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/// 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 we need to store the elements somewhere",
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 we need to store the elements somewhere",
label = "try explicitly collecting into a `Vec<{A}>`",
),
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
///
/// ```
/// 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")]
#[rustc_diagnostic_item = "from_iter_fn"]
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
///
/// ```
/// 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: Iterator> 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
///
/// ```
/// // 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);
}
}