diff options
Diffstat (limited to 'library/core/src/iter/traits')
| -rw-r--r-- | library/core/src/iter/traits/accum.rs | 231 | ||||
| -rw-r--r-- | library/core/src/iter/traits/collect.rs | 450 | ||||
| -rw-r--r-- | library/core/src/iter/traits/double_ended.rs | 374 | ||||
| -rw-r--r-- | library/core/src/iter/traits/exact_size.rs | 151 | ||||
| -rw-r--r-- | library/core/src/iter/traits/iterator.rs | 3836 | ||||
| -rw-r--r-- | library/core/src/iter/traits/marker.rs | 78 | ||||
| -rw-r--r-- | library/core/src/iter/traits/mod.rs | 21 |
7 files changed, 5141 insertions, 0 deletions
diff --git a/library/core/src/iter/traits/accum.rs b/library/core/src/iter/traits/accum.rs new file mode 100644 index 000000000..84d83ee39 --- /dev/null +++ b/library/core/src/iter/traits/accum.rs @@ -0,0 +1,231 @@ +use crate::iter; +use crate::num::Wrapping; + +/// Trait to represent types that can be created by summing up an iterator. +/// +/// This trait is used to implement [`Iterator::sum()`]. Types which implement +/// this trait can be generated by using the [`sum()`] method on an iterator. +/// Like [`FromIterator`], this trait should rarely be called directly. +/// +/// [`sum()`]: Iterator::sum +/// [`FromIterator`]: iter::FromIterator +#[stable(feature = "iter_arith_traits", since = "1.12.0")] +pub trait Sum<A = Self>: Sized { + /// Method which takes an iterator and generates `Self` from the elements by + /// "summing up" the items. + #[stable(feature = "iter_arith_traits", since = "1.12.0")] + fn sum<I: Iterator<Item = A>>(iter: I) -> Self; +} + +/// Trait to represent types that can be created by multiplying elements of an +/// iterator. +/// +/// This trait is used to implement [`Iterator::product()`]. Types which implement +/// this trait can be generated by using the [`product()`] method on an iterator. +/// Like [`FromIterator`], this trait should rarely be called directly. +/// +/// [`product()`]: Iterator::product +/// [`FromIterator`]: iter::FromIterator +#[stable(feature = "iter_arith_traits", since = "1.12.0")] +pub trait Product<A = Self>: Sized { + /// Method which takes an iterator and generates `Self` from the elements by + /// multiplying the items. + #[stable(feature = "iter_arith_traits", since = "1.12.0")] + fn product<I: Iterator<Item = A>>(iter: I) -> Self; +} + +macro_rules! integer_sum_product { + (@impls $zero:expr, $one:expr, #[$attr:meta], $($a:ty)*) => ($( + #[$attr] + impl Sum for $a { + fn sum<I: Iterator<Item=Self>>(iter: I) -> Self { + iter.fold( + $zero, + #[rustc_inherit_overflow_checks] + |a, b| a + b, + ) + } + } + + #[$attr] + impl Product for $a { + fn product<I: Iterator<Item=Self>>(iter: I) -> Self { + iter.fold( + $one, + #[rustc_inherit_overflow_checks] + |a, b| a * b, + ) + } + } + + #[$attr] + impl<'a> Sum<&'a $a> for $a { + fn sum<I: Iterator<Item=&'a Self>>(iter: I) -> Self { + iter.fold( + $zero, + #[rustc_inherit_overflow_checks] + |a, b| a + b, + ) + } + } + + #[$attr] + impl<'a> Product<&'a $a> for $a { + fn product<I: Iterator<Item=&'a Self>>(iter: I) -> Self { + iter.fold( + $one, + #[rustc_inherit_overflow_checks] + |a, b| a * b, + ) + } + } + )*); + ($($a:ty)*) => ( + integer_sum_product!(@impls 0, 1, + #[stable(feature = "iter_arith_traits", since = "1.12.0")], + $($a)*); + integer_sum_product!(@impls Wrapping(0), Wrapping(1), + #[stable(feature = "wrapping_iter_arith", since = "1.14.0")], + $(Wrapping<$a>)*); + ); +} + +macro_rules! float_sum_product { + ($($a:ident)*) => ($( + #[stable(feature = "iter_arith_traits", since = "1.12.0")] + impl Sum for $a { + fn sum<I: Iterator<Item=Self>>(iter: I) -> Self { + iter.fold( + 0.0, + #[rustc_inherit_overflow_checks] + |a, b| a + b, + ) + } + } + + #[stable(feature = "iter_arith_traits", since = "1.12.0")] + impl Product for $a { + fn product<I: Iterator<Item=Self>>(iter: I) -> Self { + iter.fold( + 1.0, + #[rustc_inherit_overflow_checks] + |a, b| a * b, + ) + } + } + + #[stable(feature = "iter_arith_traits", since = "1.12.0")] + impl<'a> Sum<&'a $a> for $a { + fn sum<I: Iterator<Item=&'a Self>>(iter: I) -> Self { + iter.fold( + 0.0, + #[rustc_inherit_overflow_checks] + |a, b| a + b, + ) + } + } + + #[stable(feature = "iter_arith_traits", since = "1.12.0")] + impl<'a> Product<&'a $a> for $a { + fn product<I: Iterator<Item=&'a Self>>(iter: I) -> Self { + iter.fold( + 1.0, + #[rustc_inherit_overflow_checks] + |a, b| a * b, + ) + } + } + )*) +} + +integer_sum_product! { i8 i16 i32 i64 i128 isize u8 u16 u32 u64 u128 usize } +float_sum_product! { f32 f64 } + +#[stable(feature = "iter_arith_traits_result", since = "1.16.0")] +impl<T, U, E> Sum<Result<U, E>> for Result<T, E> +where + T: Sum<U>, +{ + /// Takes each element in the [`Iterator`]: if it is an [`Err`], no further + /// elements are taken, and the [`Err`] is returned. Should no [`Err`] + /// occur, the sum of all elements is returned. + /// + /// # Examples + /// + /// This sums up every integer in a vector, rejecting the sum if a negative + /// element is encountered: + /// + /// ``` + /// let v = vec![1, 2]; + /// let res: Result<i32, &'static str> = v.iter().map(|&x: &i32| + /// if x < 0 { Err("Negative element found") } + /// else { Ok(x) } + /// ).sum(); + /// assert_eq!(res, Ok(3)); + /// ``` + fn sum<I>(iter: I) -> Result<T, E> + where + I: Iterator<Item = Result<U, E>>, + { + iter::try_process(iter, |i| i.sum()) + } +} + +#[stable(feature = "iter_arith_traits_result", since = "1.16.0")] +impl<T, U, E> Product<Result<U, E>> for Result<T, E> +where + T: Product<U>, +{ + /// Takes each element in the [`Iterator`]: if it is an [`Err`], no further + /// elements are taken, and the [`Err`] is returned. Should no [`Err`] + /// occur, the product of all elements is returned. + fn product<I>(iter: I) -> Result<T, E> + where + I: Iterator<Item = Result<U, E>>, + { + iter::try_process(iter, |i| i.product()) + } +} + +#[stable(feature = "iter_arith_traits_option", since = "1.37.0")] +impl<T, U> Sum<Option<U>> for Option<T> +where + T: Sum<U>, +{ + /// Takes each element in the [`Iterator`]: if it is a [`None`], no further + /// elements are taken, and the [`None`] is returned. Should no [`None`] + /// occur, the sum of all elements is returned. + /// + /// # Examples + /// + /// This sums up the position of the character 'a' in a vector of strings, + /// if a word did not have the character 'a' the operation returns `None`: + /// + /// ``` + /// let words = vec!["have", "a", "great", "day"]; + /// let total: Option<usize> = words.iter().map(|w| w.find('a')).sum(); + /// assert_eq!(total, Some(5)); + /// ``` + fn sum<I>(iter: I) -> Option<T> + where + I: Iterator<Item = Option<U>>, + { + iter::try_process(iter, |i| i.sum()) + } +} + +#[stable(feature = "iter_arith_traits_option", since = "1.37.0")] +impl<T, U> Product<Option<U>> for Option<T> +where + T: Product<U>, +{ + /// Takes each element in the [`Iterator`]: if it is a [`None`], no further + /// elements are taken, and the [`None`] is returned. Should no [`None`] + /// occur, the product of all elements is returned. + fn product<I>(iter: I) -> Option<T> + where + I: Iterator<Item = Option<U>>, + { + iter::try_process(iter, |i| i.product()) + } +} diff --git a/library/core/src/iter/traits/collect.rs b/library/core/src/iter/traits/collect.rs new file mode 100644 index 000000000..12ca508be --- /dev/null +++ b/library/core/src/iter/traits/collect.rs @@ -0,0 +1,450 @@ +/// Conversion from an [`Iterator`]. +/// +/// By implementing `FromIterator` for a type, you define how it will be +/// created from an iterator. This is common for types which describe a +/// collection of some kind. +/// +/// If you want to create a collection from the contents of an iterator, the +/// [`Iterator::collect()`] method is preferred. However, when you need to +/// specify the container type, [`FromIterator::from_iter()`] can be more +/// readable than using a turbofish (e.g. `::<Vec<_>>()`). See the +/// [`Iterator::collect()`] documentation for more examples of its use. +/// +/// See also: [`IntoIterator`]. +/// +/// # Examples +/// +/// Basic usage: +/// +/// ``` +/// let five_fives = std::iter::repeat(5).take(5); +/// +/// let v = Vec::from_iter(five_fives); +/// +/// assert_eq!(v, vec![5, 5, 5, 5, 5]); +/// ``` +/// +/// Using [`Iterator::collect()`] to implicitly use `FromIterator`: +/// +/// ``` +/// let five_fives = std::iter::repeat(5).take(5); +/// +/// let v: Vec<i32> = five_fives.collect(); +/// +/// assert_eq!(v, vec![5, 5, 5, 5, 5]); +/// ``` +/// +/// Using [`FromIterator::from_iter()`] as a more readable alternative to +/// [`Iterator::collect()`]: +/// +/// ``` +/// use std::collections::VecDeque; +/// let first = (0..10).collect::<VecDeque<i32>>(); +/// let second = VecDeque::from_iter(0..10); +/// +/// assert_eq!(first, second); +/// ``` +/// +/// Implementing `FromIterator` for your type: +/// +/// ``` +/// // A sample collection, that's just a wrapper over Vec<T> +/// #[derive(Debug)] +/// struct MyCollection(Vec<i32>); +/// +/// // Let's give it some methods so we can create one and add things +/// // to it. +/// impl MyCollection { +/// fn new() -> MyCollection { +/// MyCollection(Vec::new()) +/// } +/// +/// fn add(&mut self, elem: i32) { +/// self.0.push(elem); +/// } +/// } +/// +/// // and we'll implement FromIterator +/// impl FromIterator<i32> for MyCollection { +/// fn from_iter<I: IntoIterator<Item=i32>>(iter: I) -> Self { +/// let mut c = MyCollection::new(); +/// +/// for i in iter { +/// c.add(i); +/// } +/// +/// c +/// } +/// } +/// +/// // Now we can make a new iterator... +/// let iter = (0..5).into_iter(); +/// +/// // ... and make a MyCollection out of it +/// let c = MyCollection::from_iter(iter); +/// +/// assert_eq!(c.0, vec![0, 1, 2, 3, 4]); +/// +/// // collect works too! +/// +/// let iter = (0..5).into_iter(); +/// let c: MyCollection = iter.collect(); +/// +/// assert_eq!(c.0, vec![0, 1, 2, 3, 4]); +/// ``` +#[stable(feature = "rust1", since = "1.0.0")] +#[rustc_on_unimplemented( + on( + _Self = "[{A}]", + message = "a slice of type `{Self}` cannot be built since `{Self}` has no definite size", + label = "try explicitly collecting into a `Vec<{A}>`", + ), + on( + all(A = "{integer}", any(_Self = "[{integral}]",)), + message = "a slice of type `{Self}` cannot be built since `{Self}` has no definite size", + label = "try explicitly collecting into a `Vec<{A}>`", + ), + on( + _Self = "[{A}; _]", + message = "an array of type `{Self}` cannot be built directly from an iterator", + label = "try collecting into a `Vec<{A}>`, then using `.try_into()`", + ), + on( + all(A = "{integer}", any(_Self = "[{integral}; _]",)), + message = "an array of type `{Self}` cannot be built directly from an iterator", + label = "try collecting into a `Vec<{A}>`, then using `.try_into()`", + ), + message = "a value of type `{Self}` cannot be built from an iterator \ + over elements of type `{A}`", + label = "value of type `{Self}` cannot be built from `std::iter::Iterator<Item={A}>`" +)] +#[rustc_diagnostic_item = "FromIterator"] +pub trait FromIterator<A>: Sized { + /// Creates a value from an iterator. + /// + /// See the [module-level documentation] for more. + /// + /// [module-level documentation]: crate::iter + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let five_fives = std::iter::repeat(5).take(5); + /// + /// let v = Vec::from_iter(five_fives); + /// + /// assert_eq!(v, vec![5, 5, 5, 5, 5]); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self; +} + +/// Conversion into an [`Iterator`]. +/// +/// By implementing `IntoIterator` for a type, you define how it will be +/// converted to an iterator. This is common for types which describe a +/// collection of some kind. +/// +/// One benefit of implementing `IntoIterator` is that your type will [work +/// with Rust's `for` loop syntax](crate::iter#for-loops-and-intoiterator). +/// +/// See also: [`FromIterator`]. +/// +/// # Examples +/// +/// Basic usage: +/// +/// ``` +/// let v = [1, 2, 3]; +/// let mut iter = v.into_iter(); +/// +/// assert_eq!(Some(1), iter.next()); +/// assert_eq!(Some(2), iter.next()); +/// assert_eq!(Some(3), iter.next()); +/// assert_eq!(None, iter.next()); +/// ``` +/// Implementing `IntoIterator` for your type: +/// +/// ``` +/// // A sample collection, that's just a wrapper over Vec<T> +/// #[derive(Debug)] +/// struct MyCollection(Vec<i32>); +/// +/// // Let's give it some methods so we can create one and add things +/// // to it. +/// impl MyCollection { +/// fn new() -> MyCollection { +/// MyCollection(Vec::new()) +/// } +/// +/// fn add(&mut self, elem: i32) { +/// self.0.push(elem); +/// } +/// } +/// +/// // and we'll implement IntoIterator +/// impl IntoIterator for MyCollection { +/// type Item = i32; +/// type IntoIter = std::vec::IntoIter<Self::Item>; +/// +/// fn into_iter(self) -> Self::IntoIter { +/// self.0.into_iter() +/// } +/// } +/// +/// // Now we can make a new collection... +/// let mut c = MyCollection::new(); +/// +/// // ... add some stuff to it ... +/// c.add(0); +/// c.add(1); +/// c.add(2); +/// +/// // ... and then turn it into an Iterator: +/// for (i, n) in c.into_iter().enumerate() { +/// assert_eq!(i as i32, n); +/// } +/// ``` +/// +/// It is common to use `IntoIterator` as a trait bound. This allows +/// the input collection type to change, so long as it is still an +/// iterator. Additional bounds can be specified by restricting on +/// `Item`: +/// +/// ```rust +/// fn collect_as_strings<T>(collection: T) -> Vec<String> +/// where +/// T: IntoIterator, +/// T::Item: std::fmt::Debug, +/// { +/// collection +/// .into_iter() +/// .map(|item| format!("{item:?}")) +/// .collect() +/// } +/// ``` +#[rustc_diagnostic_item = "IntoIterator"] +#[rustc_skip_array_during_method_dispatch] +#[stable(feature = "rust1", since = "1.0.0")] +pub trait IntoIterator { + /// The type of the elements being iterated over. + #[stable(feature = "rust1", since = "1.0.0")] + type Item; + + /// Which kind of iterator are we turning this into? + #[stable(feature = "rust1", since = "1.0.0")] + type IntoIter: Iterator<Item = Self::Item>; + + /// Creates an iterator from a value. + /// + /// See the [module-level documentation] for more. + /// + /// [module-level documentation]: crate::iter + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let v = [1, 2, 3]; + /// let mut iter = v.into_iter(); + /// + /// assert_eq!(Some(1), iter.next()); + /// assert_eq!(Some(2), iter.next()); + /// assert_eq!(Some(3), iter.next()); + /// assert_eq!(None, iter.next()); + /// ``` + #[lang = "into_iter"] + #[stable(feature = "rust1", since = "1.0.0")] + fn into_iter(self) -> Self::IntoIter; +} + +#[rustc_const_unstable(feature = "const_intoiterator_identity", issue = "90603")] +#[stable(feature = "rust1", since = "1.0.0")] +impl<I: ~const Iterator> const IntoIterator for I { + type Item = I::Item; + type IntoIter = I; + + #[inline] + fn into_iter(self) -> I { + self + } +} + +/// Extend a collection with the contents of an iterator. +/// +/// Iterators produce a series of values, and collections can also be thought +/// of as a series of values. The `Extend` trait bridges this gap, allowing you +/// to extend a collection by including the contents of that iterator. When +/// extending a collection with an already existing key, that entry is updated +/// or, in the case of collections that permit multiple entries with equal +/// keys, that entry is inserted. +/// +/// # Examples +/// +/// Basic usage: +/// +/// ``` +/// // You can extend a String with some chars: +/// let mut message = String::from("The first three letters are: "); +/// +/// message.extend(&['a', 'b', 'c']); +/// +/// assert_eq!("abc", &message[29..32]); +/// ``` +/// +/// Implementing `Extend`: +/// +/// ``` +/// // A sample collection, that's just a wrapper over Vec<T> +/// #[derive(Debug)] +/// struct MyCollection(Vec<i32>); +/// +/// // Let's give it some methods so we can create one and add things +/// // to it. +/// impl MyCollection { +/// fn new() -> MyCollection { +/// MyCollection(Vec::new()) +/// } +/// +/// fn add(&mut self, elem: i32) { +/// self.0.push(elem); +/// } +/// } +/// +/// // since MyCollection has a list of i32s, we implement Extend for i32 +/// impl Extend<i32> for MyCollection { +/// +/// // This is a bit simpler with the concrete type signature: we can call +/// // extend on anything which can be turned into an Iterator which gives +/// // us i32s. Because we need i32s to put into MyCollection. +/// fn extend<T: IntoIterator<Item=i32>>(&mut self, iter: T) { +/// +/// // The implementation is very straightforward: loop through the +/// // iterator, and add() each element to ourselves. +/// for elem in iter { +/// self.add(elem); +/// } +/// } +/// } +/// +/// let mut c = MyCollection::new(); +/// +/// c.add(5); +/// c.add(6); +/// c.add(7); +/// +/// // let's extend our collection with three more numbers +/// c.extend(vec![1, 2, 3]); +/// +/// // we've added these elements onto the end +/// assert_eq!("MyCollection([5, 6, 7, 1, 2, 3])", format!("{c:?}")); +/// ``` +#[stable(feature = "rust1", since = "1.0.0")] +pub trait Extend<A> { + /// Extends a collection with the contents of an iterator. + /// + /// As this is the only required method for this trait, the [trait-level] docs + /// contain more details. + /// + /// [trait-level]: Extend + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// // You can extend a String with some chars: + /// let mut message = String::from("abc"); + /// + /// message.extend(['d', 'e', 'f'].iter()); + /// + /// assert_eq!("abcdef", &message); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T); + + /// Extends a collection with exactly one element. + #[unstable(feature = "extend_one", issue = "72631")] + fn extend_one(&mut self, item: A) { + self.extend(Some(item)); + } + + /// Reserves capacity in a collection for the given number of additional elements. + /// + /// The default implementation does nothing. + #[unstable(feature = "extend_one", issue = "72631")] + fn extend_reserve(&mut self, additional: usize) { + let _ = additional; + } +} + +#[stable(feature = "extend_for_unit", since = "1.28.0")] +impl Extend<()> for () { + fn extend<T: IntoIterator<Item = ()>>(&mut self, iter: T) { + iter.into_iter().for_each(drop) + } + fn extend_one(&mut self, _item: ()) {} +} + +#[stable(feature = "extend_for_tuple", since = "1.56.0")] +impl<A, B, ExtendA, ExtendB> Extend<(A, B)> for (ExtendA, ExtendB) +where + ExtendA: Extend<A>, + ExtendB: Extend<B>, +{ + /// Allows to `extend` a tuple of collections that also implement `Extend`. + /// + /// See also: [`Iterator::unzip`] + /// + /// # Examples + /// ``` + /// let mut tuple = (vec![0], vec![1]); + /// tuple.extend([(2, 3), (4, 5), (6, 7)]); + /// assert_eq!(tuple.0, [0, 2, 4, 6]); + /// assert_eq!(tuple.1, [1, 3, 5, 7]); + /// + /// // also allows for arbitrarily nested tuples as elements + /// let mut nested_tuple = (vec![1], (vec![2], vec![3])); + /// nested_tuple.extend([(4, (5, 6)), (7, (8, 9))]); + /// + /// let (a, (b, c)) = nested_tuple; + /// assert_eq!(a, [1, 4, 7]); + /// assert_eq!(b, [2, 5, 8]); + /// assert_eq!(c, [3, 6, 9]); + /// ``` + fn extend<T: IntoIterator<Item = (A, B)>>(&mut self, into_iter: T) { + let (a, b) = self; + let iter = into_iter.into_iter(); + + fn extend<'a, A, B>( + a: &'a mut impl Extend<A>, + b: &'a mut impl Extend<B>, + ) -> impl FnMut((), (A, B)) + 'a { + move |(), (t, u)| { + a.extend_one(t); + b.extend_one(u); + } + } + + let (lower_bound, _) = iter.size_hint(); + if lower_bound > 0 { + a.extend_reserve(lower_bound); + b.extend_reserve(lower_bound); + } + + iter.fold((), extend(a, b)); + } + + fn extend_one(&mut self, item: (A, B)) { + self.0.extend_one(item.0); + self.1.extend_one(item.1); + } + + fn extend_reserve(&mut self, additional: usize) { + self.0.extend_reserve(additional); + self.1.extend_reserve(additional); + } +} diff --git a/library/core/src/iter/traits/double_ended.rs b/library/core/src/iter/traits/double_ended.rs new file mode 100644 index 000000000..bdf94c792 --- /dev/null +++ b/library/core/src/iter/traits/double_ended.rs @@ -0,0 +1,374 @@ +use crate::ops::{ControlFlow, Try}; + +/// An iterator able to yield elements from both ends. +/// +/// Something that implements `DoubleEndedIterator` has one extra capability +/// over something that implements [`Iterator`]: the ability to also take +/// `Item`s from the back, as well as the front. +/// +/// It is important to note that both back and forth work on the same range, +/// and do not cross: iteration is over when they meet in the middle. +/// +/// In a similar fashion to the [`Iterator`] protocol, once a +/// `DoubleEndedIterator` returns [`None`] from a [`next_back()`], calling it +/// again may or may not ever return [`Some`] again. [`next()`] and +/// [`next_back()`] are interchangeable for this purpose. +/// +/// [`next_back()`]: DoubleEndedIterator::next_back +/// [`next()`]: Iterator::next +/// +/// # Examples +/// +/// Basic usage: +/// +/// ``` +/// let numbers = vec![1, 2, 3, 4, 5, 6]; +/// +/// let mut iter = numbers.iter(); +/// +/// assert_eq!(Some(&1), iter.next()); +/// assert_eq!(Some(&6), iter.next_back()); +/// assert_eq!(Some(&5), iter.next_back()); +/// assert_eq!(Some(&2), iter.next()); +/// assert_eq!(Some(&3), iter.next()); +/// assert_eq!(Some(&4), iter.next()); +/// assert_eq!(None, iter.next()); +/// assert_eq!(None, iter.next_back()); +/// ``` +#[stable(feature = "rust1", since = "1.0.0")] +#[cfg_attr(not(test), rustc_diagnostic_item = "DoubleEndedIterator")] +pub trait DoubleEndedIterator: Iterator { + /// Removes and returns an element from the end of the iterator. + /// + /// Returns `None` when there are no more elements. + /// + /// The [trait-level] docs contain more details. + /// + /// [trait-level]: DoubleEndedIterator + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let numbers = vec![1, 2, 3, 4, 5, 6]; + /// + /// let mut iter = numbers.iter(); + /// + /// assert_eq!(Some(&1), iter.next()); + /// assert_eq!(Some(&6), iter.next_back()); + /// assert_eq!(Some(&5), iter.next_back()); + /// assert_eq!(Some(&2), iter.next()); + /// assert_eq!(Some(&3), iter.next()); + /// assert_eq!(Some(&4), iter.next()); + /// assert_eq!(None, iter.next()); + /// assert_eq!(None, iter.next_back()); + /// ``` + /// + /// # Remarks + /// + /// The elements yielded by `DoubleEndedIterator`'s methods may differ from + /// the ones yielded by [`Iterator`]'s methods: + /// + /// ``` + /// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')]; + /// let uniq_by_fst_comp = || { + /// let mut seen = std::collections::HashSet::new(); + /// vec.iter().copied().filter(move |x| seen.insert(x.0)) + /// }; + /// + /// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a'))); + /// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b'))); + /// + /// assert_eq!( + /// uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}), + /// vec![(1, 'a'), (2, 'a')] + /// ); + /// assert_eq!( + /// uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}), + /// vec![(2, 'b'), (1, 'c')] + /// ); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + fn next_back(&mut self) -> Option<Self::Item>; + + /// Advances the iterator from the back by `n` elements. + /// + /// `advance_back_by` is the reverse version of [`advance_by`]. This method will + /// eagerly skip `n` elements starting from the back by calling [`next_back`] up + /// to `n` times until [`None`] is encountered. + /// + /// `advance_back_by(n)` will return [`Ok(())`] if the iterator successfully advances by + /// `n` elements, or [`Err(k)`] if [`None`] is encountered, where `k` is the number of + /// elements the iterator is advanced by before running out of elements (i.e. the length + /// of the iterator). Note that `k` is always less than `n`. + /// + /// Calling `advance_back_by(0)` can do meaningful work, for example [`Flatten`] can advance its + /// outer iterator until it finds an inner iterator that is not empty, which then often + /// allows it to return a more accurate `size_hint()` than in its initial state. + /// + /// [`advance_by`]: Iterator::advance_by + /// [`Flatten`]: crate::iter::Flatten + /// [`next_back`]: DoubleEndedIterator::next_back + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_advance_by)] + /// + /// let a = [3, 4, 5, 6]; + /// let mut iter = a.iter(); + /// + /// assert_eq!(iter.advance_back_by(2), Ok(())); + /// assert_eq!(iter.next_back(), Some(&4)); + /// assert_eq!(iter.advance_back_by(0), Ok(())); + /// assert_eq!(iter.advance_back_by(100), Err(1)); // only `&3` was skipped + /// ``` + /// + /// [`Ok(())`]: Ok + /// [`Err(k)`]: Err + #[inline] + #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")] + fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { + for i in 0..n { + self.next_back().ok_or(i)?; + } + Ok(()) + } + + /// Returns the `n`th element from the end of the iterator. + /// + /// This is essentially the reversed version of [`Iterator::nth()`]. + /// Although like most indexing operations, the count starts from zero, so + /// `nth_back(0)` returns the first value from the end, `nth_back(1)` the + /// second, and so on. + /// + /// Note that all elements between the end and the returned element will be + /// consumed, including the returned element. This also means that calling + /// `nth_back(0)` multiple times on the same iterator will return different + /// elements. + /// + /// `nth_back()` will return [`None`] if `n` is greater than or equal to the + /// length of the iterator. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// assert_eq!(a.iter().nth_back(2), Some(&1)); + /// ``` + /// + /// Calling `nth_back()` multiple times doesn't rewind the iterator: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter(); + /// + /// assert_eq!(iter.nth_back(1), Some(&2)); + /// assert_eq!(iter.nth_back(1), None); + /// ``` + /// + /// Returning `None` if there are less than `n + 1` elements: + /// + /// ``` + /// let a = [1, 2, 3]; + /// assert_eq!(a.iter().nth_back(10), None); + /// ``` + #[inline] + #[stable(feature = "iter_nth_back", since = "1.37.0")] + fn nth_back(&mut self, n: usize) -> Option<Self::Item> { + self.advance_back_by(n).ok()?; + self.next_back() + } + + /// This is the reverse version of [`Iterator::try_fold()`]: it takes + /// elements starting from the back of the iterator. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = ["1", "2", "3"]; + /// let sum = a.iter() + /// .map(|&s| s.parse::<i32>()) + /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y))); + /// assert_eq!(sum, Ok(6)); + /// ``` + /// + /// Short-circuiting: + /// + /// ``` + /// let a = ["1", "rust", "3"]; + /// let mut it = a.iter(); + /// let sum = it + /// .by_ref() + /// .map(|&s| s.parse::<i32>()) + /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y))); + /// assert!(sum.is_err()); + /// + /// // Because it short-circuited, the remaining elements are still + /// // available through the iterator. + /// assert_eq!(it.next_back(), Some(&"1")); + /// ``` + #[inline] + #[stable(feature = "iterator_try_fold", since = "1.27.0")] + fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R + where + Self: Sized, + F: FnMut(B, Self::Item) -> R, + R: Try<Output = B>, + { + let mut accum = init; + while let Some(x) = self.next_back() { + accum = f(accum, x)?; + } + try { accum } + } + + /// An iterator method that reduces the iterator's elements to a single, + /// final value, starting from the back. + /// + /// This is the reverse version of [`Iterator::fold()`]: it takes elements + /// starting from the back of the iterator. + /// + /// `rfold()` takes two arguments: an initial value, and a closure with two + /// arguments: an 'accumulator', and an element. The closure returns the value that + /// the accumulator should have for the next iteration. + /// + /// The initial value is the value the accumulator will have on the first + /// call. + /// + /// After applying this closure to every element of the iterator, `rfold()` + /// returns the accumulator. + /// + /// This operation is sometimes called 'reduce' or 'inject'. + /// + /// Folding is useful whenever you have a collection of something, and want + /// to produce a single value from it. + /// + /// Note: `rfold()` combines elements in a *right-associative* fashion. For associative + /// operators like `+`, the order the elements are combined in is not important, but for non-associative + /// operators like `-` the order will affect the final result. + /// For a *left-associative* version of `rfold()`, see [`Iterator::fold()`]. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// // the sum of all of the elements of a + /// let sum = a.iter() + /// .rfold(0, |acc, &x| acc + x); + /// + /// assert_eq!(sum, 6); + /// ``` + /// + /// This example demonstrates the right-associative nature of `rfold()`: + /// it builds a string, starting with an initial value + /// and continuing with each element from the back until the front: + /// + /// ``` + /// let numbers = [1, 2, 3, 4, 5]; + /// + /// let zero = "0".to_string(); + /// + /// let result = numbers.iter().rfold(zero, |acc, &x| { + /// format!("({x} + {acc})") + /// }); + /// + /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))"); + /// ``` + #[doc(alias = "foldr")] + #[inline] + #[stable(feature = "iter_rfold", since = "1.27.0")] + fn rfold<B, F>(mut self, init: B, mut f: F) -> B + where + Self: Sized, + F: FnMut(B, Self::Item) -> B, + { + let mut accum = init; + while let Some(x) = self.next_back() { + accum = f(accum, x); + } + accum + } + + /// Searches for an element of an iterator from the back that satisfies a predicate. + /// + /// `rfind()` takes a closure that returns `true` or `false`. It applies + /// this closure to each element of the iterator, starting at the end, and if any + /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return + /// `false`, it returns [`None`]. + /// + /// `rfind()` is short-circuiting; in other words, it will stop processing + /// as soon as the closure returns `true`. + /// + /// Because `rfind()` takes a reference, and many iterators iterate over + /// references, this leads to a possibly confusing situation where the + /// argument is a double reference. You can see this effect in the + /// examples below, with `&&x`. + /// + /// [`Some(element)`]: Some + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2)); + /// + /// assert_eq!(a.iter().rfind(|&&x| x == 5), None); + /// ``` + /// + /// Stopping at the first `true`: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter(); + /// + /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2)); + /// + /// // we can still use `iter`, as there are more elements. + /// assert_eq!(iter.next_back(), Some(&1)); + /// ``` + #[inline] + #[stable(feature = "iter_rfind", since = "1.27.0")] + fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item> + where + Self: Sized, + P: FnMut(&Self::Item) -> bool, + { + #[inline] + fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> { + move |(), x| { + if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE } + } + } + + self.try_rfold((), check(predicate)).break_value() + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I { + fn next_back(&mut self) -> Option<I::Item> { + (**self).next_back() + } + fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { + (**self).advance_back_by(n) + } + fn nth_back(&mut self, n: usize) -> Option<I::Item> { + (**self).nth_back(n) + } +} diff --git a/library/core/src/iter/traits/exact_size.rs b/library/core/src/iter/traits/exact_size.rs new file mode 100644 index 000000000..1757e37ec --- /dev/null +++ b/library/core/src/iter/traits/exact_size.rs @@ -0,0 +1,151 @@ +/// An iterator that knows its exact length. +/// +/// Many [`Iterator`]s don't know how many times they will iterate, but some do. +/// If an iterator knows how many times it can iterate, providing access to +/// that information can be useful. For example, if you want to iterate +/// backwards, a good start is to know where the end is. +/// +/// When implementing an `ExactSizeIterator`, you must also implement +/// [`Iterator`]. When doing so, the implementation of [`Iterator::size_hint`] +/// *must* return the exact size of the iterator. +/// +/// The [`len`] method has a default implementation, so you usually shouldn't +/// implement it. However, you may be able to provide a more performant +/// implementation than the default, so overriding it in this case makes sense. +/// +/// Note that this trait is a safe trait and as such does *not* and *cannot* +/// guarantee that the returned length is correct. This means that `unsafe` +/// code **must not** rely on the correctness of [`Iterator::size_hint`]. The +/// unstable and unsafe [`TrustedLen`](super::marker::TrustedLen) trait gives +/// this additional guarantee. +/// +/// [`len`]: ExactSizeIterator::len +/// +/// # Examples +/// +/// Basic usage: +/// +/// ``` +/// // a finite range knows exactly how many times it will iterate +/// let five = 0..5; +/// +/// assert_eq!(5, five.len()); +/// ``` +/// +/// In the [module-level docs], we implemented an [`Iterator`], `Counter`. +/// Let's implement `ExactSizeIterator` for it as well: +/// +/// [module-level docs]: crate::iter +/// +/// ``` +/// # struct Counter { +/// # count: usize, +/// # } +/// # impl Counter { +/// # fn new() -> Counter { +/// # Counter { count: 0 } +/// # } +/// # } +/// # impl Iterator for Counter { +/// # type Item = usize; +/// # fn next(&mut self) -> Option<Self::Item> { +/// # self.count += 1; +/// # if self.count < 6 { +/// # Some(self.count) +/// # } else { +/// # None +/// # } +/// # } +/// # } +/// impl ExactSizeIterator for Counter { +/// // We can easily calculate the remaining number of iterations. +/// fn len(&self) -> usize { +/// 5 - self.count +/// } +/// } +/// +/// // And now we can use it! +/// +/// let mut counter = Counter::new(); +/// +/// assert_eq!(5, counter.len()); +/// let _ = counter.next(); +/// assert_eq!(4, counter.len()); +/// ``` +#[stable(feature = "rust1", since = "1.0.0")] +pub trait ExactSizeIterator: Iterator { + /// Returns the exact remaining length of the iterator. + /// + /// The implementation ensures that the iterator will return exactly `len()` + /// more times a [`Some(T)`] value, before returning [`None`]. + /// This method has a default implementation, so you usually should not + /// implement it directly. However, if you can provide a more efficient + /// implementation, you can do so. See the [trait-level] docs for an + /// example. + /// + /// This function has the same safety guarantees as the + /// [`Iterator::size_hint`] function. + /// + /// [trait-level]: ExactSizeIterator + /// [`Some(T)`]: Some + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// // a finite range knows exactly how many times it will iterate + /// let mut range = 0..5; + /// + /// assert_eq!(5, range.len()); + /// let _ = range.next(); + /// assert_eq!(4, range.len()); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn len(&self) -> usize { + let (lower, upper) = self.size_hint(); + // Note: This assertion is overly defensive, but it checks the invariant + // guaranteed by the trait. If this trait were rust-internal, + // we could use debug_assert!; assert_eq! will check all Rust user + // implementations too. + assert_eq!(upper, Some(lower)); + lower + } + + /// Returns `true` if the iterator is empty. + /// + /// This method has a default implementation using + /// [`ExactSizeIterator::len()`], so you don't need to implement it yourself. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(exact_size_is_empty)] + /// + /// let mut one_element = std::iter::once(0); + /// assert!(!one_element.is_empty()); + /// + /// assert_eq!(one_element.next(), Some(0)); + /// assert!(one_element.is_empty()); + /// + /// assert_eq!(one_element.next(), None); + /// ``` + #[inline] + #[unstable(feature = "exact_size_is_empty", issue = "35428")] + fn is_empty(&self) -> bool { + self.len() == 0 + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for &mut I { + fn len(&self) -> usize { + (**self).len() + } + fn is_empty(&self) -> bool { + (**self).is_empty() + } +} diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs new file mode 100644 index 000000000..275412b57 --- /dev/null +++ b/library/core/src/iter/traits/iterator.rs @@ -0,0 +1,3836 @@ +use crate::array; +use crate::cmp::{self, Ordering}; +use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try}; + +use super::super::try_process; +use super::super::ByRefSized; +use super::super::TrustedRandomAccessNoCoerce; +use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse}; +use super::super::{FlatMap, Flatten}; +use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip}; +use super::super::{ + Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile, +}; + +fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {} + +/// An interface for dealing with iterators. +/// +/// This is the main iterator trait. For more about the concept of iterators +/// generally, please see the [module-level documentation]. In particular, you +/// may want to know how to [implement `Iterator`][impl]. +/// +/// [module-level documentation]: crate::iter +/// [impl]: crate::iter#implementing-iterator +#[stable(feature = "rust1", since = "1.0.0")] +#[rustc_on_unimplemented( + on( + _Self = "std::ops::RangeTo<Idx>", + label = "if you meant to iterate until a value, add a starting value", + note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \ + bounded `Range`: `0..end`" + ), + on( + _Self = "std::ops::RangeToInclusive<Idx>", + label = "if you meant to iterate until a value (including it), add a starting value", + note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \ + to have a bounded `RangeInclusive`: `0..=end`" + ), + on( + _Self = "[]", + label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`" + ), + on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"), + on( + _Self = "std::vec::Vec<T, A>", + label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`" + ), + on( + _Self = "&str", + label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`" + ), + on( + _Self = "std::string::String", + label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`" + ), + on( + _Self = "{integral}", + note = "if you want to iterate between `start` until a value `end`, use the exclusive range \ + syntax `start..end` or the inclusive range syntax `start..=end`" + ), + label = "`{Self}` is not an iterator", + message = "`{Self}` is not an iterator" +)] +#[doc(notable_trait)] +#[rustc_diagnostic_item = "Iterator"] +#[must_use = "iterators are lazy and do nothing unless consumed"] +pub trait Iterator { + /// The type of the elements being iterated over. + #[stable(feature = "rust1", since = "1.0.0")] + type Item; + + /// Advances the iterator and returns the next value. + /// + /// Returns [`None`] when iteration is finished. Individual iterator + /// implementations may choose to resume iteration, and so calling `next()` + /// again may or may not eventually start returning [`Some(Item)`] again at some + /// point. + /// + /// [`Some(Item)`]: Some + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter(); + /// + /// // A call to next() returns the next value... + /// assert_eq!(Some(&1), iter.next()); + /// assert_eq!(Some(&2), iter.next()); + /// assert_eq!(Some(&3), iter.next()); + /// + /// // ... and then None once it's over. + /// assert_eq!(None, iter.next()); + /// + /// // More calls may or may not return `None`. Here, they always will. + /// assert_eq!(None, iter.next()); + /// assert_eq!(None, iter.next()); + /// ``` + #[lang = "next"] + #[stable(feature = "rust1", since = "1.0.0")] + fn next(&mut self) -> Option<Self::Item>; + + /// Advances the iterator and returns an array containing the next `N` values. + /// + /// If there are not enough elements to fill the array then `Err` is returned + /// containing an iterator over the remaining elements. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_next_chunk)] + /// + /// let mut iter = "lorem".chars(); + /// + /// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']); // N is inferred as 2 + /// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']); // N is inferred as 3 + /// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4 + /// ``` + /// + /// Split a string and get the first three items. + /// + /// ``` + /// #![feature(iter_next_chunk)] + /// + /// let quote = "not all those who wander are lost"; + /// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap(); + /// assert_eq!(first, "not"); + /// assert_eq!(second, "all"); + /// assert_eq!(third, "those"); + /// ``` + #[inline] + #[unstable(feature = "iter_next_chunk", reason = "recently added", issue = "98326")] + fn next_chunk<const N: usize>( + &mut self, + ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> + where + Self: Sized, + { + array::iter_next_chunk(self) + } + + /// Returns the bounds on the remaining length of the iterator. + /// + /// Specifically, `size_hint()` returns a tuple where the first element + /// is the lower bound, and the second element is the upper bound. + /// + /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>. + /// A [`None`] here means that either there is no known upper bound, or the + /// upper bound is larger than [`usize`]. + /// + /// # Implementation notes + /// + /// It is not enforced that an iterator implementation yields the declared + /// number of elements. A buggy iterator may yield less than the lower bound + /// or more than the upper bound of elements. + /// + /// `size_hint()` is primarily intended to be used for optimizations such as + /// reserving space for the elements of the iterator, but must not be + /// trusted to e.g., omit bounds checks in unsafe code. An incorrect + /// implementation of `size_hint()` should not lead to memory safety + /// violations. + /// + /// That said, the implementation should provide a correct estimation, + /// because otherwise it would be a violation of the trait's protocol. + /// + /// The default implementation returns <code>(0, [None])</code> which is correct for any + /// iterator. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// let mut iter = a.iter(); + /// + /// assert_eq!((3, Some(3)), iter.size_hint()); + /// let _ = iter.next(); + /// assert_eq!((2, Some(2)), iter.size_hint()); + /// ``` + /// + /// A more complex example: + /// + /// ``` + /// // The even numbers in the range of zero to nine. + /// let iter = (0..10).filter(|x| x % 2 == 0); + /// + /// // We might iterate from zero to ten times. Knowing that it's five + /// // exactly wouldn't be possible without executing filter(). + /// assert_eq!((0, Some(10)), iter.size_hint()); + /// + /// // Let's add five more numbers with chain() + /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20); + /// + /// // now both bounds are increased by five + /// assert_eq!((5, Some(15)), iter.size_hint()); + /// ``` + /// + /// Returning `None` for an upper bound: + /// + /// ``` + /// // an infinite iterator has no upper bound + /// // and the maximum possible lower bound + /// let iter = 0..; + /// + /// assert_eq!((usize::MAX, None), iter.size_hint()); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn size_hint(&self) -> (usize, Option<usize>) { + (0, None) + } + + /// Consumes the iterator, counting the number of iterations and returning it. + /// + /// This method will call [`next`] repeatedly until [`None`] is encountered, + /// returning the number of times it saw [`Some`]. Note that [`next`] has to be + /// called at least once even if the iterator does not have any elements. + /// + /// [`next`]: Iterator::next + /// + /// # Overflow Behavior + /// + /// The method does no guarding against overflows, so counting elements of + /// an iterator with more than [`usize::MAX`] elements either produces the + /// wrong result or panics. If debug assertions are enabled, a panic is + /// guaranteed. + /// + /// # Panics + /// + /// This function might panic if the iterator has more than [`usize::MAX`] + /// elements. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// assert_eq!(a.iter().count(), 3); + /// + /// let a = [1, 2, 3, 4, 5]; + /// assert_eq!(a.iter().count(), 5); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn count(self) -> usize + where + Self: Sized, + { + self.fold( + 0, + #[rustc_inherit_overflow_checks] + |count, _| count + 1, + ) + } + + /// Consumes the iterator, returning the last element. + /// + /// This method will evaluate the iterator until it returns [`None`]. While + /// doing so, it keeps track of the current element. After [`None`] is + /// returned, `last()` will then return the last element it saw. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// assert_eq!(a.iter().last(), Some(&3)); + /// + /// let a = [1, 2, 3, 4, 5]; + /// assert_eq!(a.iter().last(), Some(&5)); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn last(self) -> Option<Self::Item> + where + Self: Sized, + { + #[inline] + fn some<T>(_: Option<T>, x: T) -> Option<T> { + Some(x) + } + + self.fold(None, some) + } + + /// Advances the iterator by `n` elements. + /// + /// This method will eagerly skip `n` elements by calling [`next`] up to `n` + /// times until [`None`] is encountered. + /// + /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by + /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number + /// of elements the iterator is advanced by before running out of elements (i.e. the + /// length of the iterator). Note that `k` is always less than `n`. + /// + /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`] + /// can advance its outer iterator until it finds an inner iterator that is not empty, which + /// then often allows it to return a more accurate `size_hint()` than in its initial state. + /// + /// [`Flatten`]: crate::iter::Flatten + /// [`next`]: Iterator::next + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_advance_by)] + /// + /// let a = [1, 2, 3, 4]; + /// let mut iter = a.iter(); + /// + /// assert_eq!(iter.advance_by(2), Ok(())); + /// assert_eq!(iter.next(), Some(&3)); + /// assert_eq!(iter.advance_by(0), Ok(())); + /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped + /// ``` + #[inline] + #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")] + fn advance_by(&mut self, n: usize) -> Result<(), usize> { + for i in 0..n { + self.next().ok_or(i)?; + } + Ok(()) + } + + /// Returns the `n`th element of the iterator. + /// + /// Like most indexing operations, the count starts from zero, so `nth(0)` + /// returns the first value, `nth(1)` the second, and so on. + /// + /// Note that all preceding elements, as well as the returned element, will be + /// consumed from the iterator. That means that the preceding elements will be + /// discarded, and also that calling `nth(0)` multiple times on the same iterator + /// will return different elements. + /// + /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the + /// iterator. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// assert_eq!(a.iter().nth(1), Some(&2)); + /// ``` + /// + /// Calling `nth()` multiple times doesn't rewind the iterator: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter(); + /// + /// assert_eq!(iter.nth(1), Some(&2)); + /// assert_eq!(iter.nth(1), None); + /// ``` + /// + /// Returning `None` if there are less than `n + 1` elements: + /// + /// ``` + /// let a = [1, 2, 3]; + /// assert_eq!(a.iter().nth(10), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn nth(&mut self, n: usize) -> Option<Self::Item> { + self.advance_by(n).ok()?; + self.next() + } + + /// Creates an iterator starting at the same point, but stepping by + /// the given amount at each iteration. + /// + /// Note 1: The first element of the iterator will always be returned, + /// regardless of the step given. + /// + /// Note 2: The time at which ignored elements are pulled is not fixed. + /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`, + /// `self.nth(step-1)`, …, but is also free to behave like the sequence + /// `advance_n_and_return_first(&mut self, step)`, + /// `advance_n_and_return_first(&mut self, step)`, … + /// Which way is used may change for some iterators for performance reasons. + /// The second way will advance the iterator earlier and may consume more items. + /// + /// `advance_n_and_return_first` is the equivalent of: + /// ``` + /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item> + /// where + /// I: Iterator, + /// { + /// let next = iter.next(); + /// if n > 1 { + /// iter.nth(n - 2); + /// } + /// next + /// } + /// ``` + /// + /// # Panics + /// + /// The method will panic if the given step is `0`. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [0, 1, 2, 3, 4, 5]; + /// let mut iter = a.iter().step_by(2); + /// + /// assert_eq!(iter.next(), Some(&0)); + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), Some(&4)); + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[stable(feature = "iterator_step_by", since = "1.28.0")] + fn step_by(self, step: usize) -> StepBy<Self> + where + Self: Sized, + { + StepBy::new(self, step) + } + + /// Takes two iterators and creates a new iterator over both in sequence. + /// + /// `chain()` will return a new iterator which will first iterate over + /// values from the first iterator and then over values from the second + /// iterator. + /// + /// In other words, it links two iterators together, in a chain. 🔗 + /// + /// [`once`] is commonly used to adapt a single value into a chain of + /// other kinds of iteration. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a1 = [1, 2, 3]; + /// let a2 = [4, 5, 6]; + /// + /// let mut iter = a1.iter().chain(a2.iter()); + /// + /// assert_eq!(iter.next(), Some(&1)); + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), Some(&3)); + /// assert_eq!(iter.next(), Some(&4)); + /// assert_eq!(iter.next(), Some(&5)); + /// assert_eq!(iter.next(), Some(&6)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Since the argument to `chain()` uses [`IntoIterator`], we can pass + /// anything that can be converted into an [`Iterator`], not just an + /// [`Iterator`] itself. For example, slices (`&[T]`) implement + /// [`IntoIterator`], and so can be passed to `chain()` directly: + /// + /// ``` + /// let s1 = &[1, 2, 3]; + /// let s2 = &[4, 5, 6]; + /// + /// let mut iter = s1.iter().chain(s2); + /// + /// assert_eq!(iter.next(), Some(&1)); + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), Some(&3)); + /// assert_eq!(iter.next(), Some(&4)); + /// assert_eq!(iter.next(), Some(&5)); + /// assert_eq!(iter.next(), Some(&6)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`: + /// + /// ``` + /// #[cfg(windows)] + /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> { + /// use std::os::windows::ffi::OsStrExt; + /// s.encode_wide().chain(std::iter::once(0)).collect() + /// } + /// ``` + /// + /// [`once`]: crate::iter::once + /// [`OsStr`]: ../../std/ffi/struct.OsStr.html + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter> + where + Self: Sized, + U: IntoIterator<Item = Self::Item>, + { + Chain::new(self, other.into_iter()) + } + + /// 'Zips up' two iterators into a single iterator of pairs. + /// + /// `zip()` returns a new iterator that will iterate over two other + /// iterators, returning a tuple where the first element comes from the + /// first iterator, and the second element comes from the second iterator. + /// + /// In other words, it zips two iterators together, into a single one. + /// + /// If either iterator returns [`None`], [`next`] from the zipped iterator + /// will return [`None`]. + /// If the zipped iterator has no more elements to return then each further attempt to advance + /// it will first try to advance the first iterator at most one time and if it still yielded an item + /// try to advance the second iterator at most one time. + /// + /// To 'undo' the result of zipping up two iterators, see [`unzip`]. + /// + /// [`unzip`]: Iterator::unzip + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a1 = [1, 2, 3]; + /// let a2 = [4, 5, 6]; + /// + /// let mut iter = a1.iter().zip(a2.iter()); + /// + /// assert_eq!(iter.next(), Some((&1, &4))); + /// assert_eq!(iter.next(), Some((&2, &5))); + /// assert_eq!(iter.next(), Some((&3, &6))); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Since the argument to `zip()` uses [`IntoIterator`], we can pass + /// anything that can be converted into an [`Iterator`], not just an + /// [`Iterator`] itself. For example, slices (`&[T]`) implement + /// [`IntoIterator`], and so can be passed to `zip()` directly: + /// + /// ``` + /// let s1 = &[1, 2, 3]; + /// let s2 = &[4, 5, 6]; + /// + /// let mut iter = s1.iter().zip(s2); + /// + /// assert_eq!(iter.next(), Some((&1, &4))); + /// assert_eq!(iter.next(), Some((&2, &5))); + /// assert_eq!(iter.next(), Some((&3, &6))); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// `zip()` is often used to zip an infinite iterator to a finite one. + /// This works because the finite iterator will eventually return [`None`], + /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]: + /// + /// ``` + /// let enumerate: Vec<_> = "foo".chars().enumerate().collect(); + /// + /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect(); + /// + /// assert_eq!((0, 'f'), enumerate[0]); + /// assert_eq!((0, 'f'), zipper[0]); + /// + /// assert_eq!((1, 'o'), enumerate[1]); + /// assert_eq!((1, 'o'), zipper[1]); + /// + /// assert_eq!((2, 'o'), enumerate[2]); + /// assert_eq!((2, 'o'), zipper[2]); + /// ``` + /// + /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]: + /// + /// ``` + /// use std::iter::zip; + /// + /// let a = [1, 2, 3]; + /// let b = [2, 3, 4]; + /// + /// let mut zipped = zip( + /// a.into_iter().map(|x| x * 2).skip(1), + /// b.into_iter().map(|x| x * 2).skip(1), + /// ); + /// + /// assert_eq!(zipped.next(), Some((4, 6))); + /// assert_eq!(zipped.next(), Some((6, 8))); + /// assert_eq!(zipped.next(), None); + /// ``` + /// + /// compared to: + /// + /// ``` + /// # let a = [1, 2, 3]; + /// # let b = [2, 3, 4]; + /// # + /// let mut zipped = a + /// .into_iter() + /// .map(|x| x * 2) + /// .skip(1) + /// .zip(b.into_iter().map(|x| x * 2).skip(1)); + /// # + /// # assert_eq!(zipped.next(), Some((4, 6))); + /// # assert_eq!(zipped.next(), Some((6, 8))); + /// # assert_eq!(zipped.next(), None); + /// ``` + /// + /// [`enumerate`]: Iterator::enumerate + /// [`next`]: Iterator::next + /// [`zip`]: crate::iter::zip + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter> + where + Self: Sized, + U: IntoIterator, + { + Zip::new(self, other.into_iter()) + } + + /// Creates a new iterator which places a copy of `separator` between adjacent + /// items of the original iterator. + /// + /// In case `separator` does not implement [`Clone`] or needs to be + /// computed every time, use [`intersperse_with`]. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_intersperse)] + /// + /// let mut a = [0, 1, 2].iter().intersperse(&100); + /// assert_eq!(a.next(), Some(&0)); // The first element from `a`. + /// assert_eq!(a.next(), Some(&100)); // The separator. + /// assert_eq!(a.next(), Some(&1)); // The next element from `a`. + /// assert_eq!(a.next(), Some(&100)); // The separator. + /// assert_eq!(a.next(), Some(&2)); // The last element from `a`. + /// assert_eq!(a.next(), None); // The iterator is finished. + /// ``` + /// + /// `intersperse` can be very useful to join an iterator's items using a common element: + /// ``` + /// #![feature(iter_intersperse)] + /// + /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>(); + /// assert_eq!(hello, "Hello World !"); + /// ``` + /// + /// [`Clone`]: crate::clone::Clone + /// [`intersperse_with`]: Iterator::intersperse_with + #[inline] + #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")] + fn intersperse(self, separator: Self::Item) -> Intersperse<Self> + where + Self: Sized, + Self::Item: Clone, + { + Intersperse::new(self, separator) + } + + /// Creates a new iterator which places an item generated by `separator` + /// between adjacent items of the original iterator. + /// + /// The closure will be called exactly once each time an item is placed + /// between two adjacent items from the underlying iterator; specifically, + /// the closure is not called if the underlying iterator yields less than + /// two items and after the last item is yielded. + /// + /// If the iterator's item implements [`Clone`], it may be easier to use + /// [`intersperse`]. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_intersperse)] + /// + /// #[derive(PartialEq, Debug)] + /// struct NotClone(usize); + /// + /// let v = [NotClone(0), NotClone(1), NotClone(2)]; + /// let mut it = v.into_iter().intersperse_with(|| NotClone(99)); + /// + /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`. + /// assert_eq!(it.next(), Some(NotClone(99))); // The separator. + /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`. + /// assert_eq!(it.next(), Some(NotClone(99))); // The separator. + /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from from `v`. + /// assert_eq!(it.next(), None); // The iterator is finished. + /// ``` + /// + /// `intersperse_with` can be used in situations where the separator needs + /// to be computed: + /// ``` + /// #![feature(iter_intersperse)] + /// + /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied(); + /// + /// // The closure mutably borrows its context to generate an item. + /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied(); + /// let separator = || happy_emojis.next().unwrap_or(" 🦀 "); + /// + /// let result = src.intersperse_with(separator).collect::<String>(); + /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!"); + /// ``` + /// [`Clone`]: crate::clone::Clone + /// [`intersperse`]: Iterator::intersperse + #[inline] + #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")] + fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G> + where + Self: Sized, + G: FnMut() -> Self::Item, + { + IntersperseWith::new(self, separator) + } + + /// Takes a closure and creates an iterator which calls that closure on each + /// element. + /// + /// `map()` transforms one iterator into another, by means of its argument: + /// something that implements [`FnMut`]. It produces a new iterator which + /// calls this closure on each element of the original iterator. + /// + /// If you are good at thinking in types, you can think of `map()` like this: + /// If you have an iterator that gives you elements of some type `A`, and + /// you want an iterator of some other type `B`, you can use `map()`, + /// passing a closure that takes an `A` and returns a `B`. + /// + /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is + /// lazy, it is best used when you're already working with other iterators. + /// If you're doing some sort of looping for a side effect, it's considered + /// more idiomatic to use [`for`] than `map()`. + /// + /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for + /// [`FnMut`]: crate::ops::FnMut + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter().map(|x| 2 * x); + /// + /// assert_eq!(iter.next(), Some(2)); + /// assert_eq!(iter.next(), Some(4)); + /// assert_eq!(iter.next(), Some(6)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// If you're doing some sort of side effect, prefer [`for`] to `map()`: + /// + /// ``` + /// # #![allow(unused_must_use)] + /// // don't do this: + /// (0..5).map(|x| println!("{x}")); + /// + /// // it won't even execute, as it is lazy. Rust will warn you about this. + /// + /// // Instead, use for: + /// for x in 0..5 { + /// println!("{x}"); + /// } + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn map<B, F>(self, f: F) -> Map<Self, F> + where + Self: Sized, + F: FnMut(Self::Item) -> B, + { + Map::new(self, f) + } + + /// Calls a closure on each element of an iterator. + /// + /// This is equivalent to using a [`for`] loop on the iterator, although + /// `break` and `continue` are not possible from a closure. It's generally + /// more idiomatic to use a `for` loop, but `for_each` may be more legible + /// when processing items at the end of longer iterator chains. In some + /// cases `for_each` may also be faster than a loop, because it will use + /// internal iteration on adapters like `Chain`. + /// + /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// use std::sync::mpsc::channel; + /// + /// let (tx, rx) = channel(); + /// (0..5).map(|x| x * 2 + 1) + /// .for_each(move |x| tx.send(x).unwrap()); + /// + /// let v: Vec<_> = rx.iter().collect(); + /// assert_eq!(v, vec![1, 3, 5, 7, 9]); + /// ``` + /// + /// For such a small example, a `for` loop may be cleaner, but `for_each` + /// might be preferable to keep a functional style with longer iterators: + /// + /// ``` + /// (0..5).flat_map(|x| x * 100 .. x * 110) + /// .enumerate() + /// .filter(|&(i, x)| (i + x) % 3 == 0) + /// .for_each(|(i, x)| println!("{i}:{x}")); + /// ``` + #[inline] + #[stable(feature = "iterator_for_each", since = "1.21.0")] + fn for_each<F>(self, f: F) + where + Self: Sized, + F: FnMut(Self::Item), + { + #[inline] + fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) { + move |(), item| f(item) + } + + self.fold((), call(f)); + } + + /// Creates an iterator which uses a closure to determine if an element + /// should be yielded. + /// + /// Given an element the closure must return `true` or `false`. The returned + /// iterator will yield only the elements for which the closure returns + /// true. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [0i32, 1, 2]; + /// + /// let mut iter = a.iter().filter(|x| x.is_positive()); + /// + /// assert_eq!(iter.next(), Some(&1)); + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Because the closure passed to `filter()` takes a reference, and many + /// iterators iterate over references, this leads to a possibly confusing + /// situation, where the type of the closure is a double reference: + /// + /// ``` + /// let a = [0, 1, 2]; + /// + /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s! + /// + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// It's common to instead use destructuring on the argument to strip away + /// one: + /// + /// ``` + /// let a = [0, 1, 2]; + /// + /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and * + /// + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// or both: + /// + /// ``` + /// let a = [0, 1, 2]; + /// + /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s + /// + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// of these layers. + /// + /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`. + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn filter<P>(self, predicate: P) -> Filter<Self, P> + where + Self: Sized, + P: FnMut(&Self::Item) -> bool, + { + Filter::new(self, predicate) + } + + /// Creates an iterator that both filters and maps. + /// + /// The returned iterator yields only the `value`s for which the supplied + /// closure returns `Some(value)`. + /// + /// `filter_map` can be used to make chains of [`filter`] and [`map`] more + /// concise. The example below shows how a `map().filter().map()` can be + /// shortened to a single call to `filter_map`. + /// + /// [`filter`]: Iterator::filter + /// [`map`]: Iterator::map + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = ["1", "two", "NaN", "four", "5"]; + /// + /// let mut iter = a.iter().filter_map(|s| s.parse().ok()); + /// + /// assert_eq!(iter.next(), Some(1)); + /// assert_eq!(iter.next(), Some(5)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Here's the same example, but with [`filter`] and [`map`]: + /// + /// ``` + /// let a = ["1", "two", "NaN", "four", "5"]; + /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap()); + /// assert_eq!(iter.next(), Some(1)); + /// assert_eq!(iter.next(), Some(5)); + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> + where + Self: Sized, + F: FnMut(Self::Item) -> Option<B>, + { + FilterMap::new(self, f) + } + + /// Creates an iterator which gives the current iteration count as well as + /// the next value. + /// + /// The iterator returned yields pairs `(i, val)`, where `i` is the + /// current index of iteration and `val` is the value returned by the + /// iterator. + /// + /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a + /// different sized integer, the [`zip`] function provides similar + /// functionality. + /// + /// # Overflow Behavior + /// + /// The method does no guarding against overflows, so enumerating more than + /// [`usize::MAX`] elements either produces the wrong result or panics. If + /// debug assertions are enabled, a panic is guaranteed. + /// + /// # Panics + /// + /// The returned iterator might panic if the to-be-returned index would + /// overflow a [`usize`]. + /// + /// [`zip`]: Iterator::zip + /// + /// # Examples + /// + /// ``` + /// let a = ['a', 'b', 'c']; + /// + /// let mut iter = a.iter().enumerate(); + /// + /// assert_eq!(iter.next(), Some((0, &'a'))); + /// assert_eq!(iter.next(), Some((1, &'b'))); + /// assert_eq!(iter.next(), Some((2, &'c'))); + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn enumerate(self) -> Enumerate<Self> + where + Self: Sized, + { + Enumerate::new(self) + } + + /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods + /// to look at the next element of the iterator without consuming it. See + /// their documentation for more information. + /// + /// Note that the underlying iterator is still advanced when [`peek`] or + /// [`peek_mut`] are called for the first time: In order to retrieve the + /// next element, [`next`] is called on the underlying iterator, hence any + /// side effects (i.e. anything other than fetching the next value) of + /// the [`next`] method will occur. + /// + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let xs = [1, 2, 3]; + /// + /// let mut iter = xs.iter().peekable(); + /// + /// // peek() lets us see into the future + /// assert_eq!(iter.peek(), Some(&&1)); + /// assert_eq!(iter.next(), Some(&1)); + /// + /// assert_eq!(iter.next(), Some(&2)); + /// + /// // we can peek() multiple times, the iterator won't advance + /// assert_eq!(iter.peek(), Some(&&3)); + /// assert_eq!(iter.peek(), Some(&&3)); + /// + /// assert_eq!(iter.next(), Some(&3)); + /// + /// // after the iterator is finished, so is peek() + /// assert_eq!(iter.peek(), None); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Using [`peek_mut`] to mutate the next item without advancing the + /// iterator: + /// + /// ``` + /// let xs = [1, 2, 3]; + /// + /// let mut iter = xs.iter().peekable(); + /// + /// // `peek_mut()` lets us see into the future + /// assert_eq!(iter.peek_mut(), Some(&mut &1)); + /// assert_eq!(iter.peek_mut(), Some(&mut &1)); + /// assert_eq!(iter.next(), Some(&1)); + /// + /// if let Some(mut p) = iter.peek_mut() { + /// assert_eq!(*p, &2); + /// // put a value into the iterator + /// *p = &1000; + /// } + /// + /// // The value reappears as the iterator continues + /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]); + /// ``` + /// [`peek`]: Peekable::peek + /// [`peek_mut`]: Peekable::peek_mut + /// [`next`]: Iterator::next + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn peekable(self) -> Peekable<Self> + where + Self: Sized, + { + Peekable::new(self) + } + + /// Creates an iterator that [`skip`]s elements based on a predicate. + /// + /// [`skip`]: Iterator::skip + /// + /// `skip_while()` takes a closure as an argument. It will call this + /// closure on each element of the iterator, and ignore elements + /// until it returns `false`. + /// + /// After `false` is returned, `skip_while()`'s job is over, and the + /// rest of the elements are yielded. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [-1i32, 0, 1]; + /// + /// let mut iter = a.iter().skip_while(|x| x.is_negative()); + /// + /// assert_eq!(iter.next(), Some(&0)); + /// assert_eq!(iter.next(), Some(&1)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Because the closure passed to `skip_while()` takes a reference, and many + /// iterators iterate over references, this leads to a possibly confusing + /// situation, where the type of the closure argument is a double reference: + /// + /// ``` + /// let a = [-1, 0, 1]; + /// + /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s! + /// + /// assert_eq!(iter.next(), Some(&0)); + /// assert_eq!(iter.next(), Some(&1)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Stopping after an initial `false`: + /// + /// ``` + /// let a = [-1, 0, 1, -2]; + /// + /// let mut iter = a.iter().skip_while(|x| **x < 0); + /// + /// assert_eq!(iter.next(), Some(&0)); + /// assert_eq!(iter.next(), Some(&1)); + /// + /// // while this would have been false, since we already got a false, + /// // skip_while() isn't used any more + /// assert_eq!(iter.next(), Some(&-2)); + /// + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[doc(alias = "drop_while")] + #[stable(feature = "rust1", since = "1.0.0")] + fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P> + where + Self: Sized, + P: FnMut(&Self::Item) -> bool, + { + SkipWhile::new(self, predicate) + } + + /// Creates an iterator that yields elements based on a predicate. + /// + /// `take_while()` takes a closure as an argument. It will call this + /// closure on each element of the iterator, and yield elements + /// while it returns `true`. + /// + /// After `false` is returned, `take_while()`'s job is over, and the + /// rest of the elements are ignored. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [-1i32, 0, 1]; + /// + /// let mut iter = a.iter().take_while(|x| x.is_negative()); + /// + /// assert_eq!(iter.next(), Some(&-1)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Because the closure passed to `take_while()` takes a reference, and many + /// iterators iterate over references, this leads to a possibly confusing + /// situation, where the type of the closure is a double reference: + /// + /// ``` + /// let a = [-1, 0, 1]; + /// + /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s! + /// + /// assert_eq!(iter.next(), Some(&-1)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Stopping after an initial `false`: + /// + /// ``` + /// let a = [-1, 0, 1, -2]; + /// + /// let mut iter = a.iter().take_while(|x| **x < 0); + /// + /// assert_eq!(iter.next(), Some(&-1)); + /// + /// // We have more elements that are less than zero, but since we already + /// // got a false, take_while() isn't used any more + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Because `take_while()` needs to look at the value in order to see if it + /// should be included or not, consuming iterators will see that it is + /// removed: + /// + /// ``` + /// let a = [1, 2, 3, 4]; + /// let mut iter = a.iter(); + /// + /// let result: Vec<i32> = iter.by_ref() + /// .take_while(|n| **n != 3) + /// .cloned() + /// .collect(); + /// + /// assert_eq!(result, &[1, 2]); + /// + /// let result: Vec<i32> = iter.cloned().collect(); + /// + /// assert_eq!(result, &[4]); + /// ``` + /// + /// The `3` is no longer there, because it was consumed in order to see if + /// the iteration should stop, but wasn't placed back into the iterator. + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P> + where + Self: Sized, + P: FnMut(&Self::Item) -> bool, + { + TakeWhile::new(self, predicate) + } + + /// Creates an iterator that both yields elements based on a predicate and maps. + /// + /// `map_while()` takes a closure as an argument. It will call this + /// closure on each element of the iterator, and yield elements + /// while it returns [`Some(_)`][`Some`]. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [-1i32, 4, 0, 1]; + /// + /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x)); + /// + /// assert_eq!(iter.next(), Some(-16)); + /// assert_eq!(iter.next(), Some(4)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Here's the same example, but with [`take_while`] and [`map`]: + /// + /// [`take_while`]: Iterator::take_while + /// [`map`]: Iterator::map + /// + /// ``` + /// let a = [-1i32, 4, 0, 1]; + /// + /// let mut iter = a.iter() + /// .map(|x| 16i32.checked_div(*x)) + /// .take_while(|x| x.is_some()) + /// .map(|x| x.unwrap()); + /// + /// assert_eq!(iter.next(), Some(-16)); + /// assert_eq!(iter.next(), Some(4)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// Stopping after an initial [`None`]: + /// + /// ``` + /// let a = [0, 1, 2, -3, 4, 5, -6]; + /// + /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok()); + /// let vec = iter.collect::<Vec<_>>(); + /// + /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3` + /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered. + /// assert_eq!(vec, vec![0, 1, 2]); + /// ``` + /// + /// Because `map_while()` needs to look at the value in order to see if it + /// should be included or not, consuming iterators will see that it is + /// removed: + /// + /// ``` + /// let a = [1, 2, -3, 4]; + /// let mut iter = a.iter(); + /// + /// let result: Vec<u32> = iter.by_ref() + /// .map_while(|n| u32::try_from(*n).ok()) + /// .collect(); + /// + /// assert_eq!(result, &[1, 2]); + /// + /// let result: Vec<i32> = iter.cloned().collect(); + /// + /// assert_eq!(result, &[4]); + /// ``` + /// + /// The `-3` is no longer there, because it was consumed in order to see if + /// the iteration should stop, but wasn't placed back into the iterator. + /// + /// Note that unlike [`take_while`] this iterator is **not** fused. + /// It is also not specified what this iterator returns after the first [`None`] is returned. + /// If you need fused iterator, use [`fuse`]. + /// + /// [`fuse`]: Iterator::fuse + #[inline] + #[stable(feature = "iter_map_while", since = "1.57.0")] + fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P> + where + Self: Sized, + P: FnMut(Self::Item) -> Option<B>, + { + MapWhile::new(self, predicate) + } + + /// Creates an iterator that skips the first `n` elements. + /// + /// `skip(n)` skips elements until `n` elements are skipped or the end of the + /// iterator is reached (whichever happens first). After that, all the remaining + /// elements are yielded. In particular, if the original iterator is too short, + /// then the returned iterator is empty. + /// + /// Rather than overriding this method directly, instead override the `nth` method. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter().skip(2); + /// + /// assert_eq!(iter.next(), Some(&3)); + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn skip(self, n: usize) -> Skip<Self> + where + Self: Sized, + { + Skip::new(self, n) + } + + /// Creates an iterator that yields the first `n` elements, or fewer + /// if the underlying iterator ends sooner. + /// + /// `take(n)` yields elements until `n` elements are yielded or the end of + /// the iterator is reached (whichever happens first). + /// The returned iterator is a prefix of length `n` if the original iterator + /// contains at least `n` elements, otherwise it contains all of the + /// (fewer than `n`) elements of the original iterator. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter().take(2); + /// + /// assert_eq!(iter.next(), Some(&1)); + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// `take()` is often used with an infinite iterator, to make it finite: + /// + /// ``` + /// let mut iter = (0..).take(3); + /// + /// assert_eq!(iter.next(), Some(0)); + /// assert_eq!(iter.next(), Some(1)); + /// assert_eq!(iter.next(), Some(2)); + /// assert_eq!(iter.next(), None); + /// ``` + /// + /// If less than `n` elements are available, + /// `take` will limit itself to the size of the underlying iterator: + /// + /// ``` + /// let v = [1, 2]; + /// let mut iter = v.into_iter().take(5); + /// assert_eq!(iter.next(), Some(1)); + /// assert_eq!(iter.next(), Some(2)); + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn take(self, n: usize) -> Take<Self> + where + Self: Sized, + { + Take::new(self, n) + } + + /// An iterator adapter similar to [`fold`] that holds internal state and + /// produces a new iterator. + /// + /// [`fold`]: Iterator::fold + /// + /// `scan()` takes two arguments: an initial value which seeds the internal + /// state, and a closure with two arguments, the first being a mutable + /// reference to the internal state and the second an iterator element. + /// The closure can assign to the internal state to share state between + /// iterations. + /// + /// On iteration, the closure will be applied to each element of the + /// iterator and the return value from the closure, an [`Option`], is + /// yielded by the iterator. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter().scan(1, |state, &x| { + /// // each iteration, we'll multiply the state by the element + /// *state = *state * x; + /// + /// // then, we'll yield the negation of the state + /// Some(-*state) + /// }); + /// + /// assert_eq!(iter.next(), Some(-1)); + /// assert_eq!(iter.next(), Some(-2)); + /// assert_eq!(iter.next(), Some(-6)); + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F> + where + Self: Sized, + F: FnMut(&mut St, Self::Item) -> Option<B>, + { + Scan::new(self, initial_state, f) + } + + /// Creates an iterator that works like map, but flattens nested structure. + /// + /// The [`map`] adapter is very useful, but only when the closure + /// argument produces values. If it produces an iterator instead, there's + /// an extra layer of indirection. `flat_map()` will remove this extra layer + /// on its own. + /// + /// You can think of `flat_map(f)` as the semantic equivalent + /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`. + /// + /// Another way of thinking about `flat_map()`: [`map`]'s closure returns + /// one item for each element, and `flat_map()`'s closure returns an + /// iterator for each element. + /// + /// [`map`]: Iterator::map + /// [`flatten`]: Iterator::flatten + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let words = ["alpha", "beta", "gamma"]; + /// + /// // chars() returns an iterator + /// let merged: String = words.iter() + /// .flat_map(|s| s.chars()) + /// .collect(); + /// assert_eq!(merged, "alphabetagamma"); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F> + where + Self: Sized, + U: IntoIterator, + F: FnMut(Self::Item) -> U, + { + FlatMap::new(self, f) + } + + /// Creates an iterator that flattens nested structure. + /// + /// This is useful when you have an iterator of iterators or an iterator of + /// things that can be turned into iterators and you want to remove one + /// level of indirection. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]]; + /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>(); + /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]); + /// ``` + /// + /// Mapping and then flattening: + /// + /// ``` + /// let words = ["alpha", "beta", "gamma"]; + /// + /// // chars() returns an iterator + /// let merged: String = words.iter() + /// .map(|s| s.chars()) + /// .flatten() + /// .collect(); + /// assert_eq!(merged, "alphabetagamma"); + /// ``` + /// + /// You can also rewrite this in terms of [`flat_map()`], which is preferable + /// in this case since it conveys intent more clearly: + /// + /// ``` + /// let words = ["alpha", "beta", "gamma"]; + /// + /// // chars() returns an iterator + /// let merged: String = words.iter() + /// .flat_map(|s| s.chars()) + /// .collect(); + /// assert_eq!(merged, "alphabetagamma"); + /// ``` + /// + /// Flattening only removes one level of nesting at a time: + /// + /// ``` + /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]]; + /// + /// let d2 = d3.iter().flatten().collect::<Vec<_>>(); + /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]); + /// + /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>(); + /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]); + /// ``` + /// + /// Here we see that `flatten()` does not perform a "deep" flatten. + /// Instead, only one level of nesting is removed. That is, if you + /// `flatten()` a three-dimensional array, the result will be + /// two-dimensional and not one-dimensional. To get a one-dimensional + /// structure, you have to `flatten()` again. + /// + /// [`flat_map()`]: Iterator::flat_map + #[inline] + #[stable(feature = "iterator_flatten", since = "1.29.0")] + fn flatten(self) -> Flatten<Self> + where + Self: Sized, + Self::Item: IntoIterator, + { + Flatten::new(self) + } + + /// Creates an iterator which ends after the first [`None`]. + /// + /// After an iterator returns [`None`], future calls may or may not yield + /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a + /// [`None`] is given, it will always return [`None`] forever. + /// + /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement + /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly + /// if the [`FusedIterator`] trait is improperly implemented. + /// + /// [`Some(T)`]: Some + /// [`FusedIterator`]: crate::iter::FusedIterator + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// // an iterator which alternates between Some and None + /// struct Alternate { + /// state: i32, + /// } + /// + /// impl Iterator for Alternate { + /// type Item = i32; + /// + /// fn next(&mut self) -> Option<i32> { + /// let val = self.state; + /// self.state = self.state + 1; + /// + /// // if it's even, Some(i32), else None + /// if val % 2 == 0 { + /// Some(val) + /// } else { + /// None + /// } + /// } + /// } + /// + /// let mut iter = Alternate { state: 0 }; + /// + /// // we can see our iterator going back and forth + /// assert_eq!(iter.next(), Some(0)); + /// assert_eq!(iter.next(), None); + /// assert_eq!(iter.next(), Some(2)); + /// assert_eq!(iter.next(), None); + /// + /// // however, once we fuse it... + /// let mut iter = iter.fuse(); + /// + /// assert_eq!(iter.next(), Some(4)); + /// assert_eq!(iter.next(), None); + /// + /// // it will always return `None` after the first time. + /// assert_eq!(iter.next(), None); + /// assert_eq!(iter.next(), None); + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn fuse(self) -> Fuse<Self> + where + Self: Sized, + { + Fuse::new(self) + } + + /// Does something with each element of an iterator, passing the value on. + /// + /// When using iterators, you'll often chain several of them together. + /// While working on such code, you might want to check out what's + /// happening at various parts in the pipeline. To do that, insert + /// a call to `inspect()`. + /// + /// It's more common for `inspect()` to be used as a debugging tool than to + /// exist in your final code, but applications may find it useful in certain + /// situations when errors need to be logged before being discarded. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 4, 2, 3]; + /// + /// // this iterator sequence is complex. + /// let sum = a.iter() + /// .cloned() + /// .filter(|x| x % 2 == 0) + /// .fold(0, |sum, i| sum + i); + /// + /// println!("{sum}"); + /// + /// // let's add some inspect() calls to investigate what's happening + /// let sum = a.iter() + /// .cloned() + /// .inspect(|x| println!("about to filter: {x}")) + /// .filter(|x| x % 2 == 0) + /// .inspect(|x| println!("made it through filter: {x}")) + /// .fold(0, |sum, i| sum + i); + /// + /// println!("{sum}"); + /// ``` + /// + /// This will print: + /// + /// ```text + /// 6 + /// about to filter: 1 + /// about to filter: 4 + /// made it through filter: 4 + /// about to filter: 2 + /// made it through filter: 2 + /// about to filter: 3 + /// 6 + /// ``` + /// + /// Logging errors before discarding them: + /// + /// ``` + /// let lines = ["1", "2", "a"]; + /// + /// let sum: i32 = lines + /// .iter() + /// .map(|line| line.parse::<i32>()) + /// .inspect(|num| { + /// if let Err(ref e) = *num { + /// println!("Parsing error: {e}"); + /// } + /// }) + /// .filter_map(Result::ok) + /// .sum(); + /// + /// println!("Sum: {sum}"); + /// ``` + /// + /// This will print: + /// + /// ```text + /// Parsing error: invalid digit found in string + /// Sum: 3 + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn inspect<F>(self, f: F) -> Inspect<Self, F> + where + Self: Sized, + F: FnMut(&Self::Item), + { + Inspect::new(self, f) + } + + /// Borrows an iterator, rather than consuming it. + /// + /// This is useful to allow applying iterator adapters while still + /// retaining ownership of the original iterator. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let mut words = ["hello", "world", "of", "Rust"].into_iter(); + /// + /// // Take the first two words. + /// let hello_world: Vec<_> = words.by_ref().take(2).collect(); + /// assert_eq!(hello_world, vec!["hello", "world"]); + /// + /// // Collect the rest of the words. + /// // We can only do this because we used `by_ref` earlier. + /// let of_rust: Vec<_> = words.collect(); + /// assert_eq!(of_rust, vec!["of", "Rust"]); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + fn by_ref(&mut self) -> &mut Self + where + Self: Sized, + { + self + } + + /// Transforms an iterator into a collection. + /// + /// `collect()` can take anything iterable, and turn it into a relevant + /// collection. This is one of the more powerful methods in the standard + /// library, used in a variety of contexts. + /// + /// The most basic pattern in which `collect()` is used is to turn one + /// collection into another. You take a collection, call [`iter`] on it, + /// do a bunch of transformations, and then `collect()` at the end. + /// + /// `collect()` can also create instances of types that are not typical + /// collections. For example, a [`String`] can be built from [`char`]s, + /// and an iterator of [`Result<T, E>`][`Result`] items can be collected + /// into `Result<Collection<T>, E>`. See the examples below for more. + /// + /// Because `collect()` is so general, it can cause problems with type + /// inference. As such, `collect()` is one of the few times you'll see + /// the syntax affectionately known as the 'turbofish': `::<>`. This + /// helps the inference algorithm understand specifically which collection + /// you're trying to collect into. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let doubled: Vec<i32> = a.iter() + /// .map(|&x| x * 2) + /// .collect(); + /// + /// assert_eq!(vec![2, 4, 6], doubled); + /// ``` + /// + /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because + /// we could collect into, for example, a [`VecDeque<T>`] instead: + /// + /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html + /// + /// ``` + /// use std::collections::VecDeque; + /// + /// let a = [1, 2, 3]; + /// + /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect(); + /// + /// assert_eq!(2, doubled[0]); + /// assert_eq!(4, doubled[1]); + /// assert_eq!(6, doubled[2]); + /// ``` + /// + /// Using the 'turbofish' instead of annotating `doubled`: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>(); + /// + /// assert_eq!(vec![2, 4, 6], doubled); + /// ``` + /// + /// Because `collect()` only cares about what you're collecting into, you can + /// still use a partial type hint, `_`, with the turbofish: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>(); + /// + /// assert_eq!(vec![2, 4, 6], doubled); + /// ``` + /// + /// Using `collect()` to make a [`String`]: + /// + /// ``` + /// let chars = ['g', 'd', 'k', 'k', 'n']; + /// + /// let hello: String = chars.iter() + /// .map(|&x| x as u8) + /// .map(|x| (x + 1) as char) + /// .collect(); + /// + /// assert_eq!("hello", hello); + /// ``` + /// + /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to + /// see if any of them failed: + /// + /// ``` + /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")]; + /// + /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect(); + /// + /// // gives us the first error + /// assert_eq!(Err("nope"), result); + /// + /// let results = [Ok(1), Ok(3)]; + /// + /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect(); + /// + /// // gives us the list of answers + /// assert_eq!(Ok(vec![1, 3]), result); + /// ``` + /// + /// [`iter`]: Iterator::next + /// [`String`]: ../../std/string/struct.String.html + /// [`char`]: type@char + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"] + fn collect<B: FromIterator<Self::Item>>(self) -> B + where + Self: Sized, + { + FromIterator::from_iter(self) + } + + /// Fallibly transforms an iterator into a collection, short circuiting if + /// a failure is encountered. + /// + /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible + /// conversions during collection. Its main use case is simplifying conversions from + /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`] + /// types (e.g. [`Result`]). + /// + /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`]; + /// only the inner type produced on `Try::Output` must implement it. Concretely, + /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements + /// [`FromIterator`], even though [`ControlFlow`] doesn't. + /// + /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and + /// may continue to be used, in which case it will continue iterating starting after the element that + /// triggered the failure. See the last example below for an example of how this works. + /// + /// # Examples + /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`: + /// ``` + /// #![feature(iterator_try_collect)] + /// + /// let u = vec![Some(1), Some(2), Some(3)]; + /// let v = u.into_iter().try_collect::<Vec<i32>>(); + /// assert_eq!(v, Some(vec![1, 2, 3])); + /// ``` + /// + /// Failing to collect in the same way: + /// ``` + /// #![feature(iterator_try_collect)] + /// + /// let u = vec![Some(1), Some(2), None, Some(3)]; + /// let v = u.into_iter().try_collect::<Vec<i32>>(); + /// assert_eq!(v, None); + /// ``` + /// + /// A similar example, but with `Result`: + /// ``` + /// #![feature(iterator_try_collect)] + /// + /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)]; + /// let v = u.into_iter().try_collect::<Vec<i32>>(); + /// assert_eq!(v, Ok(vec![1, 2, 3])); + /// + /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)]; + /// let v = u.into_iter().try_collect::<Vec<i32>>(); + /// assert_eq!(v, Err(())); + /// ``` + /// + /// Finally, even [`ControlFlow`] works, despite the fact that it + /// doesn't implement [`FromIterator`]. Note also that the iterator can + /// continue to be used, even if a failure is encountered: + /// + /// ``` + /// #![feature(iterator_try_collect)] + /// + /// use core::ops::ControlFlow::{Break, Continue}; + /// + /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)]; + /// let mut it = u.into_iter(); + /// + /// let v = it.try_collect::<Vec<_>>(); + /// assert_eq!(v, Break(3)); + /// + /// let v = it.try_collect::<Vec<_>>(); + /// assert_eq!(v, Continue(vec![4, 5])); + /// ``` + /// + /// [`collect`]: Iterator::collect + #[inline] + #[unstable(feature = "iterator_try_collect", issue = "94047")] + fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B> + where + Self: Sized, + <Self as Iterator>::Item: Try, + <<Self as Iterator>::Item as Try>::Residual: Residual<B>, + B: FromIterator<<Self::Item as Try>::Output>, + { + try_process(ByRefSized(self), |i| i.collect()) + } + + /// Collects all the items from an iterator into a collection. + /// + /// This method consumes the iterator and adds all its items to the + /// passed collection. The collection is then returned, so the call chain + /// can be continued. + /// + /// This is useful when you already have a collection and wants to add + /// the iterator items to it. + /// + /// This method is a convenience method to call [Extend::extend](trait.Extend.html), + /// but instead of being called on a collection, it's called on an iterator. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_collect_into)] + /// + /// let a = [1, 2, 3]; + /// let mut vec: Vec::<i32> = vec![0, 1]; + /// + /// a.iter().map(|&x| x * 2).collect_into(&mut vec); + /// a.iter().map(|&x| x * 10).collect_into(&mut vec); + /// + /// assert_eq!(vec![0, 1, 2, 4, 6, 10, 20, 30], vec); + /// ``` + /// + /// `Vec` can have a manual set capacity to avoid reallocating it: + /// + /// ``` + /// #![feature(iter_collect_into)] + /// + /// let a = [1, 2, 3]; + /// let mut vec: Vec::<i32> = Vec::with_capacity(6); + /// + /// a.iter().map(|&x| x * 2).collect_into(&mut vec); + /// a.iter().map(|&x| x * 10).collect_into(&mut vec); + /// + /// assert_eq!(6, vec.capacity()); + /// println!("{:?}", vec); + /// ``` + /// + /// The returned mutable reference can be used to continue the call chain: + /// + /// ``` + /// #![feature(iter_collect_into)] + /// + /// let a = [1, 2, 3]; + /// let mut vec: Vec::<i32> = Vec::with_capacity(6); + /// + /// let count = a.iter().collect_into(&mut vec).iter().count(); + /// + /// assert_eq!(count, vec.len()); + /// println!("Vec len is {}", count); + /// + /// let count = a.iter().collect_into(&mut vec).iter().count(); + /// + /// assert_eq!(count, vec.len()); + /// println!("Vec len now is {}", count); + /// ``` + #[inline] + #[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")] + fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E + where + Self: Sized, + { + collection.extend(self); + collection + } + + /// Consumes an iterator, creating two collections from it. + /// + /// The predicate passed to `partition()` can return `true`, or `false`. + /// `partition()` returns a pair, all of the elements for which it returned + /// `true`, and all of the elements for which it returned `false`. + /// + /// See also [`is_partitioned()`] and [`partition_in_place()`]. + /// + /// [`is_partitioned()`]: Iterator::is_partitioned + /// [`partition_in_place()`]: Iterator::partition_in_place + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let (even, odd): (Vec<_>, Vec<_>) = a + /// .into_iter() + /// .partition(|n| n % 2 == 0); + /// + /// assert_eq!(even, vec![2]); + /// assert_eq!(odd, vec![1, 3]); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + fn partition<B, F>(self, f: F) -> (B, B) + where + Self: Sized, + B: Default + Extend<Self::Item>, + F: FnMut(&Self::Item) -> bool, + { + #[inline] + fn extend<'a, T, B: Extend<T>>( + mut f: impl FnMut(&T) -> bool + 'a, + left: &'a mut B, + right: &'a mut B, + ) -> impl FnMut((), T) + 'a { + move |(), x| { + if f(&x) { + left.extend_one(x); + } else { + right.extend_one(x); + } + } + } + + let mut left: B = Default::default(); + let mut right: B = Default::default(); + + self.fold((), extend(f, &mut left, &mut right)); + + (left, right) + } + + /// Reorders the elements of this iterator *in-place* according to the given predicate, + /// such that all those that return `true` precede all those that return `false`. + /// Returns the number of `true` elements found. + /// + /// The relative order of partitioned items is not maintained. + /// + /// # Current implementation + /// + /// Current algorithms tries finding the first element for which the predicate evaluates + /// to false, and the last element for which it evaluates to true and repeatedly swaps them. + /// + /// Time complexity: *O*(*n*) + /// + /// See also [`is_partitioned()`] and [`partition()`]. + /// + /// [`is_partitioned()`]: Iterator::is_partitioned + /// [`partition()`]: Iterator::partition + /// + /// # Examples + /// + /// ``` + /// #![feature(iter_partition_in_place)] + /// + /// let mut a = [1, 2, 3, 4, 5, 6, 7]; + /// + /// // Partition in-place between evens and odds + /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0); + /// + /// assert_eq!(i, 3); + /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens + /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds + /// ``` + #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")] + fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize + where + Self: Sized + DoubleEndedIterator<Item = &'a mut T>, + P: FnMut(&T) -> bool, + { + // FIXME: should we worry about the count overflowing? The only way to have more than + // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition... + + // These closure "factory" functions exist to avoid genericity in `Self`. + + #[inline] + fn is_false<'a, T>( + predicate: &'a mut impl FnMut(&T) -> bool, + true_count: &'a mut usize, + ) -> impl FnMut(&&mut T) -> bool + 'a { + move |x| { + let p = predicate(&**x); + *true_count += p as usize; + !p + } + } + + #[inline] + fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ { + move |x| predicate(&**x) + } + + // Repeatedly find the first `false` and swap it with the last `true`. + let mut true_count = 0; + while let Some(head) = self.find(is_false(predicate, &mut true_count)) { + if let Some(tail) = self.rfind(is_true(predicate)) { + crate::mem::swap(head, tail); + true_count += 1; + } else { + break; + } + } + true_count + } + + /// Checks if the elements of this iterator are partitioned according to the given predicate, + /// such that all those that return `true` precede all those that return `false`. + /// + /// See also [`partition()`] and [`partition_in_place()`]. + /// + /// [`partition()`]: Iterator::partition + /// [`partition_in_place()`]: Iterator::partition_in_place + /// + /// # Examples + /// + /// ``` + /// #![feature(iter_is_partitioned)] + /// + /// assert!("Iterator".chars().is_partitioned(char::is_uppercase)); + /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase)); + /// ``` + #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")] + fn is_partitioned<P>(mut self, mut predicate: P) -> bool + where + Self: Sized, + P: FnMut(Self::Item) -> bool, + { + // Either all items test `true`, or the first clause stops at `false` + // and we check that there are no more `true` items after that. + self.all(&mut predicate) || !self.any(predicate) + } + + /// An iterator method that applies a function as long as it returns + /// successfully, producing a single, final value. + /// + /// `try_fold()` takes two arguments: an initial value, and a closure with + /// two arguments: an 'accumulator', and an element. The closure either + /// returns successfully, with the value that the accumulator should have + /// for the next iteration, or it returns failure, with an error value that + /// is propagated back to the caller immediately (short-circuiting). + /// + /// The initial value is the value the accumulator will have on the first + /// call. If applying the closure succeeded against every element of the + /// iterator, `try_fold()` returns the final accumulator as success. + /// + /// Folding is useful whenever you have a collection of something, and want + /// to produce a single value from it. + /// + /// # Note to Implementors + /// + /// Several of the other (forward) methods have default implementations in + /// terms of this one, so try to implement this explicitly if it can + /// do something better than the default `for` loop implementation. + /// + /// In particular, try to have this call `try_fold()` on the internal parts + /// from which this iterator is composed. If multiple calls are needed, + /// the `?` operator may be convenient for chaining the accumulator value + /// along, but beware any invariants that need to be upheld before those + /// early returns. This is a `&mut self` method, so iteration needs to be + /// resumable after hitting an error here. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// // the checked sum of all of the elements of the array + /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x)); + /// + /// assert_eq!(sum, Some(6)); + /// ``` + /// + /// Short-circuiting: + /// + /// ``` + /// let a = [10, 20, 30, 100, 40, 50]; + /// let mut it = a.iter(); + /// + /// // This sum overflows when adding the 100 element + /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x)); + /// assert_eq!(sum, None); + /// + /// // Because it short-circuited, the remaining elements are still + /// // available through the iterator. + /// assert_eq!(it.len(), 2); + /// assert_eq!(it.next(), Some(&40)); + /// ``` + /// + /// While you cannot `break` from a closure, the [`ControlFlow`] type allows + /// a similar idea: + /// + /// ``` + /// use std::ops::ControlFlow; + /// + /// let triangular = (1..30).try_fold(0_i8, |prev, x| { + /// if let Some(next) = prev.checked_add(x) { + /// ControlFlow::Continue(next) + /// } else { + /// ControlFlow::Break(prev) + /// } + /// }); + /// assert_eq!(triangular, ControlFlow::Break(120)); + /// + /// let triangular = (1..30).try_fold(0_u64, |prev, x| { + /// if let Some(next) = prev.checked_add(x) { + /// ControlFlow::Continue(next) + /// } else { + /// ControlFlow::Break(prev) + /// } + /// }); + /// assert_eq!(triangular, ControlFlow::Continue(435)); + /// ``` + #[inline] + #[stable(feature = "iterator_try_fold", since = "1.27.0")] + fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R + where + Self: Sized, + F: FnMut(B, Self::Item) -> R, + R: Try<Output = B>, + { + let mut accum = init; + while let Some(x) = self.next() { + accum = f(accum, x)?; + } + try { accum } + } + + /// An iterator method that applies a fallible function to each item in the + /// iterator, stopping at the first error and returning that error. + /// + /// This can also be thought of as the fallible form of [`for_each()`] + /// or as the stateless version of [`try_fold()`]. + /// + /// [`for_each()`]: Iterator::for_each + /// [`try_fold()`]: Iterator::try_fold + /// + /// # Examples + /// + /// ``` + /// use std::fs::rename; + /// use std::io::{stdout, Write}; + /// use std::path::Path; + /// + /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"]; + /// + /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}")); + /// assert!(res.is_ok()); + /// + /// let mut it = data.iter().cloned(); + /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old"))); + /// assert!(res.is_err()); + /// // It short-circuited, so the remaining items are still in the iterator: + /// assert_eq!(it.next(), Some("stale_bread.json")); + /// ``` + /// + /// The [`ControlFlow`] type can be used with this method for the situations + /// in which you'd use `break` and `continue` in a normal loop: + /// + /// ``` + /// use std::ops::ControlFlow; + /// + /// let r = (2..100).try_for_each(|x| { + /// if 323 % x == 0 { + /// return ControlFlow::Break(x) + /// } + /// + /// ControlFlow::Continue(()) + /// }); + /// assert_eq!(r, ControlFlow::Break(17)); + /// ``` + #[inline] + #[stable(feature = "iterator_try_fold", since = "1.27.0")] + fn try_for_each<F, R>(&mut self, f: F) -> R + where + Self: Sized, + F: FnMut(Self::Item) -> R, + R: Try<Output = ()>, + { + #[inline] + fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R { + move |(), x| f(x) + } + + self.try_fold((), call(f)) + } + + /// Folds every element into an accumulator by applying an operation, + /// returning the final result. + /// + /// `fold()` takes two arguments: an initial value, and a closure with two + /// arguments: an 'accumulator', and an element. The closure returns the value that + /// the accumulator should have for the next iteration. + /// + /// The initial value is the value the accumulator will have on the first + /// call. + /// + /// After applying this closure to every element of the iterator, `fold()` + /// returns the accumulator. + /// + /// This operation is sometimes called 'reduce' or 'inject'. + /// + /// Folding is useful whenever you have a collection of something, and want + /// to produce a single value from it. + /// + /// Note: `fold()`, and similar methods that traverse the entire iterator, + /// might not terminate for infinite iterators, even on traits for which a + /// result is determinable in finite time. + /// + /// Note: [`reduce()`] can be used to use the first element as the initial + /// value, if the accumulator type and item type is the same. + /// + /// Note: `fold()` combines elements in a *left-associative* fashion. For associative + /// operators like `+`, the order the elements are combined in is not important, but for non-associative + /// operators like `-` the order will affect the final result. + /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`]. + /// + /// # Note to Implementors + /// + /// Several of the other (forward) methods have default implementations in + /// terms of this one, so try to implement this explicitly if it can + /// do something better than the default `for` loop implementation. + /// + /// In particular, try to have this call `fold()` on the internal parts + /// from which this iterator is composed. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// // the sum of all of the elements of the array + /// let sum = a.iter().fold(0, |acc, x| acc + x); + /// + /// assert_eq!(sum, 6); + /// ``` + /// + /// Let's walk through each step of the iteration here: + /// + /// | element | acc | x | result | + /// |---------|-----|---|--------| + /// | | 0 | | | + /// | 1 | 0 | 1 | 1 | + /// | 2 | 1 | 2 | 3 | + /// | 3 | 3 | 3 | 6 | + /// + /// And so, our final result, `6`. + /// + /// This example demonstrates the left-associative nature of `fold()`: + /// it builds a string, starting with an initial value + /// and continuing with each element from the front until the back: + /// + /// ``` + /// let numbers = [1, 2, 3, 4, 5]; + /// + /// let zero = "0".to_string(); + /// + /// let result = numbers.iter().fold(zero, |acc, &x| { + /// format!("({acc} + {x})") + /// }); + /// + /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)"); + /// ``` + /// It's common for people who haven't used iterators a lot to + /// use a `for` loop with a list of things to build up a result. Those + /// can be turned into `fold()`s: + /// + /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for + /// + /// ``` + /// let numbers = [1, 2, 3, 4, 5]; + /// + /// let mut result = 0; + /// + /// // for loop: + /// for i in &numbers { + /// result = result + i; + /// } + /// + /// // fold: + /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x); + /// + /// // they're the same + /// assert_eq!(result, result2); + /// ``` + /// + /// [`reduce()`]: Iterator::reduce + #[doc(alias = "inject", alias = "foldl")] + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn fold<B, F>(mut self, init: B, mut f: F) -> B + where + Self: Sized, + F: FnMut(B, Self::Item) -> B, + { + let mut accum = init; + while let Some(x) = self.next() { + accum = f(accum, x); + } + accum + } + + /// Reduces the elements to a single one, by repeatedly applying a reducing + /// operation. + /// + /// If the iterator is empty, returns [`None`]; otherwise, returns the + /// result of the reduction. + /// + /// The reducing function is a closure with two arguments: an 'accumulator', and an element. + /// For iterators with at least one element, this is the same as [`fold()`] + /// with the first element of the iterator as the initial accumulator value, folding + /// every subsequent element into it. + /// + /// [`fold()`]: Iterator::fold + /// + /// # Example + /// + /// Find the maximum value: + /// + /// ``` + /// fn find_max<I>(iter: I) -> Option<I::Item> + /// where I: Iterator, + /// I::Item: Ord, + /// { + /// iter.reduce(|accum, item| { + /// if accum >= item { accum } else { item } + /// }) + /// } + /// let a = [10, 20, 5, -23, 0]; + /// let b: [u32; 0] = []; + /// + /// assert_eq!(find_max(a.iter()), Some(&20)); + /// assert_eq!(find_max(b.iter()), None); + /// ``` + #[inline] + #[stable(feature = "iterator_fold_self", since = "1.51.0")] + fn reduce<F>(mut self, f: F) -> Option<Self::Item> + where + Self: Sized, + F: FnMut(Self::Item, Self::Item) -> Self::Item, + { + let first = self.next()?; + Some(self.fold(first, f)) + } + + /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the + /// closure returns a failure, the failure is propagated back to the caller immediately. + /// + /// The return type of this method depends on the return type of the closure. If the closure + /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>, + /// E>`. If the closure returns `Option<Self::Item>`, then this function will return + /// `Option<Option<Self::Item>>`. + /// + /// When called on an empty iterator, this function will return either `Some(None)` or + /// `Ok(None)` depending on the type of the provided closure. + /// + /// For iterators with at least one element, this is essentially the same as calling + /// [`try_fold()`] with the first element of the iterator as the initial accumulator value. + /// + /// [`try_fold()`]: Iterator::try_fold + /// + /// # Examples + /// + /// Safely calculate the sum of a series of numbers: + /// + /// ``` + /// #![feature(iterator_try_reduce)] + /// + /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0]; + /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y)); + /// assert_eq!(sum, Some(Some(58))); + /// ``` + /// + /// Determine when a reduction short circuited: + /// + /// ``` + /// #![feature(iterator_try_reduce)] + /// + /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5]; + /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y)); + /// assert_eq!(sum, None); + /// ``` + /// + /// Determine when a reduction was not performed because there are no elements: + /// + /// ``` + /// #![feature(iterator_try_reduce)] + /// + /// let numbers: Vec<usize> = Vec::new(); + /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y)); + /// assert_eq!(sum, Some(None)); + /// ``` + /// + /// Use a [`Result`] instead of an [`Option`]: + /// + /// ``` + /// #![feature(iterator_try_reduce)] + /// + /// let numbers = vec!["1", "2", "3", "4", "5"]; + /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> = + /// numbers.into_iter().try_reduce(|x, y| { + /// if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) } + /// }); + /// assert_eq!(max, Ok(Some("5"))); + /// ``` + #[inline] + #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")] + fn try_reduce<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<R::Output>> + where + Self: Sized, + F: FnMut(Self::Item, Self::Item) -> R, + R: Try<Output = Self::Item>, + R::Residual: Residual<Option<Self::Item>>, + { + let first = match self.next() { + Some(i) => i, + None => return Try::from_output(None), + }; + + match self.try_fold(first, f).branch() { + ControlFlow::Break(r) => FromResidual::from_residual(r), + ControlFlow::Continue(i) => Try::from_output(Some(i)), + } + } + + /// Tests if every element of the iterator matches a predicate. + /// + /// `all()` takes a closure that returns `true` or `false`. It applies + /// this closure to each element of the iterator, and if they all return + /// `true`, then so does `all()`. If any of them return `false`, it + /// returns `false`. + /// + /// `all()` is short-circuiting; in other words, it will stop processing + /// as soon as it finds a `false`, given that no matter what else happens, + /// the result will also be `false`. + /// + /// An empty iterator returns `true`. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// assert!(a.iter().all(|&x| x > 0)); + /// + /// assert!(!a.iter().all(|&x| x > 2)); + /// ``` + /// + /// Stopping at the first `false`: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter(); + /// + /// assert!(!iter.all(|&x| x != 2)); + /// + /// // we can still use `iter`, as there are more elements. + /// assert_eq!(iter.next(), Some(&3)); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn all<F>(&mut self, f: F) -> bool + where + Self: Sized, + F: FnMut(Self::Item) -> bool, + { + #[inline] + fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> { + move |(), x| { + if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK } + } + } + self.try_fold((), check(f)) == ControlFlow::CONTINUE + } + + /// Tests if any element of the iterator matches a predicate. + /// + /// `any()` takes a closure that returns `true` or `false`. It applies + /// this closure to each element of the iterator, and if any of them return + /// `true`, then so does `any()`. If they all return `false`, it + /// returns `false`. + /// + /// `any()` is short-circuiting; in other words, it will stop processing + /// as soon as it finds a `true`, given that no matter what else happens, + /// the result will also be `true`. + /// + /// An empty iterator returns `false`. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// assert!(a.iter().any(|&x| x > 0)); + /// + /// assert!(!a.iter().any(|&x| x > 5)); + /// ``` + /// + /// Stopping at the first `true`: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter(); + /// + /// assert!(iter.any(|&x| x != 2)); + /// + /// // we can still use `iter`, as there are more elements. + /// assert_eq!(iter.next(), Some(&2)); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn any<F>(&mut self, f: F) -> bool + where + Self: Sized, + F: FnMut(Self::Item) -> bool, + { + #[inline] + fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> { + move |(), x| { + if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE } + } + } + + self.try_fold((), check(f)) == ControlFlow::BREAK + } + + /// Searches for an element of an iterator that satisfies a predicate. + /// + /// `find()` takes a closure that returns `true` or `false`. It applies + /// this closure to each element of the iterator, and if any of them return + /// `true`, then `find()` returns [`Some(element)`]. If they all return + /// `false`, it returns [`None`]. + /// + /// `find()` is short-circuiting; in other words, it will stop processing + /// as soon as the closure returns `true`. + /// + /// Because `find()` takes a reference, and many iterators iterate over + /// references, this leads to a possibly confusing situation where the + /// argument is a double reference. You can see this effect in the + /// examples below, with `&&x`. + /// + /// [`Some(element)`]: Some + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2)); + /// + /// assert_eq!(a.iter().find(|&&x| x == 5), None); + /// ``` + /// + /// Stopping at the first `true`: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter(); + /// + /// assert_eq!(iter.find(|&&x| x == 2), Some(&2)); + /// + /// // we can still use `iter`, as there are more elements. + /// assert_eq!(iter.next(), Some(&3)); + /// ``` + /// + /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`. + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn find<P>(&mut self, predicate: P) -> Option<Self::Item> + where + Self: Sized, + P: FnMut(&Self::Item) -> bool, + { + #[inline] + fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> { + move |(), x| { + if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE } + } + } + + self.try_fold((), check(predicate)).break_value() + } + + /// Applies function to the elements of iterator and returns + /// the first non-none result. + /// + /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`. + /// + /// # Examples + /// + /// ``` + /// let a = ["lol", "NaN", "2", "5"]; + /// + /// let first_number = a.iter().find_map(|s| s.parse().ok()); + /// + /// assert_eq!(first_number, Some(2)); + /// ``` + #[inline] + #[stable(feature = "iterator_find_map", since = "1.30.0")] + fn find_map<B, F>(&mut self, f: F) -> Option<B> + where + Self: Sized, + F: FnMut(Self::Item) -> Option<B>, + { + #[inline] + fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> { + move |(), x| match f(x) { + Some(x) => ControlFlow::Break(x), + None => ControlFlow::CONTINUE, + } + } + + self.try_fold((), check(f)).break_value() + } + + /// Applies function to the elements of iterator and returns + /// the first true result or the first error. + /// + /// The return type of this method depends on the return type of the closure. + /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>; E>`. + /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`. + /// + /// # Examples + /// + /// ``` + /// #![feature(try_find)] + /// + /// let a = ["1", "2", "lol", "NaN", "5"]; + /// + /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> { + /// Ok(s.parse::<i32>()? == search) + /// }; + /// + /// let result = a.iter().try_find(|&&s| is_my_num(s, 2)); + /// assert_eq!(result, Ok(Some(&"2"))); + /// + /// let result = a.iter().try_find(|&&s| is_my_num(s, 5)); + /// assert!(result.is_err()); + /// ``` + /// + /// This also supports other types which implement `Try`, not just `Result`. + /// ``` + /// #![feature(try_find)] + /// + /// use std::num::NonZeroU32; + /// let a = [3, 5, 7, 4, 9, 0, 11]; + /// let result = a.iter().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two())); + /// assert_eq!(result, Some(Some(&4))); + /// let result = a.iter().take(3).try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two())); + /// assert_eq!(result, Some(None)); + /// let result = a.iter().rev().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two())); + /// assert_eq!(result, None); + /// ``` + #[inline] + #[unstable(feature = "try_find", reason = "new API", issue = "63178")] + fn try_find<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<Self::Item>> + where + Self: Sized, + F: FnMut(&Self::Item) -> R, + R: Try<Output = bool>, + R::Residual: Residual<Option<Self::Item>>, + { + #[inline] + fn check<I, V, R>( + mut f: impl FnMut(&I) -> V, + ) -> impl FnMut((), I) -> ControlFlow<R::TryType> + where + V: Try<Output = bool, Residual = R>, + R: Residual<Option<I>>, + { + move |(), x| match f(&x).branch() { + ControlFlow::Continue(false) => ControlFlow::CONTINUE, + ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))), + ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)), + } + } + + match self.try_fold((), check(f)) { + ControlFlow::Break(x) => x, + ControlFlow::Continue(()) => Try::from_output(None), + } + } + + /// Searches for an element in an iterator, returning its index. + /// + /// `position()` takes a closure that returns `true` or `false`. It applies + /// this closure to each element of the iterator, and if one of them + /// returns `true`, then `position()` returns [`Some(index)`]. If all of + /// them return `false`, it returns [`None`]. + /// + /// `position()` is short-circuiting; in other words, it will stop + /// processing as soon as it finds a `true`. + /// + /// # Overflow Behavior + /// + /// The method does no guarding against overflows, so if there are more + /// than [`usize::MAX`] non-matching elements, it either produces the wrong + /// result or panics. If debug assertions are enabled, a panic is + /// guaranteed. + /// + /// # Panics + /// + /// This function might panic if the iterator has more than `usize::MAX` + /// non-matching elements. + /// + /// [`Some(index)`]: Some + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// assert_eq!(a.iter().position(|&x| x == 2), Some(1)); + /// + /// assert_eq!(a.iter().position(|&x| x == 5), None); + /// ``` + /// + /// Stopping at the first `true`: + /// + /// ``` + /// let a = [1, 2, 3, 4]; + /// + /// let mut iter = a.iter(); + /// + /// assert_eq!(iter.position(|&x| x >= 2), Some(1)); + /// + /// // we can still use `iter`, as there are more elements. + /// assert_eq!(iter.next(), Some(&3)); + /// + /// // The returned index depends on iterator state + /// assert_eq!(iter.position(|&x| x == 4), Some(0)); + /// + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn position<P>(&mut self, predicate: P) -> Option<usize> + where + Self: Sized, + P: FnMut(Self::Item) -> bool, + { + #[inline] + fn check<T>( + mut predicate: impl FnMut(T) -> bool, + ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> { + #[rustc_inherit_overflow_checks] + move |i, x| { + if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) } + } + } + + self.try_fold(0, check(predicate)).break_value() + } + + /// Searches for an element in an iterator from the right, returning its + /// index. + /// + /// `rposition()` takes a closure that returns `true` or `false`. It applies + /// this closure to each element of the iterator, starting from the end, + /// and if one of them returns `true`, then `rposition()` returns + /// [`Some(index)`]. If all of them return `false`, it returns [`None`]. + /// + /// `rposition()` is short-circuiting; in other words, it will stop + /// processing as soon as it finds a `true`. + /// + /// [`Some(index)`]: Some + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2)); + /// + /// assert_eq!(a.iter().rposition(|&x| x == 5), None); + /// ``` + /// + /// Stopping at the first `true`: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter(); + /// + /// assert_eq!(iter.rposition(|&x| x == 2), Some(1)); + /// + /// // we can still use `iter`, as there are more elements. + /// assert_eq!(iter.next(), Some(&1)); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn rposition<P>(&mut self, predicate: P) -> Option<usize> + where + P: FnMut(Self::Item) -> bool, + Self: Sized + ExactSizeIterator + DoubleEndedIterator, + { + // No need for an overflow check here, because `ExactSizeIterator` + // implies that the number of elements fits into a `usize`. + #[inline] + fn check<T>( + mut predicate: impl FnMut(T) -> bool, + ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> { + move |i, x| { + let i = i - 1; + if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) } + } + } + + let n = self.len(); + self.try_rfold(n, check(predicate)).break_value() + } + + /// Returns the maximum element of an iterator. + /// + /// If several elements are equally maximum, the last element is + /// returned. If the iterator is empty, [`None`] is returned. + /// + /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being + /// incomparable. You can work around this by using [`Iterator::reduce`]: + /// ``` + /// assert_eq!( + /// [2.4, f32::NAN, 1.3] + /// .into_iter() + /// .reduce(f32::max) + /// .unwrap(), + /// 2.4 + /// ); + /// ``` + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// let b: Vec<u32> = Vec::new(); + /// + /// assert_eq!(a.iter().max(), Some(&3)); + /// assert_eq!(b.iter().max(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn max(self) -> Option<Self::Item> + where + Self: Sized, + Self::Item: Ord, + { + self.max_by(Ord::cmp) + } + + /// Returns the minimum element of an iterator. + /// + /// If several elements are equally minimum, the first element is returned. + /// If the iterator is empty, [`None`] is returned. + /// + /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being + /// incomparable. You can work around this by using [`Iterator::reduce`]: + /// ``` + /// assert_eq!( + /// [2.4, f32::NAN, 1.3] + /// .into_iter() + /// .reduce(f32::min) + /// .unwrap(), + /// 1.3 + /// ); + /// ``` + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// let b: Vec<u32> = Vec::new(); + /// + /// assert_eq!(a.iter().min(), Some(&1)); + /// assert_eq!(b.iter().min(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + fn min(self) -> Option<Self::Item> + where + Self: Sized, + Self::Item: Ord, + { + self.min_by(Ord::cmp) + } + + /// Returns the element that gives the maximum value from the + /// specified function. + /// + /// If several elements are equally maximum, the last element is + /// returned. If the iterator is empty, [`None`] is returned. + /// + /// # Examples + /// + /// ``` + /// let a = [-3_i32, 0, 1, 5, -10]; + /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10); + /// ``` + #[inline] + #[stable(feature = "iter_cmp_by_key", since = "1.6.0")] + fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item> + where + Self: Sized, + F: FnMut(&Self::Item) -> B, + { + #[inline] + fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) { + move |x| (f(&x), x) + } + + #[inline] + fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering { + x_p.cmp(y_p) + } + + let (_, x) = self.map(key(f)).max_by(compare)?; + Some(x) + } + + /// Returns the element that gives the maximum value with respect to the + /// specified comparison function. + /// + /// If several elements are equally maximum, the last element is + /// returned. If the iterator is empty, [`None`] is returned. + /// + /// # Examples + /// + /// ``` + /// let a = [-3_i32, 0, 1, 5, -10]; + /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5); + /// ``` + #[inline] + #[stable(feature = "iter_max_by", since = "1.15.0")] + fn max_by<F>(self, compare: F) -> Option<Self::Item> + where + Self: Sized, + F: FnMut(&Self::Item, &Self::Item) -> Ordering, + { + #[inline] + fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T { + move |x, y| cmp::max_by(x, y, &mut compare) + } + + self.reduce(fold(compare)) + } + + /// Returns the element that gives the minimum value from the + /// specified function. + /// + /// If several elements are equally minimum, the first element is + /// returned. If the iterator is empty, [`None`] is returned. + /// + /// # Examples + /// + /// ``` + /// let a = [-3_i32, 0, 1, 5, -10]; + /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0); + /// ``` + #[inline] + #[stable(feature = "iter_cmp_by_key", since = "1.6.0")] + fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item> + where + Self: Sized, + F: FnMut(&Self::Item) -> B, + { + #[inline] + fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) { + move |x| (f(&x), x) + } + + #[inline] + fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering { + x_p.cmp(y_p) + } + + let (_, x) = self.map(key(f)).min_by(compare)?; + Some(x) + } + + /// Returns the element that gives the minimum value with respect to the + /// specified comparison function. + /// + /// If several elements are equally minimum, the first element is + /// returned. If the iterator is empty, [`None`] is returned. + /// + /// # Examples + /// + /// ``` + /// let a = [-3_i32, 0, 1, 5, -10]; + /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10); + /// ``` + #[inline] + #[stable(feature = "iter_min_by", since = "1.15.0")] + fn min_by<F>(self, compare: F) -> Option<Self::Item> + where + Self: Sized, + F: FnMut(&Self::Item, &Self::Item) -> Ordering, + { + #[inline] + fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T { + move |x, y| cmp::min_by(x, y, &mut compare) + } + + self.reduce(fold(compare)) + } + + /// Reverses an iterator's direction. + /// + /// Usually, iterators iterate from left to right. After using `rev()`, + /// an iterator will instead iterate from right to left. + /// + /// This is only possible if the iterator has an end, so `rev()` only + /// works on [`DoubleEndedIterator`]s. + /// + /// # Examples + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut iter = a.iter().rev(); + /// + /// assert_eq!(iter.next(), Some(&3)); + /// assert_eq!(iter.next(), Some(&2)); + /// assert_eq!(iter.next(), Some(&1)); + /// + /// assert_eq!(iter.next(), None); + /// ``` + #[inline] + #[doc(alias = "reverse")] + #[stable(feature = "rust1", since = "1.0.0")] + fn rev(self) -> Rev<Self> + where + Self: Sized + DoubleEndedIterator, + { + Rev::new(self) + } + + /// Converts an iterator of pairs into a pair of containers. + /// + /// `unzip()` consumes an entire iterator of pairs, producing two + /// collections: one from the left elements of the pairs, and one + /// from the right elements. + /// + /// This function is, in some sense, the opposite of [`zip`]. + /// + /// [`zip`]: Iterator::zip + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [(1, 2), (3, 4), (5, 6)]; + /// + /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip(); + /// + /// assert_eq!(left, [1, 3, 5]); + /// assert_eq!(right, [2, 4, 6]); + /// + /// // you can also unzip multiple nested tuples at once + /// let a = [(1, (2, 3)), (4, (5, 6))]; + /// + /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip(); + /// assert_eq!(x, [1, 4]); + /// assert_eq!(y, [2, 5]); + /// assert_eq!(z, [3, 6]); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB) + where + FromA: Default + Extend<A>, + FromB: Default + Extend<B>, + Self: Sized + Iterator<Item = (A, B)>, + { + let mut unzipped: (FromA, FromB) = Default::default(); + unzipped.extend(self); + unzipped + } + + /// Creates an iterator which copies all of its elements. + /// + /// This is useful when you have an iterator over `&T`, but you need an + /// iterator over `T`. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let v_copied: Vec<_> = a.iter().copied().collect(); + /// + /// // copied is the same as .map(|&x| x) + /// let v_map: Vec<_> = a.iter().map(|&x| x).collect(); + /// + /// assert_eq!(v_copied, vec![1, 2, 3]); + /// assert_eq!(v_map, vec![1, 2, 3]); + /// ``` + #[stable(feature = "iter_copied", since = "1.36.0")] + fn copied<'a, T: 'a>(self) -> Copied<Self> + where + Self: Sized + Iterator<Item = &'a T>, + T: Copy, + { + Copied::new(self) + } + + /// Creates an iterator which [`clone`]s all of its elements. + /// + /// This is useful when you have an iterator over `&T`, but you need an + /// iterator over `T`. + /// + /// There is no guarantee whatsoever about the `clone` method actually + /// being called *or* optimized away. So code should not depend on + /// either. + /// + /// [`clone`]: Clone::clone + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let v_cloned: Vec<_> = a.iter().cloned().collect(); + /// + /// // cloned is the same as .map(|&x| x), for integers + /// let v_map: Vec<_> = a.iter().map(|&x| x).collect(); + /// + /// assert_eq!(v_cloned, vec![1, 2, 3]); + /// assert_eq!(v_map, vec![1, 2, 3]); + /// ``` + /// + /// To get the best performance, try to clone late: + /// + /// ``` + /// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]]; + /// // don't do this: + /// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect(); + /// assert_eq!(&[vec![23]], &slower[..]); + /// // instead call `cloned` late + /// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect(); + /// assert_eq!(&[vec![23]], &faster[..]); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + fn cloned<'a, T: 'a>(self) -> Cloned<Self> + where + Self: Sized + Iterator<Item = &'a T>, + T: Clone, + { + Cloned::new(self) + } + + /// Repeats an iterator endlessly. + /// + /// Instead of stopping at [`None`], the iterator will instead start again, + /// from the beginning. After iterating again, it will start at the + /// beginning again. And again. And again. Forever. Note that in case the + /// original iterator is empty, the resulting iterator will also be empty. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// + /// let mut it = a.iter().cycle(); + /// + /// assert_eq!(it.next(), Some(&1)); + /// assert_eq!(it.next(), Some(&2)); + /// assert_eq!(it.next(), Some(&3)); + /// assert_eq!(it.next(), Some(&1)); + /// assert_eq!(it.next(), Some(&2)); + /// assert_eq!(it.next(), Some(&3)); + /// assert_eq!(it.next(), Some(&1)); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + #[inline] + fn cycle(self) -> Cycle<Self> + where + Self: Sized + Clone, + { + Cycle::new(self) + } + + /// Sums the elements of an iterator. + /// + /// Takes each element, adds them together, and returns the result. + /// + /// An empty iterator returns the zero value of the type. + /// + /// # Panics + /// + /// When calling `sum()` and a primitive integer type is being returned, this + /// method will panic if the computation overflows and debug assertions are + /// enabled. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let a = [1, 2, 3]; + /// let sum: i32 = a.iter().sum(); + /// + /// assert_eq!(sum, 6); + /// ``` + #[stable(feature = "iter_arith", since = "1.11.0")] + fn sum<S>(self) -> S + where + Self: Sized, + S: Sum<Self::Item>, + { + Sum::sum(self) + } + + /// Iterates over the entire iterator, multiplying all the elements + /// + /// An empty iterator returns the one value of the type. + /// + /// # Panics + /// + /// When calling `product()` and a primitive integer type is being returned, + /// method will panic if the computation overflows and debug assertions are + /// enabled. + /// + /// # Examples + /// + /// ``` + /// fn factorial(n: u32) -> u32 { + /// (1..=n).product() + /// } + /// assert_eq!(factorial(0), 1); + /// assert_eq!(factorial(1), 1); + /// assert_eq!(factorial(5), 120); + /// ``` + #[stable(feature = "iter_arith", since = "1.11.0")] + fn product<P>(self) -> P + where + Self: Sized, + P: Product<Self::Item>, + { + Product::product(self) + } + + /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those + /// of another. + /// + /// # Examples + /// + /// ``` + /// use std::cmp::Ordering; + /// + /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal); + /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less); + /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater); + /// ``` + #[stable(feature = "iter_order", since = "1.5.0")] + fn cmp<I>(self, other: I) -> Ordering + where + I: IntoIterator<Item = Self::Item>, + Self::Item: Ord, + Self: Sized, + { + self.cmp_by(other, |x, y| x.cmp(&y)) + } + + /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those + /// of another with respect to the specified comparison function. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_order_by)] + /// + /// use std::cmp::Ordering; + /// + /// let xs = [1, 2, 3, 4]; + /// let ys = [1, 4, 9, 16]; + /// + /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less); + /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal); + /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater); + /// ``` + #[unstable(feature = "iter_order_by", issue = "64295")] + fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering + where + Self: Sized, + I: IntoIterator, + F: FnMut(Self::Item, I::Item) -> Ordering, + { + let mut other = other.into_iter(); + + loop { + let x = match self.next() { + None => { + if other.next().is_none() { + return Ordering::Equal; + } else { + return Ordering::Less; + } + } + Some(val) => val, + }; + + let y = match other.next() { + None => return Ordering::Greater, + Some(val) => val, + }; + + match cmp(x, y) { + Ordering::Equal => (), + non_eq => return non_eq, + } + } + } + + /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those + /// of another. + /// + /// # Examples + /// + /// ``` + /// use std::cmp::Ordering; + /// + /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal)); + /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less)); + /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater)); + /// + /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None); + /// ``` + #[stable(feature = "iter_order", since = "1.5.0")] + fn partial_cmp<I>(self, other: I) -> Option<Ordering> + where + I: IntoIterator, + Self::Item: PartialOrd<I::Item>, + Self: Sized, + { + self.partial_cmp_by(other, |x, y| x.partial_cmp(&y)) + } + + /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those + /// of another with respect to the specified comparison function. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_order_by)] + /// + /// use std::cmp::Ordering; + /// + /// let xs = [1.0, 2.0, 3.0, 4.0]; + /// let ys = [1.0, 4.0, 9.0, 16.0]; + /// + /// assert_eq!( + /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)), + /// Some(Ordering::Less) + /// ); + /// assert_eq!( + /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)), + /// Some(Ordering::Equal) + /// ); + /// assert_eq!( + /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)), + /// Some(Ordering::Greater) + /// ); + /// ``` + #[unstable(feature = "iter_order_by", issue = "64295")] + fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering> + where + Self: Sized, + I: IntoIterator, + F: FnMut(Self::Item, I::Item) -> Option<Ordering>, + { + let mut other = other.into_iter(); + + loop { + let x = match self.next() { + None => { + if other.next().is_none() { + return Some(Ordering::Equal); + } else { + return Some(Ordering::Less); + } + } + Some(val) => val, + }; + + let y = match other.next() { + None => return Some(Ordering::Greater), + Some(val) => val, + }; + + match partial_cmp(x, y) { + Some(Ordering::Equal) => (), + non_eq => return non_eq, + } + } + } + + /// Determines if the elements of this [`Iterator`] are equal to those of + /// another. + /// + /// # Examples + /// + /// ``` + /// assert_eq!([1].iter().eq([1].iter()), true); + /// assert_eq!([1].iter().eq([1, 2].iter()), false); + /// ``` + #[stable(feature = "iter_order", since = "1.5.0")] + fn eq<I>(self, other: I) -> bool + where + I: IntoIterator, + Self::Item: PartialEq<I::Item>, + Self: Sized, + { + self.eq_by(other, |x, y| x == y) + } + + /// Determines if the elements of this [`Iterator`] are equal to those of + /// another with respect to the specified equality function. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// #![feature(iter_order_by)] + /// + /// let xs = [1, 2, 3, 4]; + /// let ys = [1, 4, 9, 16]; + /// + /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y)); + /// ``` + #[unstable(feature = "iter_order_by", issue = "64295")] + fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool + where + Self: Sized, + I: IntoIterator, + F: FnMut(Self::Item, I::Item) -> bool, + { + let mut other = other.into_iter(); + + loop { + let x = match self.next() { + None => return other.next().is_none(), + Some(val) => val, + }; + + let y = match other.next() { + None => return false, + Some(val) => val, + }; + + if !eq(x, y) { + return false; + } + } + } + + /// Determines if the elements of this [`Iterator`] are unequal to those of + /// another. + /// + /// # Examples + /// + /// ``` + /// assert_eq!([1].iter().ne([1].iter()), false); + /// assert_eq!([1].iter().ne([1, 2].iter()), true); + /// ``` + #[stable(feature = "iter_order", since = "1.5.0")] + fn ne<I>(self, other: I) -> bool + where + I: IntoIterator, + Self::Item: PartialEq<I::Item>, + Self: Sized, + { + !self.eq(other) + } + + /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison) + /// less than those of another. + /// + /// # Examples + /// + /// ``` + /// assert_eq!([1].iter().lt([1].iter()), false); + /// assert_eq!([1].iter().lt([1, 2].iter()), true); + /// assert_eq!([1, 2].iter().lt([1].iter()), false); + /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false); + /// ``` + #[stable(feature = "iter_order", since = "1.5.0")] + fn lt<I>(self, other: I) -> bool + where + I: IntoIterator, + Self::Item: PartialOrd<I::Item>, + Self: Sized, + { + self.partial_cmp(other) == Some(Ordering::Less) + } + + /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison) + /// less or equal to those of another. + /// + /// # Examples + /// + /// ``` + /// assert_eq!([1].iter().le([1].iter()), true); + /// assert_eq!([1].iter().le([1, 2].iter()), true); + /// assert_eq!([1, 2].iter().le([1].iter()), false); + /// assert_eq!([1, 2].iter().le([1, 2].iter()), true); + /// ``` + #[stable(feature = "iter_order", since = "1.5.0")] + fn le<I>(self, other: I) -> bool + where + I: IntoIterator, + Self::Item: PartialOrd<I::Item>, + Self: Sized, + { + matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal)) + } + + /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison) + /// greater than those of another. + /// + /// # Examples + /// + /// ``` + /// assert_eq!([1].iter().gt([1].iter()), false); + /// assert_eq!([1].iter().gt([1, 2].iter()), false); + /// assert_eq!([1, 2].iter().gt([1].iter()), true); + /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false); + /// ``` + #[stable(feature = "iter_order", since = "1.5.0")] + fn gt<I>(self, other: I) -> bool + where + I: IntoIterator, + Self::Item: PartialOrd<I::Item>, + Self: Sized, + { + self.partial_cmp(other) == Some(Ordering::Greater) + } + + /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison) + /// greater than or equal to those of another. + /// + /// # Examples + /// + /// ``` + /// assert_eq!([1].iter().ge([1].iter()), true); + /// assert_eq!([1].iter().ge([1, 2].iter()), false); + /// assert_eq!([1, 2].iter().ge([1].iter()), true); + /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true); + /// ``` + #[stable(feature = "iter_order", since = "1.5.0")] + fn ge<I>(self, other: I) -> bool + where + I: IntoIterator, + Self::Item: PartialOrd<I::Item>, + Self: Sized, + { + matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal)) + } + + /// Checks if the elements of this iterator are sorted. + /// + /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the + /// iterator yields exactly zero or one element, `true` is returned. + /// + /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition + /// implies that this function returns `false` if any two consecutive items are not + /// comparable. + /// + /// # Examples + /// + /// ``` + /// #![feature(is_sorted)] + /// + /// assert!([1, 2, 2, 9].iter().is_sorted()); + /// assert!(![1, 3, 2, 4].iter().is_sorted()); + /// assert!([0].iter().is_sorted()); + /// assert!(std::iter::empty::<i32>().is_sorted()); + /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted()); + /// ``` + #[inline] + #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")] + fn is_sorted(self) -> bool + where + Self: Sized, + Self::Item: PartialOrd, + { + self.is_sorted_by(PartialOrd::partial_cmp) + } + + /// Checks if the elements of this iterator are sorted using the given comparator function. + /// + /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare` + /// function to determine the ordering of two elements. Apart from that, it's equivalent to + /// [`is_sorted`]; see its documentation for more information. + /// + /// # Examples + /// + /// ``` + /// #![feature(is_sorted)] + /// + /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b))); + /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b))); + /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b))); + /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b))); + /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b))); + /// ``` + /// + /// [`is_sorted`]: Iterator::is_sorted + #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")] + fn is_sorted_by<F>(mut self, compare: F) -> bool + where + Self: Sized, + F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>, + { + #[inline] + fn check<'a, T>( + last: &'a mut T, + mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a, + ) -> impl FnMut(T) -> bool + 'a { + move |curr| { + if let Some(Ordering::Greater) | None = compare(&last, &curr) { + return false; + } + *last = curr; + true + } + } + + let mut last = match self.next() { + Some(e) => e, + None => return true, + }; + + self.all(check(&mut last, compare)) + } + + /// Checks if the elements of this iterator are sorted using the given key extraction + /// function. + /// + /// Instead of comparing the iterator's elements directly, this function compares the keys of + /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see + /// its documentation for more information. + /// + /// [`is_sorted`]: Iterator::is_sorted + /// + /// # Examples + /// + /// ``` + /// #![feature(is_sorted)] + /// + /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len())); + /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs())); + /// ``` + #[inline] + #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")] + fn is_sorted_by_key<F, K>(self, f: F) -> bool + where + Self: Sized, + F: FnMut(Self::Item) -> K, + K: PartialOrd, + { + self.map(f).is_sorted() + } + + /// See [TrustedRandomAccess][super::super::TrustedRandomAccess] + // The unusual name is to avoid name collisions in method resolution + // see #76479. + #[inline] + #[doc(hidden)] + #[unstable(feature = "trusted_random_access", issue = "none")] + unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item + where + Self: TrustedRandomAccessNoCoerce, + { + unreachable!("Always specialized"); + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<I: Iterator + ?Sized> Iterator for &mut I { + type Item = I::Item; + #[inline] + fn next(&mut self) -> Option<I::Item> { + (**self).next() + } + fn size_hint(&self) -> (usize, Option<usize>) { + (**self).size_hint() + } + fn advance_by(&mut self, n: usize) -> Result<(), usize> { + (**self).advance_by(n) + } + fn nth(&mut self, n: usize) -> Option<Self::Item> { + (**self).nth(n) + } +} diff --git a/library/core/src/iter/traits/marker.rs b/library/core/src/iter/traits/marker.rs new file mode 100644 index 000000000..da7537457 --- /dev/null +++ b/library/core/src/iter/traits/marker.rs @@ -0,0 +1,78 @@ +use crate::iter::Step; + +/// An iterator that always continues to yield `None` when exhausted. +/// +/// Calling next on a fused iterator that has returned `None` once is guaranteed +/// to return [`None`] again. This trait should be implemented by all iterators +/// that behave this way because it allows optimizing [`Iterator::fuse()`]. +/// +/// Note: In general, you should not use `FusedIterator` in generic bounds if +/// you need a fused iterator. Instead, you should just call [`Iterator::fuse()`] +/// on the iterator. If the iterator is already fused, the additional [`Fuse`] +/// wrapper will be a no-op with no performance penalty. +/// +/// [`Fuse`]: crate::iter::Fuse +#[stable(feature = "fused", since = "1.26.0")] +#[rustc_unsafe_specialization_marker] +pub trait FusedIterator: Iterator {} + +#[stable(feature = "fused", since = "1.26.0")] +impl<I: FusedIterator + ?Sized> FusedIterator for &mut I {} + +/// An iterator that reports an accurate length using size_hint. +/// +/// The iterator reports a size hint where it is either exact +/// (lower bound is equal to upper bound), or the upper bound is [`None`]. +/// The upper bound must only be [`None`] if the actual iterator length is +/// larger than [`usize::MAX`]. In that case, the lower bound must be +/// [`usize::MAX`], resulting in an [`Iterator::size_hint()`] of +/// `(usize::MAX, None)`. +/// +/// The iterator must produce exactly the number of elements it reported +/// or diverge before reaching the end. +/// +/// # Safety +/// +/// This trait must only be implemented when the contract is upheld. Consumers +/// of this trait must inspect [`Iterator::size_hint()`]’s upper bound. +#[unstable(feature = "trusted_len", issue = "37572")] +#[rustc_unsafe_specialization_marker] +pub unsafe trait TrustedLen: Iterator {} + +#[unstable(feature = "trusted_len", issue = "37572")] +unsafe impl<I: TrustedLen + ?Sized> TrustedLen for &mut I {} + +/// An iterator that when yielding an item will have taken at least one element +/// from its underlying [`SourceIter`]. +/// +/// Calling any method that advances the iterator, e.g. [`next()`] or [`try_fold()`], +/// guarantees that for each step at least one value of the iterator's underlying source +/// has been moved out and the result of the iterator chain could be inserted +/// in its place, assuming structural constraints of the source allow such an insertion. +/// In other words this trait indicates that an iterator pipeline can be collected in place. +/// +/// The primary use of this trait is in-place iteration. Refer to the [`vec::in_place_collect`] +/// module documentation for more information. +/// +/// [`vec::in_place_collect`]: ../../../../alloc/vec/in_place_collect/index.html +/// [`SourceIter`]: crate::iter::SourceIter +/// [`next()`]: Iterator::next +/// [`try_fold()`]: Iterator::try_fold +#[unstable(issue = "none", feature = "inplace_iteration")] +#[doc(hidden)] +pub unsafe trait InPlaceIterable: Iterator {} + +/// A type that upholds all invariants of [`Step`]. +/// +/// The invariants of [`Step::steps_between()`] are a superset of the invariants +/// of [`TrustedLen`]. As such, [`TrustedLen`] is implemented for all range +/// types with the same generic type argument. +/// +/// # Safety +/// +/// The implementation of [`Step`] for the given type must guarantee all +/// invariants of all methods are upheld. See the [`Step`] trait's documentation +/// for details. Consumers are free to rely on the invariants in unsafe code. +#[unstable(feature = "trusted_step", issue = "85731")] +#[rustc_specialization_trait] +pub unsafe trait TrustedStep: Step {} diff --git a/library/core/src/iter/traits/mod.rs b/library/core/src/iter/traits/mod.rs new file mode 100644 index 000000000..ed0fb634d --- /dev/null +++ b/library/core/src/iter/traits/mod.rs @@ -0,0 +1,21 @@ +mod accum; +mod collect; +mod double_ended; +mod exact_size; +mod iterator; +mod marker; + +#[stable(feature = "rust1", since = "1.0.0")] +pub use self::{ + accum::{Product, Sum}, + collect::{Extend, FromIterator, IntoIterator}, + double_ended::DoubleEndedIterator, + exact_size::ExactSizeIterator, + iterator::Iterator, + marker::{FusedIterator, TrustedLen}, +}; + +#[unstable(issue = "none", feature = "inplace_iteration")] +pub use self::marker::InPlaceIterable; +#[unstable(feature = "trusted_step", issue = "85731")] +pub use self::marker::TrustedStep; |