// SPDX-License-Identifier: Apache-2.0 OR MIT //! 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::Deref;#[cfg(not(no_global_oom_handling))]use core::ops::{Add, AddAssign}; #[stable(feature = "rust1", since = "1.0.0")]pub use core::borrow::{Borrow, BorrowMut}; use core::fmt;#[cfg(not(no_global_oom_handling))]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.#[cfg_attr(not(test), rustc_diagnostic_item = "ToOwned")]#[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 Twhere    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.////// If you need reference-counting pointers, note that/// [`Rc::make_mut`][crate::rc::Rc::make_mut] and/// [`Arc::make_mut`][crate::sync::Arc::make_mut] can provide clone-on-write/// functionality as well.////// # 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's 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")]#[cfg_attr(not(test), rustc_diagnostic_item = "Cow")]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")]    #[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]    pub const 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")]    #[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]    pub const 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` returns a clone of the borrowed data:    ///    /// ```    /// 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` returns the owned data. The data is moved out of the    /// `Cow` without being cloned.    ///    /// ```    /// 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")]#[rustc_const_unstable(feature = "const_deref", issue = "88955")]impl<B: ?Sized + ToOwned> const Deref for Cow<'_, B>where    B::Owned: ~const Borrow<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    }} #[cfg(not(no_global_oom_handling))]#[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    }} #[cfg(not(no_global_oom_handling))]#[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    }} #[cfg(not(no_global_oom_handling))]#[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);        }    }} #[cfg(not(no_global_oom_handling))]#[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);        }    }}