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    //! A module for working with borrowed data.

    #![stable(feature = "rust1", since = "1.0.0")]

    use core::cmp::Ordering;
    use core::hash::{Hash, Hasher};
    use core::ops::{Add, AddAssign, Deref};

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::borrow::{Borrow, BorrowMut};

    use crate::fmt;
    use crate::string::String;

    use Cow::*;

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
    where
        B: ToOwned,
        <B as ToOwned>::Owned: 'a,
    {
        fn borrow(&self) -> &B {
            &**self
        }
    }

    /// A generalization of `Clone` to borrowed data.
    ///
    /// Some types make it possible to go from borrowed to owned, usually by
    /// implementing the `Clone` trait. But `Clone` works only for going from `&T`
    /// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
    /// from any borrow of a given type.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait ToOwned {
        /// The resulting type after obtaining ownership.
        #[stable(feature = "rust1", since = "1.0.0")]
        type Owned: Borrow<Self>;

        /// Creates owned data from borrowed data, usually by cloning.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s: &str = "a";
        /// let ss: String = s.to_owned();
        ///
        /// let v: &[i32] = &[1, 2];
        /// let vv: Vec<i32> = v.to_owned();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[must_use = "cloning is often expensive and is not expected to have side effects"]
        fn to_owned(&self) -> Self::Owned;

        /// Uses borrowed data to replace owned data, usually by cloning.
        ///
        /// This is borrow-generalized version of `Clone::clone_from`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// # #![feature(toowned_clone_into)]
        /// let mut s: String = String::new();
        /// "hello".clone_into(&mut s);
        ///
        /// let mut v: Vec<i32> = Vec::new();
        /// [1, 2][..].clone_into(&mut v);
        /// ```
        #[unstable(feature = "toowned_clone_into", reason = "recently added", issue = "41263")]
        fn clone_into(&self, target: &mut Self::Owned) {
            *target = self.to_owned();
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ToOwned for T
    where
        T: Clone,
    {
        type Owned = T;
        fn to_owned(&self) -> T {
            self.clone()
        }

        fn clone_into(&self, target: &mut T) {
            target.clone_from(self);
        }
    }

    /// A clone-on-write smart pointer.
    ///
    /// The type `Cow` is a smart pointer providing clone-on-write functionality: it
    /// can enclose and provide immutable access to borrowed data, and clone the
    /// data lazily when mutation or ownership is required. The type is designed to
    /// work with general borrowed data via the `Borrow` trait.
    ///
    /// `Cow` implements `Deref`, which means that you can call
    /// non-mutating methods directly on the data it encloses. If mutation
    /// is desired, `to_mut` will obtain a mutable reference to an owned
    /// value, cloning if necessary.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::borrow::Cow;
    ///
    /// fn abs_all(input: &mut Cow<[i32]>) {
    ///     for i in 0..input.len() {
    ///         let v = input[i];
    ///         if v < 0 {
    ///             // Clones into a vector if not already owned.
    ///             input.to_mut()[i] = -v;
    ///         }
    ///     }
    /// }
    ///
    /// // No clone occurs because `input` doesn't need to be mutated.
    /// let slice = [0, 1, 2];
    /// let mut input = Cow::from(&slice[..]);
    /// abs_all(&mut input);
    ///
    /// // Clone occurs because `input` needs to be mutated.
    /// let slice = [-1, 0, 1];
    /// let mut input = Cow::from(&slice[..]);
    /// abs_all(&mut input);
    ///
    /// // No clone occurs because `input` is already owned.
    /// let mut input = Cow::from(vec![-1, 0, 1]);
    /// abs_all(&mut input);
    /// ```
    ///
    /// Another example showing how to keep `Cow` in a struct:
    ///
    /// ```
    /// use std::borrow::Cow;
    ///
    /// struct Items<'a, X: 'a> where [X]: ToOwned<Owned = Vec<X>> {
    ///     values: Cow<'a, [X]>,
    /// }
    ///
    /// impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
    ///     fn new(v: Cow<'a, [X]>) -> Self {
    ///         Items { values: v }
    ///     }
    /// }
    ///
    /// // Creates a container from borrowed values of a slice
    /// let readonly = [1, 2];
    /// let borrowed = Items::new((&readonly[..]).into());
    /// match borrowed {
    ///     Items { values: Cow::Borrowed(b) } => println!("borrowed {:?}", b),
    ///     _ => panic!("expect borrowed value"),
    /// }
    ///
    /// let mut clone_on_write = borrowed;
    /// // Mutates the data from slice into owned vec and pushes a new value on top
    /// clone_on_write.values.to_mut().push(3);
    /// println!("clone_on_write = {:?}", clone_on_write.values);
    ///
    /// // The data was mutated. Let check it out.
    /// match clone_on_write {
    ///     Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
    ///     _ => panic!("expect owned data"),
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum Cow<'a, B: ?Sized + 'a>
    where
        B: ToOwned,
    {
        /// Borrowed data.
        #[stable(feature = "rust1", since = "1.0.0")]
        Borrowed(#[stable(feature = "rust1", since = "1.0.0")] &'a B),

        /// Owned data.
        #[stable(feature = "rust1", since = "1.0.0")]
        Owned(#[stable(feature = "rust1", since = "1.0.0")] <B as ToOwned>::Owned),
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: ?Sized + ToOwned> Clone for Cow<'_, B> {
        fn clone(&self) -> Self {
            match *self {
                Borrowed(b) => Borrowed(b),
                Owned(ref o) => {
                    let b: &B = o.borrow();
                    Owned(b.to_owned())
                }
            }
        }

        fn clone_from(&mut self, source: &Self) {
            match (self, source) {
                (&mut Owned(ref mut dest), &Owned(ref o)) => o.borrow().clone_into(dest),
                (t, s) => *t = s.clone(),
            }
        }
    }

    impl<B: ?Sized + ToOwned> Cow<'_, B> {
        /// Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(cow_is_borrowed)]
        /// use std::borrow::Cow;
        ///
        /// let cow = Cow::Borrowed("moo");
        /// assert!(cow.is_borrowed());
        ///
        /// let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
        /// assert!(!bull.is_borrowed());
        /// ```
        #[unstable(feature = "cow_is_borrowed", issue = "65143")]
        pub fn is_borrowed(&self) -> bool {
            match *self {
                Borrowed(_) => true,
                Owned(_) => false,
            }
        }

        /// Returns true if the data is owned, i.e. if `to_mut` would be a no-op.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(cow_is_borrowed)]
        /// use std::borrow::Cow;
        ///
        /// let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
        /// assert!(cow.is_owned());
        ///
        /// let bull = Cow::Borrowed("...moo?");
        /// assert!(!bull.is_owned());
        /// ```
        #[unstable(feature = "cow_is_borrowed", issue = "65143")]
        pub fn is_owned(&self) -> bool {
            !self.is_borrowed()
        }

        /// Acquires a mutable reference to the owned form of the data.
        ///
        /// Clones the data if it is not already owned.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::borrow::Cow;
        ///
        /// let mut cow = Cow::Borrowed("foo");
        /// cow.to_mut().make_ascii_uppercase();
        ///
        /// assert_eq!(
        ///   cow,
        ///   Cow::Owned(String::from("FOO")) as Cow<str>
        /// );
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
            match *self {
                Borrowed(borrowed) => {
                    *self = Owned(borrowed.to_owned());
                    match *self {
                        Borrowed(..) => unreachable!(),
                        Owned(ref mut owned) => owned,
                    }
                }
                Owned(ref mut owned) => owned,
            }
        }

        /// Extracts the owned data.
        ///
        /// Clones the data if it is not already owned.
        ///
        /// # Examples
        ///
        /// Calling `into_owned` on a `Cow::Borrowed` clones the underlying data
        /// and becomes a `Cow::Owned`:
        ///
        /// ```
        /// use std::borrow::Cow;
        ///
        /// let s = "Hello world!";
        /// let cow = Cow::Borrowed(s);
        ///
        /// assert_eq!(
        ///   cow.into_owned(),
        ///   String::from(s)
        /// );
        /// ```
        ///
        /// Calling `into_owned` on a `Cow::Owned` is a no-op:
        ///
        /// ```
        /// use std::borrow::Cow;
        ///
        /// let s = "Hello world!";
        /// let cow: Cow<str> = Cow::Owned(String::from(s));
        ///
        /// assert_eq!(
        ///   cow.into_owned(),
        ///   String::from(s)
        /// );
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_owned(self) -> <B as ToOwned>::Owned {
            match self {
                Borrowed(borrowed) => borrowed.to_owned(),
                Owned(owned) => owned,
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: ?Sized + ToOwned> Deref for Cow<'_, B> {
        type Target = B;

        fn deref(&self) -> &B {
            match *self {
                Borrowed(borrowed) => borrowed,
                Owned(ref owned) => owned.borrow(),
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: ?Sized> Ord for Cow<'_, B>
    where
        B: Ord + ToOwned,
    {
        #[inline]
        fn cmp(&self, other: &Self) -> Ordering {
            Ord::cmp(&**self, &**other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B>
    where
        B: PartialEq<C> + ToOwned,
        C: ToOwned,
    {
        #[inline]
        fn eq(&self, other: &Cow<'b, C>) -> bool {
            PartialEq::eq(&**self, &**other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, B: ?Sized> PartialOrd for Cow<'a, B>
    where
        B: PartialOrd + ToOwned,
    {
        #[inline]
        fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
            PartialOrd::partial_cmp(&**self, &**other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: ?Sized> fmt::Debug for Cow<'_, B>
    where
        B: fmt::Debug + ToOwned<Owned: fmt::Debug>,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                Borrowed(ref b) => fmt::Debug::fmt(b, f),
                Owned(ref o) => fmt::Debug::fmt(o, f),
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: ?Sized> fmt::Display for Cow<'_, B>
    where
        B: fmt::Display + ToOwned<Owned: fmt::Display>,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                Borrowed(ref b) => fmt::Display::fmt(b, f),
                Owned(ref o) => fmt::Display::fmt(o, f),
            }
        }
    }

    #[stable(feature = "default", since = "1.11.0")]
    impl<B: ?Sized> Default for Cow<'_, B>
    where
        B: ToOwned<Owned: Default>,
    {
        /// Creates an owned Cow<'a, B> with the default value for the contained owned value.
        fn default() -> Self {
            Owned(<B as ToOwned>::Owned::default())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: ?Sized> Hash for Cow<'_, B>
    where
        B: Hash + ToOwned,
    {
        #[inline]
        fn hash<H: Hasher>(&self, state: &mut H) {
            Hash::hash(&**self, state)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
        fn as_ref(&self) -> &T {
            self
        }
    }

    #[stable(feature = "cow_add", since = "1.14.0")]
    impl<'a> Add<&'a str> for Cow<'a, str> {
        type Output = Cow<'a, str>;

        #[inline]
        fn add(mut self, rhs: &'a str) -> Self::Output {
            self += rhs;
            self
        }
    }

    #[stable(feature = "cow_add", since = "1.14.0")]
    impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
        type Output = Cow<'a, str>;

        #[inline]
        fn add(mut self, rhs: Cow<'a, str>) -> Self::Output {
            self += rhs;
            self
        }
    }

    #[stable(feature = "cow_add", since = "1.14.0")]
    impl<'a> AddAssign<&'a str> for Cow<'a, str> {
        fn add_assign(&mut self, rhs: &'a str) {
            if self.is_empty() {
                *self = Cow::Borrowed(rhs)
            } else if !rhs.is_empty() {
                if let Cow::Borrowed(lhs) = *self {
                    let mut s = String::with_capacity(lhs.len() + rhs.len());
                    s.push_str(lhs);
                    *self = Cow::Owned(s);
                }
                self.to_mut().push_str(rhs);
            }
        }
    }

    #[stable(feature = "cow_add", since = "1.14.0")]
    impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
        fn add_assign(&mut self, rhs: Cow<'a, str>) {
            if self.is_empty() {
                *self = rhs
            } else if !rhs.is_empty() {
                if let Cow::Borrowed(lhs) = *self {
                    let mut s = String::with_capacity(lhs.len() + rhs.len());
                    s.push_str(lhs);
                    *self = Cow::Owned(s);
                }
                self.to_mut().push_str(&rhs);
            }
        }
    }