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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.
    ///
    /// `FromIterator`'s [`from_iter`] is rarely called explicitly, and is instead
    /// used through [`Iterator`]'s [`collect`] method. See [`collect`]'s
    /// documentation for more examples.
    ///
    /// [`from_iter`]: #tymethod.from_iter
    /// [`Iterator`]: trait.Iterator.html
    /// [`collect`]: trait.Iterator.html#method.collect
    ///
    /// See also: [`IntoIterator`].
    ///
    /// [`IntoIterator`]: trait.IntoIterator.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter::FromIterator;
    ///
    /// 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 [`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]);
    /// ```
    ///
    /// Implementing `FromIterator` for your type:
    ///
    /// ```
    /// use std::iter::FromIterator;
    ///
    /// // 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(
        message="a collection of type `{Self}` cannot be built from an iterator \
                 over elements of type `{A}`",
        label="a collection of type `{Self}` cannot be built from `std::iter::Iterator<Item={A}>`",
    )]
    pub trait FromIterator<A>: Sized {
        /// Creates a value from an iterator.
        ///
        /// See the [module-level documentation] for more.
        ///
        /// [module-level documentation]: index.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::iter::FromIterator;
        ///
        /// 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](index.html#for-loops-and-intoiterator).
    ///
    /// See also: [`FromIterator`].
    ///
    /// [`FromIterator`]: trait.FromIterator.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v = vec![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()
    /// }
    /// ```
    #[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]: index.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let v = vec![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());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn into_iter(self) -> Self::IntoIter;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator> IntoIterator for I {
        type Item = I::Item;
        type IntoIter = I;

        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 method for this trait, the [trait-level] docs
        /// contain more details.
        ///
        /// [trait-level]: trait.Extend.html
        ///
        /// # 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);
    }

    #[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)
        }
    }

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