, 1, 2, 4, 6, 10, 20, 30]);
    2144    /// ```
    2145    ///
    2146    /// `Vec` can have a manual set capacity to avoid reallocating it:
    2147    ///
    2148    /// ```
    2149    /// #![feature(iter_collect_into)]
    2150    ///
    2151    /// let a = [1, 2, 3];
    2152    /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
    2153    ///
    2154    /// a.iter().map(|x| x * 2).collect_into(&mut vec);
    2155    /// a.iter().map(|x| x * 10).collect_into(&mut vec);
    2156    ///
    2157    /// assert_eq!(6, vec.capacity());
    2158    /// assert_eq!(vec, vec![2, 4, 6, 10, 20, 30]);
    2159    /// ```
    2160    ///
    2161    /// The returned mutable reference can be used to continue the call chain:
    2162    ///
    2163    /// ```
    2164    /// #![feature(iter_collect_into)]
    2165    ///
    2166    /// let a = [1, 2, 3];
    2167    /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
    2168    ///
    2169    /// let count = a.iter().collect_into(&mut vec).iter().count();
    2170    ///
    2171    /// assert_eq!(count, vec.len());
    2172    /// assert_eq!(vec, vec![1, 2, 3]);
    2173    ///
    2174    /// let count = a.iter().collect_into(&mut vec).iter().count();
    2175    ///
    2176    /// assert_eq!(count, vec.len());
    2177    /// assert_eq!(vec, vec![1, 2, 3, 1, 2, 3]);
    2178    /// ```
    2179    #[inline]
    2180    #[unstable(feature = "iter_collect_into", issue = "94780")]
    2181    fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
    2182    where
    2183        Self: Sized,
    2184    {
    2185        collection.extend(self);
    2186        collection
    2187    }
    2188
    2189    /// Consumes an iterator, creating two collections from it.
    2190    ///
    2191    /// The predicate passed to `partition()` can return `true`, or `false`.
    2192    /// `partition()` returns a pair, all of the elements for which it returned
    2193    /// `true`, and all of the elements for which it returned `false`.
    2194    ///
    2195    /// See also [`is_partitioned()`] and [`partition_in_place()`].
    2196    ///
    2197    /// [`is_partitioned()`]: Iterator::is_partitioned
    2198    /// [`partition_in_place()`]: Iterator::partition_in_place
    2199    ///
    2200    /// # Examples
    2201    ///
    2202    /// ```
    2203    /// let a = [1, 2, 3];
    2204    ///
    2205    /// let (even, odd): (Vec<_>, Vec<_>) = a
    2206    ///     .into_iter()
    2207    ///     .partition(|n| n % 2 == 0);
    2208    ///
    2209    /// assert_eq!(even, [2]);
    2210    /// assert_eq!(odd, [1, 3]);
    2211    /// ```
    2212    #[stable(feature = "rust1", since = "1.0.0")]
    2213    fn partition<B, F>(self, f: F) -> (B, B)
    2214    where
    2215        Self: Sized,
    2216        B: Default + Extend<Self::Item>,
    2217        F: FnMut(&Self::Item) -> bool,
    2218    {
    2219        #[inline]
    2220        fn extend<'a, T, B: Extend<T>>(
    2221            mut f: impl FnMut(&T) -> bool + 'a,
    2222            left: &'a mut B,
    2223            right: &'a mut B,
    2224        ) -> impl FnMut((), T) + 'a {
    2225            move |(), x| {
    2226                if f(&x) {
    2227                    left.extend_one(x);
    2228                } else {
    2229                    right.extend_one(x);
    2230                }
    2231            }
    2232        }
    2233
    2234        let mut left: B = Default::default();
    2235        let mut right: B = Default::default();
    2236
    2237        self.fold((), extend(f, &mut left, &mut right));
    2238
    2239        (left, right)
    2240    }
    2241
    2242    /// Reorders the elements of this iterator *in-place* according to the given predicate,
    2243    /// such that all those that return `true` precede all those that return `false`.
    2244    /// Returns the number of `true` elements found.
    2245    ///
    2246    /// The relative order of partitioned items is not maintained.
    2247    ///
    2248    /// # Current implementation
    2249    ///
    2250    /// The current algorithm tries to find the first element for which the predicate evaluates
    2251    /// to false and the last element for which it evaluates to true, and repeatedly swaps them.
    2252    ///
    2253    /// Time complexity: *O*(*n*)
    2254    ///
    2255    /// See also [`is_partitioned()`] and [`partition()`].
    2256    ///
    2257    /// [`is_partitioned()`]: Iterator::is_partitioned
    2258    /// [`partition()`]: Iterator::partition
    2259    ///
    2260    /// # Examples
    2261    ///
    2262    /// ```
    2263    /// #![feature(iter_partition_in_place)]
    2264    ///
    2265    /// let mut a = [1, 2, 3, 4, 5, 6, 7];
    2266    ///
    2267    /// // Partition in-place between evens and odds
    2268    /// let i = a.iter_mut().partition_in_place(|n| n % 2 == 0);
    2269    ///
    2270    /// assert_eq!(i, 3);
    2271    /// assert!(a[..i].iter().all(|n| n % 2 == 0)); // evens
    2272    /// assert!(a[i..].iter().all(|n| n % 2 == 1)); // odds
    2273    /// ```
    2274    #[unstable(feature = "iter_partition_in_place", issue = "62543")]
    2275    fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
    2276    where
    2277        Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
    2278        P: FnMut(&T) -> bool,
    2279    {
    2280        // FIXME: should we worry about the count overflowing? The only way to have more than
    2281        // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
    2282
    2283        // These closure "factory" functions exist to avoid genericity in `Self`.
    2284
    2285        #[inline]
    2286        fn is_false<'a, T>(
    2287            predicate: &'a mut impl FnMut(&T) -> bool,
    2288            true_count: &'a mut usize,
    2289        ) -> impl FnMut(&&mut T) -> bool + 'a {
    2290            move |x| {
    2291                let p = predicate(&**x);
    2292                *true_count += p as usize;
    2293                !p
    2294            }
    2295        }
    2296
    2297        #[inline]
    2298        fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
    2299            move |x| predicate(&**x)
    2300        }
    2301
    2302        // Repeatedly find the first `false` and swap it with the last `true`.
    2303        let mut true_count = 0;
    2304        while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
    2305            if let Some(tail) = self.rfind(is_true(predicate)) {
    2306                crate::mem::swap(head, tail);
    2307                true_count += 1;
    2308            } else {
    2309                break;
    2310            }
    2311        }
    2312        true_count
    2313    }
    2314
    2315    /// Checks if the elements of this iterator are partitioned according to the given predicate,
    2316    /// such that all those that return `true` precede all those that return `false`.
    2317    ///
    2318    /// See also [`partition()`] and [`partition_in_place()`].
    2319    ///
    2320    /// [`partition()`]: Iterator::partition
    2321    /// [`partition_in_place()`]: Iterator::partition_in_place
    2322    ///
    2323    /// # Examples
    2324    ///
    2325    /// ```
    2326    /// #![feature(iter_is_partitioned)]
    2327    ///
    2328    /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
    2329    /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
    2330    /// ```
    2331    #[unstable(feature = "iter_is_partitioned", issue = "62544")]
    2332    fn is_partitioned<P>(mut self, mut predicate: P) -> bool
    2333    where
    2334        Self: Sized,
    2335        P: FnMut(Self::Item) -> bool,
    2336    {
    2337        // Either all items test `true`, or the first clause stops at `false`
    2338        // and we check that there are no more `true` items after that.
    2339        self.all(&mut predicate) || !self.any(predicate)
    2340    }
    2341
    2342    /// An iterator method that applies a function as long as it returns
    2343    /// successfully, producing a single, final value.
    2344    ///
    2345    /// `try_fold()` takes two arguments: an initial value, and a closure with
    2346    /// two arguments: an 'accumulator', and an element. The closure either
    2347    /// returns successfully, with the value that the accumulator should have
    2348    /// for the next iteration, or it returns failure, with an error value that
    2349    /// is propagated back to the caller immediately (short-circuiting).
    2350    ///
    2351    /// The initial value is the value the accumulator will have on the first
    2352    /// call. If applying the closure succeeded against every element of the
    2353    /// iterator, `try_fold()` returns the final accumulator as success.
    2354    ///
    2355    /// Folding is useful whenever you have a collection of something, and want
    2356    /// to produce a single value from it.
    2357    ///
    2358    /// # Note to Implementors
    2359    ///
    2360    /// Several of the other (forward) methods have default implementations in
    2361    /// terms of this one, so try to implement this explicitly if it can
    2362    /// do something better than the default `for` loop implementation.
    2363    ///
    2364    /// In particular, try to have this call `try_fold()` on the internal parts
    2365    /// from which this iterator is composed. If multiple calls are needed,
    2366    /// the `?` operator may be convenient for chaining the accumulator value
    2367    /// along, but beware any invariants that need to be upheld before those
    2368    /// early returns. This is a `&mut self` method, so iteration needs to be
    2369    /// resumable after hitting an error here.
    2370    ///
    2371    /// # Examples
    2372    ///
    2373    /// Basic usage:
    2374    ///
    2375    /// ```
    2376    /// let a = [1, 2, 3];
    2377    ///
    2378    /// // the checked sum of all of the elements of the array
    2379    /// let sum = a.into_iter().try_fold(0i8, |acc, x| acc.checked_add(x));
    2380    ///
    2381    /// assert_eq!(sum, Some(6));
    2382    /// ```
    2383    ///
    2384    /// Short-circuiting:
    2385    ///
    2386    /// ```
    2387    /// let a = [10, 20, 30, 100, 40, 50];
    2388    /// let mut iter = a.into_iter();
    2389    ///
    2390    /// // This sum overflows when adding the 100 element
    2391    /// let sum = iter.try_fold(0i8, |acc, x| acc.checked_add(x));
    2392    /// assert_eq!(sum, None);
    2393    ///
    2394    /// // Because it short-circuited, the remaining elements are still
    2395    /// // available through the iterator.
    2396    /// assert_eq!(iter.len(), 2);
    2397    /// assert_eq!(iter.next(), Some(40));
    2398    /// ```
    2399    ///
    2400    /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
    2401    /// a similar idea:
    2402    ///
    2403    /// ```
    2404    /// use std::ops::ControlFlow;
    2405    ///
    2406    /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
    2407    ///     if let Some(next) = prev.checked_add(x) {
    2408    ///         ControlFlow::Continue(next)
    2409    ///     } else {
    2410    ///         ControlFlow::Break(prev)
    2411    ///     }
    2412    /// });
    2413    /// assert_eq!(triangular, ControlFlow::Break(120));
    2414    ///
    2415    /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
    2416    ///     if let Some(next) = prev.checked_add(x) {
    2417    ///         ControlFlow::Continue(next)
    2418    ///     } else {
    2419    ///         ControlFlow::Break(prev)
    2420    ///     }
    2421    /// });
    2422    /// assert_eq!(triangular, ControlFlow::Continue(435));
    2423    /// ```
    2424    #[inline]
    2425    #[stable(feature = "iterator_try_fold", since = "1.27.0")]
    2426    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
    2427    where
    2428        Self: Sized,
    2429        F: FnMut(B, Self::Item) -> R,
    2430        R: Try<Output = B>,
    2431    {
    2432        let mut accum = init;
    2433        while let Some(x) = self.next() {
    2434            accum = f(accum, x)?;
    2435        }
    2436        try { accum }
    2437    }
    2438
    2439    /// An iterator method that applies a fallible function to each item in the
    2440    /// iterator, stopping at the first error and returning that error.
    2441    ///
    2442    /// This can also be thought of as the fallible form of [`for_each()`]
    2443    /// or as the stateless version of [`try_fold()`].
    2444    ///
    2445    /// [`for_each()`]: Iterator::for_each
    2446    /// [`try_fold()`]: Iterator::try_fold
    2447    ///
    2448    /// # Examples
    2449    ///
    2450    /// ```
    2451    /// use std::fs::rename;
    2452    /// use std::io::{stdout, Write};
    2453    /// use std::path::Path;
    2454    ///
    2455    /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
    2456    ///
    2457    /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}"));
    2458    /// assert!(res.is_ok());
    2459    ///
    2460    /// let mut it = data.iter().cloned();
    2461    /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
    2462    /// assert!(res.is_err());
    2463    /// // It short-circuited, so the remaining items are still in the iterator:
    2464    /// assert_eq!(it.next(), Some("stale_bread.json"));
    2465    /// ```
    2466    ///
    2467    /// The [`ControlFlow`] type can be used with this method for the situations
    2468    /// in which you'd use `break` and `continue` in a normal loop:
    2469    ///
    2470    /// ```
    2471    /// use std::ops::ControlFlow;
    2472    ///
    2473    /// let r = (2..100).try_for_each(|x| {
    2474    ///     if 323 % x == 0 {
    2475    ///         return ControlFlow::Break(x)
    2476    ///     }
    2477    ///
    2478    ///     ControlFlow::Continue(())
    2479    /// });
    2480    /// assert_eq!(r, ControlFlow::Break(17));
    2481    /// ```
    2482    #[inline]
    2483    #[stable(feature = "iterator_try_fold", since = "1.27.0")]
    2484    fn try_for_each<F, R>(&mut self, f: F) -> R
    2485    where
    2486        Self: Sized,
    2487        F: FnMut(Self::Item) -> R,
    2488        R: Try<Output = ()>,
    2489    {
    2490        #[inline]
    2491        fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
    2492            move |(), x| f(x)
    2493        }
    2494
    2495        self.try_fold((), call(f))
    2496    }
    2497
    2498    /// Folds every element into an accumulator by applying an operation,
    2499    /// returning the final result.
    2500    ///
    2501    /// `fold()` takes two arguments: an initial value, and a closure with two
    2502    /// arguments: an 'accumulator', and an element. The closure returns the value that
    2503    /// the accumulator should have for the next iteration.
    2504    ///
    2505    /// The initial value is the value the accumulator will have on the first
    2506    /// call.
    2507    ///
    2508    /// After applying this closure to every element of the iterator, `fold()`
    2509    /// returns the accumulator.
    2510    ///
    2511    /// This operation is sometimes called 'reduce' or 'inject'.
    2512    ///
    2513    /// Folding is useful whenever you have a collection of something, and want
    2514    /// to produce a single value from it.
    2515    ///
    2516    /// Note: `fold()`, and similar methods that traverse the entire iterator,
    2517    /// might not terminate for infinite iterators, even on traits for which a
    2518    /// result is determinable in finite time.
    2519    ///
    2520    /// Note: [`reduce()`] can be used to use the first element as the initial
    2521    /// value, if the accumulator type and item type is the same.
    2522    ///
    2523    /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
    2524    /// operators like `+`, the order the elements are combined in is not important, but for non-associative
    2525    /// operators like `-` the order will affect the final result.
    2526    /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
    2527    ///
    2528    /// # Note to Implementors
    2529    ///
    2530    /// Several of the other (forward) methods have default implementations in
    2531    /// terms of this one, so try to implement this explicitly if it can
    2532    /// do something better than the default `for` loop implementation.
    2533    ///
    2534    /// In particular, try to have this call `fold()` on the internal parts
    2535    /// from which this iterator is composed.
    2536    ///
    2537    /// # Examples
    2538    ///
    2539    /// Basic usage:
    2540    ///
    2541    /// ```
    2542    /// let a = [1, 2, 3];
    2543    ///
    2544    /// // the sum of all of the elements of the array
    2545    /// let sum = a.iter().fold(0, |acc, x| acc + x);
    2546    ///
    2547    /// assert_eq!(sum, 6);
    2548    /// ```
    2549    ///
    2550    /// Let's walk through each step of the iteration here:
    2551    ///
    2552    /// | element | acc | x | result |
    2553    /// |---------|-----|---|--------|
    2554    /// |         | 0   |   |        |
    2555    /// | 1       | 0   | 1 | 1      |
    2556    /// | 2       | 1   | 2 | 3      |
    2557    /// | 3       | 3   | 3 | 6      |
    2558    ///
    2559    /// And so, our final result, `6`.
    2560    ///
    2561    /// This example demonstrates the left-associative nature of `fold()`:
    2562    /// it builds a string, starting with an initial value
    2563    /// and continuing with each element from the front until the back:
    2564    ///
    2565    /// ```
    2566    /// let numbers = [1, 2, 3, 4, 5];
    2567    ///
    2568    /// let zero = "0".to_string();
    2569    ///
    2570    /// let result = numbers.iter().fold(zero, |acc, &x| {
    2571    ///     format!("({acc} + {x})")
    2572    /// });
    2573    ///
    2574    /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
    2575    /// ```
    2576    /// It's common for people who haven't used iterators a lot to
    2577    /// use a `for` loop with a list of things to build up a result. Those
    2578    /// can be turned into `fold()`s:
    2579    ///
    2580    /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
    2581    ///
    2582    /// ```
    2583    /// let numbers = [1, 2, 3, 4, 5];
    2584    ///
    2585    /// let mut result = 0;
    2586    ///
    2587    /// // for loop:
    2588    /// for i in &numbers {
    2589    ///     result = result + i;
    2590    /// }
    2591    ///
    2592    /// // fold:
    2593    /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
    2594    ///
    2595    /// // they're the same
    2596    /// assert_eq!(result, result2);
    2597    /// ```
    2598    ///
    2599    /// [`reduce()`]: Iterator::reduce
    2600    #[doc(alias = "inject", alias = "foldl")]
    2601    #[inline]
    2602    #[stable(feature = "rust1", since = "1.0.0")]
    2603    fn fold<B, F>(mut self, init: B, mut f: F) -> B
    2604    where
    2605        Self: Sized,
    2606        F: FnMut(B, Self::Item) -> B,
    2607    {
    2608        let mut accum = init;
    2609        while let Some(x) = self.next() {
    2610            accum = f(accum, x);
    2611        }
    2612        accum
    2613    }
    2614
    2615    /// Reduces the elements to a single one, by repeatedly applying a reducing
    2616    /// operation.
    2617    ///
    2618    /// If the iterator is empty, returns [`None`]; otherwise, returns the
    2619    /// result of the reduction.
    2620    ///
    2621    /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
    2622    /// For iterators with at least one element, this is the same as [`fold()`]
    2623    /// with the first element of the iterator as the initial accumulator value, folding
    2624    /// every subsequent element into it.
    2625    ///
    2626    /// [`fold()`]: Iterator::fold
    2627    ///
    2628    /// # Example
    2629    ///
    2630    /// ```
    2631    /// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap_or(0);
    2632    /// assert_eq!(reduced, 45);
    2633    ///
    2634    /// // Which is equivalent to doing it with `fold`:
    2635    /// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
    2636    /// assert_eq!(reduced, folded);
    2637    /// ```
    2638    #[inline]
    2639    #[stable(feature = "iterator_fold_self", since = "1.51.0")]
    2640    fn reduce<F>(mut self, f: F) -> Option<Self::Item>
    2641    where
    2642        Self: Sized,
    2643        F: FnMut(Self::Item, Self::Item) -> Self::Item,
    2644    {
    2645        let first = self.next()?;
    2646        Some(self.fold(first, f))
    2647    }
    2648
    2649    /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
    2650    /// closure returns a failure, the failure is propagated back to the caller immediately.
    2651    ///
    2652    /// The return type of this method depends on the return type of the closure. If the closure
    2653    /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
    2654    /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
    2655    /// `Option<Option<Self::Item>>`.
    2656    ///
    2657    /// When called on an empty iterator, this function will return either `Some(None)` or
    2658    /// `Ok(None)` depending on the type of the provided closure.
    2659    ///
    2660    /// For iterators with at least one element, this is essentially the same as calling
    2661    /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
    2662    ///
    2663    /// [`try_fold()`]: Iterator::try_fold
    2664    ///
    2665    /// # Examples
    2666    ///
    2667    /// Safely calculate the sum of a series of numbers:
    2668    ///
    2669    /// ```
    2670    /// #![feature(iterator_try_reduce)]
    2671    ///
    2672    /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
    2673    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
    2674    /// assert_eq!(sum, Some(Some(58)));
    2675    /// ```
    2676    ///
    2677    /// Determine when a reduction short circuited:
    2678    ///
    2679    /// ```
    2680    /// #![feature(iterator_try_reduce)]
    2681    ///
    2682    /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
    2683    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
    2684    /// assert_eq!(sum, None);
    2685    /// ```
    2686    ///
    2687    /// Determine when a reduction was not performed because there are no elements:
    2688    ///
    2689    /// ```
    2690    /// #![feature(iterator_try_reduce)]
    2691    ///
    2692    /// let numbers: Vec<usize> = Vec::new();
    2693    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
    2694    /// assert_eq!(sum, Some(None));
    2695    /// ```
    2696    ///
    2697    /// Use a [`Result`] instead of an [`Option`]:
    2698    ///
    2699    /// ```
    2700    /// #![feature(iterator_try_reduce)]
    2701    ///
    2702    /// let numbers = vec!["1", "2", "3", "4", "5"];
    2703    /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
    2704    ///     numbers.into_iter().try_reduce(|x, y| {
    2705    ///         if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
    2706    ///     });
    2707    /// assert_eq!(max, Ok(Some("5")));
    2708    /// ```
    2709    #[inline]
    2710    #[unstable(feature = "iterator_try_reduce", issue = "87053")]
    2711    fn try_reduce<R>(
    2712        &mut self,
    2713        f: impl FnMut(Self::Item, Self::Item) -> R,
    2714    ) -> ChangeOutputType<R, Option<R::Output>>
    2715    where
    2716        Self: Sized,
    2717        R: Try<Output = Self::Item, Residual: Residual<Option<Self::Item>>>,
    2718    {
    2719        let first = match self.next() {
    2720            Some(i) => i,
    2721            None => return Try::from_output(None),
    2722        };
    2723
    2724        match self.try_fold(first, f).branch() {
    2725            ControlFlow::Break(r) => FromResidual::from_residual(r),
    2726            ControlFlow::Continue(i) => Try::from_output(Some(i)),
    2727        }
    2728    }
    2729
    2730    /// Tests if every element of the iterator matches a predicate.
    2731    ///
    2732    /// `all()` takes a closure that returns `true` or `false`. It applies
    2733    /// this closure to each element of the iterator, and if they all return
    2734    /// `true`, then so does `all()`. If any of them return `false`, it
    2735    /// returns `false`.
    2736    ///
    2737    /// `all()` is short-circuiting; in other words, it will stop processing
    2738    /// as soon as it finds a `false`, given that no matter what else happens,
    2739    /// the result will also be `false`.
    2740    ///
    2741    /// An empty iterator returns `true`.
    2742    ///
    2743    /// # Examples
    2744    ///
    2745    /// Basic usage:
    2746    ///
    2747    /// ```
    2748    /// let a = [1, 2, 3];
    2749    ///
    2750    /// assert!(a.into_iter().all(|x| x > 0));
    2751    ///
    2752    /// assert!(!a.into_iter().all(|x| x > 2));
    2753    /// ```
    2754    ///
    2755    /// Stopping at the first `false`:
    2756    ///
    2757    /// ```
    2758    /// let a = [1, 2, 3];
    2759    ///
    2760    /// let mut iter = a.into_iter();
    2761    ///
    2762    /// assert!(!iter.all(|x| x != 2));
    2763    ///
    2764    /// // we can still use `iter`, as there are more elements.
    2765    /// assert_eq!(iter.next(), Some(3));
    2766    /// ```
    2767    #[inline]
    2768    #[stable(feature = "rust1", since = "1.0.0")]
    2769    fn all<F>(&mut self, f: F) -> bool
    2770    where
    2771        Self: Sized,
    2772        F: FnMut(Self::Item) -> bool,
    2773    {
    2774        #[inline]
    2775        fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
    2776            move |(), x| {
    2777                if f(x) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
    2778            }
    2779        }
    2780        self.try_fold((), check(f)) == ControlFlow::Continue(())
    2781    }
    2782
    2783    /// Tests if any element of the iterator matches a predicate.
    2784    ///
    2785    /// `any()` takes a closure that returns `true` or `false`. It applies
    2786    /// this closure to each element of the iterator, and if any of them return
    2787    /// `true`, then so does `any()`. If they all return `false`, it
    2788    /// returns `false`.
    2789    ///
    2790    /// `any()` is short-circuiting; in other words, it will stop processing
    2791    /// as soon as it finds a `true`, given that no matter what else happens,
    2792    /// the result will also be `true`.
    2793    ///
    2794    /// An empty iterator returns `false`.
    2795    ///
    2796    /// # Examples
    2797    ///
    2798    /// Basic usage:
    2799    ///
    2800    /// ```
    2801    /// let a = [1, 2, 3];
    2802    ///
    2803    /// assert!(a.into_iter().any(|x| x > 0));
    2804    ///
    2805    /// assert!(!a.into_iter().any(|x| x > 5));
    2806    /// ```
    2807    ///
    2808    /// Stopping at the first `true`:
    2809    ///
    2810    /// ```
    2811    /// let a = [1, 2, 3];
    2812    ///
    2813    /// let mut iter = a.into_iter();
    2814    ///
    2815    /// assert!(iter.any(|x| x != 2));
    2816    ///
    2817    /// // we can still use `iter`, as there are more elements.
    2818    /// assert_eq!(iter.next(), Some(2));
    2819    /// ```
    2820    #[inline]
    2821    #[stable(feature = "rust1", since = "1.0.0")]
    2822    fn any<F>(&mut self, f: F) -> bool
    2823    where
    2824        Self: Sized,
    2825        F: FnMut(Self::Item) -> bool,
    2826    {
    2827        #[inline]
    2828        fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
    2829            move |(), x| {
    2830                if f(x) { ControlFlow::Break(()) } else { ControlFlow::Continue(()) }
    2831            }
    2832        }
    2833
    2834        self.try_fold((), check(f)) == ControlFlow::Break(())
    2835    }
    2836
    2837    /// Searches for an element of an iterator that satisfies a predicate.
    2838    ///
    2839    /// `find()` takes a closure that returns `true` or `false`. It applies
    2840    /// this closure to each element of the iterator, and if any of them return
    2841    /// `true`, then `find()` returns [`Some(element)`]. If they all return
    2842    /// `false`, it returns [`None`].
    2843    ///
    2844    /// `find()` is short-circuiting; in other words, it will stop processing
    2845    /// as soon as the closure returns `true`.
    2846    ///
    2847    /// Because `find()` takes a reference, and many iterators iterate over
    2848    /// references, this leads to a possibly confusing situation where the
    2849    /// argument is a double reference. You can see this effect in the
    2850    /// examples below, with `&&x`.
    2851    ///
    2852    /// If you need the index of the element, see [`position()`].
    2853    ///
    2854    /// [`Some(element)`]: Some
    2855    /// [`position()`]: Iterator::position
    2856    ///
    2857    /// # Examples
    2858    ///
    2859    /// Basic usage:
    2860    ///
    2861    /// ```
    2862    /// let a = [1, 2, 3];
    2863    ///
    2864    /// assert_eq!(a.into_iter().find(|&x| x == 2), Some(2));
    2865    /// assert_eq!(a.into_iter().find(|&x| x == 5), None);
    2866    /// ```
    2867    ///
    2868    /// Iterating over references:
    2869    ///
    2870    /// ```
    2871    /// let a = [1, 2, 3];
    2872    ///
    2873    /// // `iter()` yields references i.e. `&i32` and `find()` takes a
    2874    /// // reference to each element.
    2875    /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
    2876    /// assert_eq!(a.iter().find(|&&x| x == 5), None);
    2877    /// ```
    2878    ///
    2879    /// Stopping at the first `true`:
    2880    ///
    2881    /// ```
    2882    /// let a = [1, 2, 3];
    2883    ///
    2884    /// let mut iter = a.into_iter();
    2885    ///
    2886    /// assert_eq!(iter.find(|&x| x == 2), Some(2));
    2887    ///
    2888    /// // we can still use `iter`, as there are more elements.
    2889    /// assert_eq!(iter.next(), Some(3));
    2890    /// ```
    2891    ///
    2892    /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
    2893    #[inline]
    2894    #[stable(feature = "rust1", since = "1.0.0")]
    2895    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
    2896    where
    2897        Self: Sized,
    2898        P: FnMut(&Self::Item) -> bool,
    2899    {
    2900        #[inline]
    2901        fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
    2902            move |(), x| {
    2903                if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::Continue(()) }
    2904            }
    2905        }
    2906
    2907        self.try_fold((), check(predicate)).break_value()
    2908    }
    2909
    2910    /// Applies function to the elements of iterator and returns
    2911    /// the first non-none result.
    2912    ///
    2913    /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
    2914    ///
    2915    /// # Examples
    2916    ///
    2917    /// ```
    2918    /// let a = ["lol", "NaN", "2", "5"];
    2919    ///
    2920    /// let first_number = a.iter().find_map(|s| s.parse().ok());
    2921    ///
    2922    /// assert_eq!(first_number, Some(2));
    2923    /// ```
    2924    #[inline]
    2925    #[stable(feature = "iterator_find_map", since = "1.30.0")]
    2926    fn find_map<B, F>(&mut self, f: F) -> Option<B>
    2927    where
    2928        Self: Sized,
    2929        F: FnMut(Self::Item) -> Option<B>,
    2930    {
    2931        #[inline]
    2932        fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
    2933            move |(), x| match f(x) {
    2934                Some(x) => ControlFlow::Break(x),
    2935                None => ControlFlow::Continue(()),
    2936            }
    2937        }
    2938
    2939        self.try_fold((), check(f)).break_value()
    2940    }
    2941
    2942    /// Applies function to the elements of iterator and returns
    2943    /// the first true result or the first error.
    2944    ///
    2945    /// The return type of this method depends on the return type of the closure.
    2946    /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>, E>`.
    2947    /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
    2948    ///
    2949    /// # Examples
    2950    ///
    2951    /// ```
    2952    /// #![feature(try_find)]
    2953    ///
    2954    /// let a = ["1", "2", "lol", "NaN", "5"];
    2955    ///
    2956    /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
    2957    ///     Ok(s.parse::<i32>()? == search)
    2958    /// };
    2959    ///
    2960    /// let result = a.into_iter().try_find(|&s| is_my_num(s, 2));
    2961    /// assert_eq!(result, Ok(Some("2")));
    2962    ///
    2963    /// let result = a.into_iter().try_find(|&s| is_my_num(s, 5));
    2964    /// assert!(result.is_err());
    2965    /// ```
    2966    ///
    2967    /// This also supports other types which implement [`Try`], not just [`Result`].
    2968    ///
    2969    /// ```
    2970    /// #![feature(try_find)]
    2971    ///
    2972    /// use std::num::NonZero;
    2973    ///
    2974    /// let a = [3, 5, 7, 4, 9, 0, 11u32];
    2975    /// let result = a.into_iter().try_find(|&x| NonZero::new(x).map(|y| y.is_power_of_two()));
    2976    /// assert_eq!(result, Some(Some(4)));
    2977    /// let result = a.into_iter().take(3).try_find(|&x| NonZero::new(x).map(|y| y.is_power_of_two()));
    2978    /// assert_eq!(result, Some(None));
    2979    /// let result = a.into_iter().rev().try_find(|&x| NonZero::new(x).map(|y| y.is_power_of_two()));
    2980    /// assert_eq!(result, None);
    2981    /// ```
    2982    #[inline]
    2983    #[unstable(feature = "try_find", issue = "63178")]
    2984    fn try_find<R>(
    2985        &mut self,
    2986        f: impl FnMut(&Self::Item) -> R,
    2987    ) -> ChangeOutputType<R, Option<Self::Item>>
    2988    where
    2989        Self: Sized,
    2990        R: Try<Output = bool, Residual: Residual<Option<Self::Item>>>,
    2991    {
    2992        #[inline]
    2993        fn check<I, V, R>(
    2994            mut f: impl FnMut(&I) -> V,
    2995        ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
    2996        where
    2997            V: Try<Output = bool, Residual = R>,
    2998            R: Residual<Option<I>>,
    2999        {
    3000            move |(), x| match f(&x).branch() {
    3001                ControlFlow::Continue(false) => ControlFlow::Continue(()),
    3002                ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
    3003                ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
    3004            }
    3005        }
    3006
    3007        match self.try_fold((), check(f)) {
    3008            ControlFlow::Break(x) => x,
    3009            ControlFlow::Continue(()) => Try::from_output(None),
    3010        }
    3011    }
    3012
    3013    /// Searches for an element in an iterator, returning its index.
    3014    ///
    3015    /// `position()` takes a closure that returns `true` or `false`. It applies
    3016    /// this closure to each element of the iterator, and if one of them
    3017    /// returns `true`, then `position()` returns [`Some(index)`]. If all of
    3018    /// them return `false`, it returns [`None`].
    3019    ///
    3020    /// `position()` is short-circuiting; in other words, it will stop
    3021    /// processing as soon as it finds a `true`.
    3022    ///
    3023    /// # Overflow Behavior
    3024    ///
    3025    /// The method does no guarding against overflows, so if there are more
    3026    /// than [`usize::MAX`] non-matching elements, it either produces the wrong
    3027    /// result or panics. If overflow checks are enabled, a panic is
    3028    /// guaranteed.
    3029    ///
    3030    /// # Panics
    3031    ///
    3032    /// This function might panic if the iterator has more than `usize::MAX`
    3033    /// non-matching elements.
    3034    ///
    3035    /// [`Some(index)`]: Some
    3036    ///
    3037    /// # Examples
    3038    ///
    3039    /// Basic usage:
    3040    ///
    3041    /// ```
    3042    /// let a = [1, 2, 3];
    3043    ///
    3044    /// assert_eq!(a.into_iter().position(|x| x == 2), Some(1));
    3045    ///
    3046    /// assert_eq!(a.into_iter().position(|x| x == 5), None);
    3047    /// ```
    3048    ///
    3049    /// Stopping at the first `true`:
    3050    ///
    3051    /// ```
    3052    /// let a = [1, 2, 3, 4];
    3053    ///
    3054    /// let mut iter = a.into_iter();
    3055    ///
    3056    /// assert_eq!(iter.position(|x| x >= 2), Some(1));
    3057    ///
    3058    /// // we can still use `iter`, as there are more elements.
    3059    /// assert_eq!(iter.next(), Some(3));
    3060    ///
    3061    /// // The returned index depends on iterator state
    3062    /// assert_eq!(iter.position(|x| x == 4), Some(0));
    3063    ///
    3064    /// ```
    3065    #[inline]
    3066    #[stable(feature = "rust1", since = "1.0.0")]
    3067    fn position<P>(&mut self, predicate: P) -> Option<usize>
    3068    where
    3069        Self: Sized,
    3070        P: FnMut(Self::Item) -> bool,
    3071    {
    3072        #[inline]
    3073        fn check<'a, T>(
    3074            mut predicate: impl FnMut(T) -> bool + 'a,
    3075            acc: &'a mut usize,
    3076        ) -> impl FnMut((), T) -> ControlFlow<usize, ()> + 'a {
    3077            #[rustc_inherit_overflow_checks]
    3078            move |_, x| {
    3079                if predicate(x) {
    3080                    ControlFlow::Break(*acc)
    3081                } else {
    3082                    *acc += 1;
    3083                    ControlFlow::Continue(())
    3084                }
    3085            }
    3086        }
    3087
    3088        let mut acc = 0;
    3089        self.try_fold((), check(predicate, &mut acc)).break_value()
    3090    }
    3091
    3092    /// Searches for an element in an iterator from the right, returning its
    3093    /// index.
    3094    ///
    3095    /// `rposition()` takes a closure that returns `true` or `false`. It applies
    3096    /// this closure to each element of the iterator, starting from the end,
    3097    /// and if one of them returns `true`, then `rposition()` returns
    3098    /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
    3099    ///
    3100    /// `rposition()` is short-circuiting; in other words, it will stop
    3101    /// processing as soon as it finds a `true`.
    3102    ///
    3103    /// [`Some(index)`]: Some
    3104    ///
    3105    /// # Examples
    3106    ///
    3107    /// Basic usage:
    3108    ///
    3109    /// ```
    3110    /// let a = [1, 2, 3];
    3111    ///
    3112    /// assert_eq!(a.into_iter().rposition(|x| x == 3), Some(2));
    3113    ///
    3114    /// assert_eq!(a.into_iter().rposition(|x| x == 5), None);
    3115    /// ```
    3116    ///
    3117    /// Stopping at the first `true`:
    3118    ///
    3119    /// ```
    3120    /// let a = [-1, 2, 3, 4];
    3121    ///
    3122    /// let mut iter = a.into_iter();
    3123    ///
    3124    /// assert_eq!(iter.rposition(|x| x >= 2), Some(3));
    3125    ///
    3126    /// // we can still use `iter`, as there are more elements.
    3127    /// assert_eq!(iter.next(), Some(-1));
    3128    /// assert_eq!(iter.next_back(), Some(3));
    3129    /// ```
    3130    #[inline]
    3131    #[stable(feature = "rust1", since = "1.0.0")]
    3132    fn rposition<P>(&mut self, predicate: P) -> Option<usize>
    3133    where
    3134        P: FnMut(Self::Item) -> bool,
    3135        Self: Sized + ExactSizeIterator + DoubleEndedIterator,
    3136    {
    3137        // No need for an overflow check here, because `ExactSizeIterator`
    3138        // implies that the number of elements fits into a `usize`.
    3139        #[inline]
    3140        fn check<T>(
    3141            mut predicate: impl FnMut(T) -> bool,
    3142        ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
    3143            move |i, x| {
    3144                let i = i - 1;
    3145                if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
    3146            }
    3147        }
    3148
    3149        let n = self.len();
    3150        self.try_rfold(n, check(predicate)).break_value()
    3151    }
    3152
    3153    /// Returns the maximum element of an iterator.
    3154    ///
    3155    /// If several elements are equally maximum, the last element is
    3156    /// returned. If the iterator is empty, [`None`] is returned.
    3157    ///
    3158    /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
    3159    /// incomparable. You can work around this by using [`Iterator::reduce`]:
    3160    /// ```
    3161    /// assert_eq!(
    3162    ///     [2.4, f32::NAN, 1.3]
    3163    ///         .into_iter()
    3164    ///         .reduce(f32::max)
    3165    ///         .unwrap_or(0.),
    3166    ///     2.4
    3167    /// );
    3168    /// ```
    3169    ///
    3170    /// # Examples
    3171    ///
    3172    /// ```
    3173    /// let a = [1, 2, 3];
    3174    /// let b: [u32; 0] = [];
    3175    ///
    3176    /// assert_eq!(a.into_iter().max(), Some(3));
    3177    /// assert_eq!(b.into_iter().max(), None);
    3178    /// ```
    3179    #[inline]
    3180    #[stable(feature = "rust1", since = "1.0.0")]
    3181    fn max(self) -> Option<Self::Item>
    3182    where
    3183        Self: Sized,
    3184        Self::Item: Ord,
    3185    {
    3186        self.max_by(Ord::cmp)
    3187    }
    3188
    3189    /// Returns the minimum element of an iterator.
    3190    ///
    3191    /// If several elements are equally minimum, the first element is returned.
    3192    /// If the iterator is empty, [`None`] is returned.
    3193    ///
    3194    /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
    3195    /// incomparable. You can work around this by using [`Iterator::reduce`]:
    3196    /// ```
    3197    /// assert_eq!(
    3198    ///     [2.4, f32::NAN, 1.3]
    3199    ///         .into_iter()
    3200    ///         .reduce(f32::min)
    3201    ///         .unwrap_or(0.),
    3202    ///     1.3
    3203    /// );
    3204    /// ```
    3205    ///
    3206    /// # Examples
    3207    ///
    3208    /// ```
    3209    /// let a = [1, 2, 3];
    3210    /// let b: [u32; 0] = [];
    3211    ///
    3212    /// assert_eq!(a.into_iter().min(), Some(1));
    3213    /// assert_eq!(b.into_iter().min(), None);
    3214    /// ```
    3215    #[inline]
    3216    #[stable(feature = "rust1", since = "1.0.0")]
    3217    fn min(self) -> Option<Self::Item>
    3218    where
    3219        Self: Sized,
    3220        Self::Item: Ord,
    3221    {
    3222        self.min_by(Ord::cmp)
    3223    }
    3224
    3225    /// Returns the element that gives the maximum value from the
    3226    /// specified function.
    3227    ///
    3228    /// If several elements are equally maximum, the last element is
    3229    /// returned. If the iterator is empty, [`None`] is returned.
    3230    ///
    3231    /// # Examples
    3232    ///
    3233    /// ```
    3234    /// let a = [-3_i32, 0, 1, 5, -10];
    3235    /// assert_eq!(a.into_iter().max_by_key(|x| x.abs()).unwrap(), -10);
    3236    /// ```
    3237    #[inline]
    3238    #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
    3239    fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
    3240    where
    3241        Self: Sized,
    3242        F: FnMut(&Self::Item) -> B,
    3243    {
    3244        #[inline]
    3245        fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
    3246            move |x| (f(&x), x)
    3247        }
    3248
    3249        #[inline]
    3250        fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
    3251            x_p.cmp(y_p)
    3252        }
    3253
    3254        let (_, x) = self.map(key(f)).max_by(compare)?;
    3255        Some(x)
    3256    }
    3257
    3258    /// Returns the element that gives the maximum value with respect to the
    3259    /// specified comparison function.
    3260    ///
    3261    /// If several elements are equally maximum, the last element is
    3262    /// returned. If the iterator is empty, [`None`] is returned.
    3263    ///
    3264    /// # Examples
    3265    ///
    3266    /// ```
    3267    /// let a = [-3_i32, 0, 1, 5, -10];
    3268    /// assert_eq!(a.into_iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
    3269    /// ```
    3270    #[inline]
    3271    #[stable(feature = "iter_max_by", since = "1.15.0")]
    3272    fn max_by<F>(self, compare: F) -> Option<Self::Item>
    3273    where
    3274        Self: Sized,
    3275        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    3276    {
    3277        #[inline]
    3278        fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
    3279            move |x, y| cmp::max_by(x, y, &mut compare)
    3280        }
    3281
    3282        self.reduce(fold(compare))
    3283    }
    3284
    3285    /// Returns the element that gives the minimum value from the
    3286    /// specified function.
    3287    ///
    3288    /// If several elements are equally minimum, the first element is
    3289    /// returned. If the iterator is empty, [`None`] is returned.
    3290    ///
    3291    /// # Examples
    3292    ///
    3293    /// ```
    3294    /// let a = [-3_i32, 0, 1, 5, -10];
    3295    /// assert_eq!(a.into_iter().min_by_key(|x| x.abs()).unwrap(), 0);
    3296    /// ```
    3297    #[inline]
    3298    #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
    3299    fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
    3300    where
    3301        Self: Sized,
    3302        F: FnMut(&Self::Item) -> B,
    3303    {
    3304        #[inline]
    3305        fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
    3306            move |x| (f(&x), x)
    3307        }
    3308
    3309        #[inline]
    3310        fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
    3311            x_p.cmp(y_p)
    3312        }
    3313
    3314        let (_, x) = self.map(key(f)).min_by(compare)?;
    3315        Some(x)
    3316    }
    3317
    3318    /// Returns the element that gives the minimum value with respect to the
    3319    /// specified comparison function.
    3320    ///
    3321    /// If several elements are equally minimum, the first element is
    3322    /// returned. If the iterator is empty, [`None`] is returned.
    3323    ///
    3324    /// # Examples
    3325    ///
    3326    /// ```
    3327    /// let a = [-3_i32, 0, 1, 5, -10];
    3328    /// assert_eq!(a.into_iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
    3329    /// ```
    3330    #[inline]
    3331    #[stable(feature = "iter_min_by", since = "1.15.0")]
    3332    fn min_by<F>(self, compare: F) -> Option<Self::Item>
    3333    where
    3334        Self: Sized,
    3335        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    3336    {
    3337        #[inline]
    3338        fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
    3339            move |x, y| cmp::min_by(x, y, &mut compare)
    3340        }
    3341
    3342        self.reduce(fold(compare))
    3343    }
    3344
    3345    /// Reverses an iterator's direction.
    3346    ///
    3347    /// Usually, iterators iterate from left to right. After using `rev()`,
    3348    /// an iterator will instead iterate from right to left.
    3349    ///
    3350    /// This is only possible if the iterator has an end, so `rev()` only
    3351    /// works on [`DoubleEndedIterator`]s.
    3352    ///
    3353    /// # Examples
    3354    ///
    3355    /// ```
    3356    /// let a = [1, 2, 3];
    3357    ///
    3358    /// let mut iter = a.into_iter().rev();
    3359    ///
    3360    /// assert_eq!(iter.next(), Some(3));
    3361    /// assert_eq!(iter.next(), Some(2));
    3362    /// assert_eq!(iter.next(), Some(1));
    3363    ///
    3364    /// assert_eq!(iter.next(), None);
    3365    /// ```
    3366    #[inline]
    3367    #[doc(alias = "reverse")]
    3368    #[stable(feature = "rust1", since = "1.0.0")]
    3369    fn rev(self) -> Rev<Self>
    3370    where
    3371        Self: Sized + DoubleEndedIterator,
    3372    {
    3373        Rev::new(self)
    3374    }
    3375
    3376    /// Converts an iterator of pairs into a pair of containers.
    3377    ///
    3378    /// `unzip()` consumes an entire iterator of pairs, producing two
    3379    /// collections: one from the left elements of the pairs, and one
    3380    /// from the right elements.
    3381    ///
    3382    /// This function is, in some sense, the opposite of [`zip`].
    3383    ///
    3384    /// [`zip`]: Iterator::zip
    3385    ///
    3386    /// # Examples
    3387    ///
    3388    /// ```
    3389    /// let a = [(1, 2), (3, 4), (5, 6)];
    3390    ///
    3391    /// let (left, right): (Vec<_>, Vec<_>) = a.into_iter().unzip();
    3392    ///
    3393    /// assert_eq!(left, [1, 3, 5]);
    3394    /// assert_eq!(right, [2, 4, 6]);
    3395    ///
    3396    /// // you can also unzip multiple nested tuples at once
    3397    /// let a = [(1, (2, 3)), (4, (5, 6))];
    3398    ///
    3399    /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.into_iter().unzip();
    3400    /// assert_eq!(x, [1, 4]);
    3401    /// assert_eq!(y, [2, 5]);
    3402    /// assert_eq!(z, [3, 6]);
    3403    /// ```
    3404    #[stable(feature = "rust1", since = "1.0.0")]
    3405    fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
    3406    where
    3407        FromA: Default + Extend<A>,
    3408        FromB: Default + Extend<B>,
    3409        Self: Sized + Iterator<Item = (A, B)>,
    3410    {
    3411        let mut unzipped: (FromA, FromB) = Default::default();
    3412        unzipped.extend(self);
    3413        unzipped
    3414    }
    3415
    3416    /// Creates an iterator which copies all of its elements.
    3417    ///
    3418    /// This is useful when you have an iterator over `&T`, but you need an
    3419    /// iterator over `T`.
    3420    ///
    3421    /// # Examples
    3422    ///
    3423    /// ```
    3424    /// let a = [1, 2, 3];
    3425    ///
    3426    /// let v_copied: Vec<_> = a.iter().copied().collect();
    3427    ///
    3428    /// // copied is the same as .map(|&x| x)
    3429    /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
    3430    ///
    3431    /// assert_eq!(v_copied, [1, 2, 3]);
    3432    /// assert_eq!(v_map, [1, 2, 3]);
    3433    /// ```
    3434    #[stable(feature = "iter_copied", since = "1.36.0")]
    3435    #[rustc_diagnostic_item = "iter_copied"]
    3436    fn copied<'a, T>(self) -> Copied<Self>
    3437    where
    3438        T: Copy + 'a,
    3439        Self: Sized + Iterator<Item = &'a T>,
    3440    {
    3441        Copied::new(self)
    3442    }
    3443
    3444    /// Creates an iterator which [`clone`]s all of its elements.
    3445    ///
    3446    /// This is useful when you have an iterator over `&T`, but you need an
    3447    /// iterator over `T`.
    3448    ///
    3449    /// There is no guarantee whatsoever about the `clone` method actually
    3450    /// being called *or* optimized away. So code should not depend on
    3451    /// either.
    3452    ///
    3453    /// [`clone`]: Clone::clone
    3454    ///
    3455    /// # Examples
    3456    ///
    3457    /// Basic usage:
    3458    ///
    3459    /// ```
    3460    /// let a = [1, 2, 3];
    3461    ///
    3462    /// let v_cloned: Vec<_> = a.iter().cloned().collect();
    3463    ///
    3464    /// // cloned is the same as .map(|&x| x), for integers
    3465    /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
    3466    ///
    3467    /// assert_eq!(v_cloned, [1, 2, 3]);
    3468    /// assert_eq!(v_map, [1, 2, 3]);
    3469    /// ```
    3470    ///
    3471    /// To get the best performance, try to clone late:
    3472    ///
    3473    /// ```
    3474    /// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]];
    3475    /// // don't do this:
    3476    /// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect();
    3477    /// assert_eq!(&[vec![23]], &slower[..]);
    3478    /// // instead call `cloned` late
    3479    /// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect();
    3480    /// assert_eq!(&[vec![23]], &faster[..]);
    3481    /// ```
    3482    #[stable(feature = "rust1", since = "1.0.0")]
    3483    #[rustc_diagnostic_item = "iter_cloned"]
    3484    fn cloned<'a, T>(self) -> Cloned<Self>
    3485    where
    3486        T: Clone + 'a,
    3487        Self: Sized + Iterator<Item = &'a T>,
    3488    {
    3489        Cloned::new(self)
    3490    }
    3491
    3492    /// Repeats an iterator endlessly.
    3493    ///
    3494    /// Instead of stopping at [`None`], the iterator will instead start again,
    3495    /// from the beginning. After iterating again, it will start at the
    3496    /// beginning again. And again. And again. Forever. Note that in case the
    3497    /// original iterator is empty, the resulting iterator will also be empty.
    3498    ///
    3499    /// # Examples
    3500    ///
    3501    /// ```
    3502    /// let a = [1, 2, 3];
    3503    ///
    3504    /// let mut iter = a.into_iter().cycle();
    3505    ///
    3506    /// loop {
    3507    ///     assert_eq!(iter.next(), Some(1));
    3508    ///     assert_eq!(iter.next(), Some(2));
    3509    ///     assert_eq!(iter.next(), Some(3));
    3510    /// #   break;
    3511    /// }
    3512    /// ```
    3513    #[stable(feature = "rust1", since = "1.0.0")]
    3514    #[inline]
    3515    fn cycle(self) -> Cycle<Self>
    3516    where
    3517        Self: Sized + Clone,
    3518    {
    3519        Cycle::new(self)
    3520    }
    3521
    3522    /// Returns an iterator over `N` elements of the iterator at a time.
    3523    ///
    3524    /// The chunks do not overlap. If `N` does not divide the length of the
    3525    /// iterator, then the last up to `N-1` elements will be omitted and can be
    3526    /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
    3527    /// function of the iterator.
    3528    ///
    3529    /// # Panics
    3530    ///
    3531    /// Panics if `N` is zero.
    3532    ///
    3533    /// # Examples
    3534    ///
    3535    /// Basic usage:
    3536    ///
    3537    /// ```
    3538    /// #![feature(iter_array_chunks)]
    3539    ///
    3540    /// let mut iter = "lorem".chars().array_chunks();
    3541    /// assert_eq!(iter.next(), Some(['l', 'o']));
    3542    /// assert_eq!(iter.next(), Some(['r', 'e']));
    3543    /// assert_eq!(iter.next(), None);
    3544    /// assert_eq!(iter.into_remainder().as_slice(), &['m']);
    3545    /// ```
    3546    ///
    3547    /// ```
    3548    /// #![feature(iter_array_chunks)]
    3549    ///
    3550    /// let data = [1, 1, 2, -2, 6, 0, 3, 1];
    3551    /// //          ^-----^  ^------^
    3552    /// for [x, y, z] in data.iter().array_chunks() {
    3553    ///     assert_eq!(x + y + z, 4);
    3554    /// }
    3555    /// ```
    3556    #[track_caller]
    3557    #[unstable(feature = "iter_array_chunks", issue = "100450")]
    3558    fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
    3559    where
    3560        Self: Sized,
    3561    {
    3562        ArrayChunks::new(self)
    3563    }
    3564
    3565    /// Sums the elements of an iterator.
    3566    ///
    3567    /// Takes each element, adds them together, and returns the result.
    3568    ///
    3569    /// An empty iterator returns the *additive identity* ("zero") of the type,
    3570    /// which is `0` for integers and `-0.0` for floats.
    3571    ///
    3572    /// `sum()` can be used to sum any type implementing [`Sum`][`core::iter::Sum`],
    3573    /// including [`Option`][`Option::sum`] and [`Result`][`Result::sum`].
    3574    ///
    3575    /// # Panics
    3576    ///
    3577    /// When calling `sum()` and a primitive integer type is being returned, this
    3578    /// method will panic if the computation overflows and overflow checks are
    3579    /// enabled.
    3580    ///
    3581    /// # Examples
    3582    ///
    3583    /// ```
    3584    /// let a = [1, 2, 3];
    3585    /// let sum: i32 = a.iter().sum();
    3586    ///
    3587    /// assert_eq!(sum, 6);
    3588    ///
    3589    /// let b: Vec<f32> = vec![];
    3590    /// let sum: f32 = b.iter().sum();
    3591    /// assert_eq!(sum, -0.0_f32);
    3592    /// ```
    3593    #[stable(feature = "iter_arith", since = "1.11.0")]
    3594    fn sum<S>(self) -> S
    3595    where
    3596        Self: Sized,
    3597        S: Sum<Self::Item>,
    3598    {
    3599        Sum::sum(self)
    3600    }
    3601
    3602    /// Iterates over the entire iterator, multiplying all the elements
    3603    ///
    3604    /// An empty iterator returns the one value of the type.
    3605    ///
    3606    /// `product()` can be used to multiply any type implementing [`Product`][`core::iter::Product`],
    3607    /// including [`Option`][`Option::product`] and [`Result`][`Result::product`].
    3608    ///
    3609    /// # Panics
    3610    ///
    3611    /// When calling `product()` and a primitive integer type is being returned,
    3612    /// method will panic if the computation overflows and overflow checks are
    3613    /// enabled.
    3614    ///
    3615    /// # Examples
    3616    ///
    3617    /// ```
    3618    /// fn factorial(n: u32) -> u32 {
    3619    ///     (1..=n).product()
    3620    /// }
    3621    /// assert_eq!(factorial(0), 1);
    3622    /// assert_eq!(factorial(1), 1);
    3623    /// assert_eq!(factorial(5), 120);
    3624    /// ```
    3625    #[stable(feature = "iter_arith", since = "1.11.0")]
    3626    fn product<P>(self) -> P
    3627    where
    3628        Self: Sized,
    3629        P: Product<Self::Item>,
    3630    {
    3631        Product::product(self)
    3632    }
    3633
    3634    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
    3635    /// of another.
    3636    ///
    3637    /// # Examples
    3638    ///
    3639    /// ```
    3640    /// use std::cmp::Ordering;
    3641    ///
    3642    /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
    3643    /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
    3644    /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
    3645    /// ```
    3646    #[stable(feature = "iter_order", since = "1.5.0")]
    3647    fn cmp<I>(self, other: I) -> Ordering
    3648    where
    3649        I: IntoIterator<Item = Self::Item>,
    3650        Self::Item: Ord,
    3651        Self: Sized,
    3652    {
    3653        self.cmp_by(other, |x, y| x.cmp(&y))
    3654    }
    3655
    3656    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
    3657    /// of another with respect to the specified comparison function.
    3658    ///
    3659    /// # Examples
    3660    ///
    3661    /// ```
    3662    /// #![feature(iter_order_by)]
    3663    ///
    3664    /// use std::cmp::Ordering;
    3665    ///
    3666    /// let xs = [1, 2, 3, 4];
    3667    /// let ys = [1, 4, 9, 16];
    3668    ///
    3669    /// assert_eq!(xs.into_iter().cmp_by(ys, |x, y| x.cmp(&y)), Ordering::Less);
    3670    /// assert_eq!(xs.into_iter().cmp_by(ys, |x, y| (x * x).cmp(&y)), Ordering::Equal);
    3671    /// assert_eq!(xs.into_iter().cmp_by(ys, |x, y| (2 * x).cmp(&y)), Ordering::Greater);
    3672    /// ```
    3673    #[unstable(feature = "iter_order_by", issue = "64295")]
    3674    fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
    3675    where
    3676        Self: Sized,
    3677        I: IntoIterator,
    3678        F: FnMut(Self::Item, I::Item) -> Ordering,
    3679    {
    3680        #[inline]
    3681        fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
    3682        where
    3683            F: FnMut(X, Y) -> Ordering,
    3684        {
    3685            move |x, y| match cmp(x, y) {
    3686                Ordering::Equal => ControlFlow::Continue(()),
    3687                non_eq => ControlFlow::Break(non_eq),
    3688            }
    3689        }
    3690
    3691        match iter_compare(self, other.into_iter(), compare(cmp)) {
    3692            ControlFlow::Continue(ord) => ord,
    3693            ControlFlow::Break(ord) => ord,
    3694        }
    3695    }
    3696
    3697    /// [Lexicographically](Ord#lexicographical-comparison) compares the [`PartialOrd`] elements of
    3698    /// this [`Iterator`] with those of another. The comparison works like short-circuit
    3699    /// evaluation, returning a result without comparing the remaining elements.
    3700    /// As soon as an order can be determined, the evaluation stops and a result is returned.
    3701    ///
    3702    /// # Examples
    3703    ///
    3704    /// ```
    3705    /// use std::cmp::Ordering;
    3706    ///
    3707    /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
    3708    /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
    3709    /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
    3710    /// ```
    3711    ///
    3712    /// For floating-point numbers, NaN does not have a total order and will result
    3713    /// in `None` when compared:
    3714    ///
    3715    /// ```
    3716    /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
    3717    /// ```
    3718    ///
    3719    /// The results are determined by the order of evaluation.
    3720    ///
    3721    /// ```
    3722    /// use std::cmp::Ordering;
    3723    ///
    3724    /// assert_eq!([1.0, f64::NAN].iter().partial_cmp([2.0, f64::NAN].iter()), Some(Ordering::Less));
    3725    /// assert_eq!([2.0, f64::NAN].iter().partial_cmp([1.0, f64::NAN].iter()), Some(Ordering::Greater));
    3726    /// assert_eq!([f64::NAN, 1.0].iter().partial_cmp([f64::NAN, 2.0].iter()), None);
    3727    /// ```
    3728    ///
    3729    #[stable(feature = "iter_order", since = "1.5.0")]
    3730    fn partial_cmp<I>(self, other: I) -> Option<Ordering>
    3731    where
    3732        I: IntoIterator,
    3733        Self::Item: PartialOrd<I::Item>,
    3734        Self: Sized,
    3735    {
    3736        self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
    3737    }
    3738
    3739    /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
    3740    /// of another with respect to the specified comparison function.
    3741    ///
    3742    /// # Examples
    3743    ///
    3744    /// ```
    3745    /// #![feature(iter_order_by)]
    3746    ///
    3747    /// use std::cmp::Ordering;
    3748    ///
    3749    /// let xs = [1.0, 2.0, 3.0, 4.0];
    3750    /// let ys = [1.0, 4.0, 9.0, 16.0];
    3751    ///
    3752    /// assert_eq!(
    3753    ///     xs.iter().partial_cmp_by(ys, |x, y| x.partial_cmp(&y)),
    3754    ///     Some(Ordering::Less)
    3755    /// );
    3756    /// assert_eq!(
    3757    ///     xs.iter().partial_cmp_by(ys, |x, y| (x * x).partial_cmp(&y)),
    3758    ///     Some(Ordering::Equal)
    3759    /// );
    3760    /// assert_eq!(
    3761    ///     xs.iter().partial_cmp_by(ys, |x, y| (2.0 * x).partial_cmp(&y)),
    3762    ///     Some(Ordering::Greater)
    3763    /// );
    3764    /// ```
    3765    #[unstable(feature = "iter_order_by", issue = "64295")]
    3766    fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
    3767    where
    3768        Self: Sized,
    3769        I: IntoIterator,
    3770        F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
    3771    {
    3772        #[inline]
    3773        fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
    3774        where
    3775            F: FnMut(X, Y) -> Option<Ordering>,
    3776        {
    3777            move |x, y| match partial_cmp(x, y) {
    3778                Some(Ordering::Equal) => ControlFlow::Continue(()),
    3779                non_eq => ControlFlow::Break(non_eq),
    3780            }
    3781        }
    3782
    3783        match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
    3784            ControlFlow::Continue(ord) => Some(ord),
    3785            ControlFlow::Break(ord) => ord,
    3786        }
    3787    }
    3788
    3789    /// Determines if the elements of this [`Iterator`] are equal to those of
    3790    /// another.
    3791    ///
    3792    /// # Examples
    3793    ///
    3794    /// ```
    3795    /// assert_eq!([1].iter().eq([1].iter()), true);
    3796    /// assert_eq!([1].iter().eq([1, 2].iter()), false);
    3797    /// ```
    3798    #[stable(feature = "iter_order", since = "1.5.0")]
    3799    fn eq<I>(self, other: I) -> bool
    3800    where
    3801        I: IntoIterator,
    3802        Self::Item: PartialEq<I::Item>,
    3803        Self: Sized,
    3804    {
    3805        self.eq_by(other, |x, y| x == y)
    3806    }
    3807
    3808    /// Determines if the elements of this [`Iterator`] are equal to those of
    3809    /// another with respect to the specified equality function.
    3810    ///
    3811    /// # Examples
    3812    ///
    3813    /// ```
    3814    /// #![feature(iter_order_by)]
    3815    ///
    3816    /// let xs = [1, 2, 3, 4];
    3817    /// let ys = [1, 4, 9, 16];
    3818    ///
    3819    /// assert!(xs.iter().eq_by(ys, |x, y| x * x == y));
    3820    /// ```
    3821    #[unstable(feature = "iter_order_by", issue = "64295")]
    3822    fn eq_by<I, F>(self, other: I, eq: F) -> bool
    3823    where
    3824        Self: Sized,
    3825        I: IntoIterator,
    3826        F: FnMut(Self::Item, I::Item) -> bool,
    3827    {
    3828        #[inline]
    3829        fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
    3830        where
    3831            F: FnMut(X, Y) -> bool,
    3832        {
    3833            move |x, y| {
    3834                if eq(x, y) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
    3835            }
    3836        }
    3837
    3838        SpecIterEq::spec_iter_eq(self, other.into_iter(), compare(eq))
    3839    }
    3840
    3841    /// Determines if the elements of this [`Iterator`] are not equal to those of
    3842    /// another.
    3843    ///
    3844    /// # Examples
    3845    ///
    3846    /// ```
    3847    /// assert_eq!([1].iter().ne([1].iter()), false);
    3848    /// assert_eq!([1].iter().ne([1, 2].iter()), true);
    3849    /// ```
    3850    #[stable(feature = "iter_order", since = "1.5.0")]
    3851    fn ne<I>(self, other: I) -> bool
    3852    where
    3853        I: IntoIterator,
    3854        Self::Item: PartialEq<I::Item>,
    3855        Self: Sized,
    3856    {
    3857        !self.eq(other)
    3858    }
    3859
    3860    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
    3861    /// less than those of another.
    3862    ///
    3863    /// # Examples
    3864    ///
    3865    /// ```
    3866    /// assert_eq!([1].iter().lt([1].iter()), false);
    3867    /// assert_eq!([1].iter().lt([1, 2].iter()), true);
    3868    /// assert_eq!([1, 2].iter().lt([1].iter()), false);
    3869    /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
    3870    /// ```
    3871    #[stable(feature = "iter_order", since = "1.5.0")]
    3872    fn lt<I>(self, other: I) -> bool
    3873    where
    3874        I: IntoIterator,
    3875        Self::Item: PartialOrd<I::Item>,
    3876        Self: Sized,
    3877    {
    3878        self.partial_cmp(other) == Some(Ordering::Less)
    3879    }
    3880
    3881    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
    3882    /// less or equal to those of another.
    3883    ///
    3884    /// # Examples
    3885    ///
    3886    /// ```
    3887    /// assert_eq!([1].iter().le([1].iter()), true);
    3888    /// assert_eq!([1].iter().le([1, 2].iter()), true);
    3889    /// assert_eq!([1, 2].iter().le([1].iter()), false);
    3890    /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
    3891    /// ```
    3892    #[stable(feature = "iter_order", since = "1.5.0")]
    3893    fn le<I>(self, other: I) -> bool
    3894    where
    3895        I: IntoIterator,
    3896        Self::Item: PartialOrd<I::Item>,
    3897        Self: Sized,
    3898    {
    3899        matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
    3900    }
    3901
    3902    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
    3903    /// greater than those of another.
    3904    ///
    3905    /// # Examples
    3906    ///
    3907    /// ```
    3908    /// assert_eq!([1].iter().gt([1].iter()), false);
    3909    /// assert_eq!([1].iter().gt([1, 2].iter()), false);
    3910    /// assert_eq!([1, 2].iter().gt([1].iter()), true);
    3911    /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
    3912    /// ```
    3913    #[stable(feature = "iter_order", since = "1.5.0")]
    3914    fn gt<I>(self, other: I) -> bool
    3915    where
    3916        I: IntoIterator,
    3917        Self::Item: PartialOrd<I::Item>,
    3918        Self: Sized,
    3919    {
    3920        self.partial_cmp(other) == Some(Ordering::Greater)
    3921    }
    3922
    3923    /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
    3924    /// greater than or equal to those of another.
    3925    ///
    3926    /// # Examples
    3927    ///
    3928    /// ```
    3929    /// assert_eq!([1].iter().ge([1].iter()), true);
    3930    /// assert_eq!([1].iter().ge([1, 2].iter()), false);
    3931    /// assert_eq!([1, 2].iter().ge([1].iter()), true);
    3932    /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
    3933    /// ```
    3934    #[stable(feature = "iter_order", since = "1.5.0")]
    3935    fn ge<I>(self, other: I) -> bool
    3936    where
    3937        I: IntoIterator,
    3938        Self::Item: PartialOrd<I::Item>,
    3939        Self: Sized,
    3940    {
    3941        matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
    3942    }
    3943
    3944    /// Checks if the elements of this iterator are sorted.
    3945    ///
    3946    /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
    3947    /// iterator yields exactly zero or one element, `true` is returned.
    3948    ///
    3949    /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
    3950    /// implies that this function returns `false` if any two consecutive items are not
    3951    /// comparable.
    3952    ///
    3953    /// # Examples
    3954    ///
    3955    /// ```
    3956    /// assert!([1, 2, 2, 9].iter().is_sorted());
    3957    /// assert!(![1, 3, 2, 4].iter().is_sorted());
    3958    /// assert!([0].iter().is_sorted());
    3959    /// assert!(std::iter::empty::<i32>().is_sorted());
    3960    /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
    3961    /// ```
    3962    #[inline]
    3963    #[stable(feature = "is_sorted", since = "1.82.0")]
    3964    fn is_sorted(self) -> bool
    3965    where
    3966        Self: Sized,
    3967        Self::Item: PartialOrd,
    3968    {
    3969        self.is_sorted_by(|a, b| a <= b)
    3970    }
    3971
    3972    /// Checks if the elements of this iterator are sorted using the given comparator function.
    3973    ///
    3974    /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
    3975    /// function to determine whether two elements are to be considered in sorted order.
    3976    ///
    3977    /// # Examples
    3978    ///
    3979    /// ```
    3980    /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a <= b));
    3981    /// assert!(![1, 2, 2, 9].iter().is_sorted_by(|a, b| a < b));
    3982    ///
    3983    /// assert!([0].iter().is_sorted_by(|a, b| true));
    3984    /// assert!([0].iter().is_sorted_by(|a, b| false));
    3985    ///
    3986    /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| false));
    3987    /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| true));
    3988    /// ```
    3989    #[stable(feature = "is_sorted", since = "1.82.0")]
    3990    fn is_sorted_by<F>(mut self, compare: F) -> bool
    3991    where
    3992        Self: Sized,
    3993        F: FnMut(&Self::Item, &Self::Item) -> bool,
    3994    {
    3995        #[inline]
    3996        fn check<'a, T>(
    3997            last: &'a mut T,
    3998            mut compare: impl FnMut(&T, &T) -> bool + 'a,
    3999        ) -> impl FnMut(T) -> bool + 'a {
    4000            move |curr| {
    4001                if !compare(&last, &curr) {
    4002                    return false;
    4003                }
    4004                *last = curr;
    4005                true
    4006            }
    4007        }
    4008
    4009        let mut last = match self.next() {
    4010            Some(e) => e,
    4011            None => return true,
    4012        };
    4013
    4014        self.all(check(&mut last, compare))
    4015    }
    4016
    4017    /// Checks if the elements of this iterator are sorted using the given key extraction
    4018    /// function.
    4019    ///
    4020    /// Instead of comparing the iterator's elements directly, this function compares the keys of
    4021    /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
    4022    /// its documentation for more information.
    4023    ///
    4024    /// [`is_sorted`]: Iterator::is_sorted
    4025    ///
    4026    /// # Examples
    4027    ///
    4028    /// ```
    4029    /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
    4030    /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
    4031    /// ```
    4032    #[inline]
    4033    #[stable(feature = "is_sorted", since = "1.82.0")]
    4034    fn is_sorted_by_key<F, K>(self, f: F) -> bool
    4035    where
    4036        Self: Sized,
    4037        F: FnMut(Self::Item) -> K,
    4038        K: PartialOrd,
    4039    {
    4040        self.map(f).is_sorted()
    4041    }
    4042
    4043    /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
    4044    // The unusual name is to avoid name collisions in method resolution
    4045    // see #76479.
    4046    #[inline]
    4047    #[doc(hidden)]
    4048    #[unstable(feature = "trusted_random_access", issue = "none")]
    4049    unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
    4050    where
    4051        Self: TrustedRandomAccessNoCoerce,
    4052    {
    4053        unreachable!("Always specialized");
    4054    }
    4055}
    4056
    4057trait SpecIterEq<B: Iterator>: Iterator {
    4058    fn spec_iter_eq<F>(self, b: B, f: F) -> bool
    4059    where
    4060        F: FnMut(Self::Item, <B as Iterator>::Item) -> ControlFlow<()>;
    4061}
    4062
    4063impl<A: Iterator, B: Iterator> SpecIterEq<B> for A {
    4064    #[inline]
    4065    default fn spec_iter_eq<F>(self, b: B, f: F) -> bool
    4066    where
    4067        F: FnMut(Self::Item, <B as Iterator>::Item) -> ControlFlow<()>,
    4068    {
    4069        iter_eq(self, b, f)
    4070    }
    4071}
    4072
    4073impl<A: Iterator + TrustedLen, B: Iterator + TrustedLen> SpecIterEq<B> for A {
    4074    #[inline]
    4075    fn spec_iter_eq<F>(self, b: B, f: F) -> bool
    4076    where
    4077        F: FnMut(Self::Item, <B as Iterator>::Item) -> ControlFlow<()>,
    4078    {
    4079        // we *can't* short-circuit if:
    4080        match (self.size_hint(), b.size_hint()) {
    4081            // ... both iterators have the same length
    4082            ((_, Some(a)), (_, Some(b))) if a == b => {}
    4083            // ... or both of them are longer than `usize::MAX` (i.e. have an unknown length).
    4084            ((_, None), (_, None)) => {}
    4085            // otherwise, we can ascertain that they are unequal without actually comparing items
    4086            _ => return false,
    4087        }
    4088
    4089        iter_eq(self, b, f)
    4090    }
    4091}
    4092
    4093/// Compares two iterators element-wise using the given function.
    4094///
    4095/// If `ControlFlow::Continue(())` is returned from the function, the comparison moves on to the next
    4096/// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
    4097/// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
    4098/// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
    4099/// the iterators.
    4100///
    4101/// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
    4102/// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
    4103#[inline]
    4104fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
    4105where
    4106    A: Iterator,
    4107    B: Iterator,
    4108    F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
    4109{
    4110    #[inline]
    4111    fn compare<'a, B, X, T>(
    4112        b: &'a mut B,
    4113        mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
    4114    ) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
    4115    where
    4116        B: Iterator,
    4117    {
    4118        move |x| match b.next() {
    4119            None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
    4120            Some(y) => f(x, y).map_break(ControlFlow::Break),
    4121        }
    4122    }
    4123
    4124    match a.try_for_each(compare(&mut b, f)) {
    4125        ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
    4126            None => Ordering::Equal,
    4127            Some(_) => Ordering::Less,
    4128        }),
    4129        ControlFlow::Break(x) => x,
    4130    }
    4131}
    4132
    4133#[inline]
    4134fn iter_eq<A, B, F>(a: A, b: B, f: F) -> bool
    4135where
    4136    A: Iterator,
    4137    B: Iterator,
    4138    F: FnMut(A::Item, B::Item) -> ControlFlow<()>,
    4139{
    4140    iter_compare(a, b, f).continue_value().is_some_and(|ord| ord == Ordering::Equal)
    4141}
    4142
    4143/// Implements `Iterator` for mutable references to iterators, such as those produced by [`Iterator::by_ref`].
    4144///
    4145/// This implementation passes all method calls on to the original iterator.
    4146#[stable(feature = "rust1", since = "1.0.0")]
    4147impl<I: Iterator + ?Sized> Iterator for &mut I {
    4148    type Item = I::Item;
    4149    #[inline]
    4150    fn next(&mut self) -> Option<I::Item> {
    4151        (**self).next()
    4152    }
    4153    fn size_hint(&self) -> (usize, Option<usize>) {
    4154        (**self).size_hint()
    4155    }
    4156    fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
    4157        (**self).advance_by(n)
    4158    }
    4159    fn nth(&mut self, n: usize) -> Option<Self::Item> {
    4160        (**self).nth(n)
    4161    }
    4162    fn fold<B, F>(self, init: B, f: F) -> B
    4163    where
    4164        F: FnMut(B, Self::Item) -> B,
    4165    {
    4166        self.spec_fold(init, f)
    4167    }
    4168    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    4169    where
    4170        F: FnMut(B, Self::Item) -> R,
    4171        R: Try<Output = B>,
    4172    {
    4173        self.spec_try_fold(init, f)
    4174    }
    4175}
    4176
    4177/// Helper trait to specialize `fold` and `try_fold` for `&mut I where I: Sized`
    4178trait IteratorRefSpec: Iterator {
    4179    fn spec_fold<B, F>(self, init: B, f: F) -> B
    4180    where
    4181        F: FnMut(B, Self::Item) -> B;
    4182
    4183    fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    4184    where
    4185        F: FnMut(B, Self::Item) -> R,
    4186        R: Try<Output = B>;
    4187}
    4188
    4189impl<I: Iterator + ?Sized> IteratorRefSpec for &mut I {
    4190    default fn spec_fold<B, F>(self, init: B, mut f: F) -> B
    4191    where
    4192        F: FnMut(B, Self::Item) -> B,
    4193    {
    4194        let mut accum = init;
    4195        while let Some(x) = self.next() {
    4196            accum = f(accum, x);
    4197        }
    4198        accum
    4199    }
    4200
    4201    default fn spec_try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
    4202    where
    4203        F: FnMut(B, Self::Item) -> R,
    4204        R: Try<Output = B>,
    4205    {
    4206        let mut accum = init;
    4207        while let Some(x) = self.next() {
    4208            accum = f(accum, x)?;
    4209        }
    4210        try { accum }
    4211    }
    4212}
    4213
    4214impl<I: Iterator> IteratorRefSpec for &mut I {
    4215    impl_fold_via_try_fold! { spec_fold -> spec_try_fold }
    4216
    4217    fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    4218    where
    4219        F: FnMut(B, Self::Item) -> R,
    4220        R: Try<Output = B>,
    4221    {
    4222        (**self).try_fold(init, f)
    4223    }
    4224}