borrow.rs.html -- source
Description: Source to the Rust file `src/libcore/borrow.rs`.
Keywords: rust, rustlang, rust-lang

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    //! A module for working with borrowed data.

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

    /// A trait for borrowing data.
    ///
    /// In Rust, it is common to provide different representations of a type for
    /// different use cases. For instance, storage location and management for a
    /// value can be specifically chosen as appropriate for a particular use via
    /// pointer types such as [`Box<T>`] or [`Rc<T>`]. Beyond these generic
    /// wrappers that can be used with any type, some types provide optional
    /// facets providing potentially costly functionality. An example for such a
    /// type is [`String`] which adds the ability to extend a string to the basic
    /// [`str`]. This requires keeping additional information unnecessary for a
    /// simple, immutable string.
    ///
    /// These types provide access to the underlying data through references
    /// to the type of that data. They are said to be ‘borrowed as’ that type.
    /// For instance, a [`Box<T>`] can be borrowed as `T` while a [`String`]
    /// can be borrowed as `str`.
    ///
    /// Types express that they can be borrowed as some type `T` by implementing
    /// `Borrow<T>`, providing a reference to a `T` in the trait’s
    /// [`borrow`] method. A type is free to borrow as several different types.
    /// If it wishes to mutably borrow as the type – allowing the underlying data
    /// to be modified, it can additionally implement [`BorrowMut<T>`].
    ///
    /// Further, when providing implementations for additional traits, it needs
    /// to be considered whether they should behave identical to those of the
    /// underlying type as a consequence of acting as a representation of that
    /// underlying type. Generic code typically uses `Borrow<T>` when it relies
    /// on the identical behavior of these additional trait implementations.
    /// These traits will likely appear as additional trait bounds.
    ///
    /// If generic code merely needs to work for all types that can
    /// provide a reference to related type `T`, it is often better to use
    /// [`AsRef<T>`] as more types can safely implement it.
    ///
    /// [`AsRef<T>`]: ../../std/convert/trait.AsRef.html
    /// [`BorrowMut<T>`]: trait.BorrowMut.html
    /// [`Box<T>`]: ../../std/boxed/struct.Box.html
    /// [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
    /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
    /// [`str`]: ../../std/primitive.str.html
    /// [`String`]: ../../std/string/struct.String.html
    /// [`borrow`]: #tymethod.borrow
    ///
    /// # Examples
    ///
    /// As a data collection, [`HashMap<K, V>`] owns both keys and values. If
    /// the key’s actual data is wrapped in a managing type of some kind, it
    /// should, however, still be possible to search for a value using a
    /// reference to the key’s data. For instance, if the key is a string, then
    /// it is likely stored with the hash map as a [`String`], while it should
    /// be possible to search using a [`&str`][`str`]. Thus, `insert` needs to
    /// operate on a `String` while `get` needs to be able to use a `&str`.
    ///
    /// Slightly simplified, the relevant parts of `HashMap<K, V>` look like
    /// this:
    ///
    /// ```
    /// use std::borrow::Borrow;
    /// use std::hash::Hash;
    ///
    /// pub struct HashMap<K, V> {
    ///     # marker: ::std::marker::PhantomData<(K, V)>,
    ///     // fields omitted
    /// }
    ///
    /// impl<K, V> HashMap<K, V> {
    ///     pub fn insert(&self, key: K, value: V) -> Option<V>
    ///     where K: Hash + Eq
    ///     {
    ///         # unimplemented!()
    ///         // ...
    ///     }
    ///
    ///     pub fn get<Q>(&self, k: &Q) -> Option<&V>
    ///     where
    ///         K: Borrow<Q>,
    ///         Q: Hash + Eq + ?Sized
    ///     {
    ///         # unimplemented!()
    ///         // ...
    ///     }
    /// }
    /// ```
    ///
    /// The entire hash map is generic over a key type `K`. Because these keys
    /// are stored with the hash map, this type has to own the key’s data.
    /// When inserting a key-value pair, the map is given such a `K` and needs
    /// to find the correct hash bucket and check if the key is already present
    /// based on that `K`. It therefore requires `K: Hash + Eq`.
    ///
    /// When searching for a value in the map, however, having to provide a
    /// reference to a `K` as the key to search for would require to always
    /// create such an owned value. For string keys, this would mean a `String`
    /// value needs to be created just for the search for cases where only a
    /// `str` is available.
    ///
    /// Instead, the `get` method is generic over the type of the underlying key
    /// data, called `Q` in the method signature above. It states that `K`
    /// borrows as a `Q` by requiring that `K: Borrow<Q>`. By additionally
    /// requiring `Q: Hash + Eq`, it signals the requirement that `K` and `Q`
    /// have implementations of the `Hash` and `Eq` traits that produce identical
    /// results.
    ///
    /// The implementation of `get` relies in particular on identical
    /// implementations of `Hash` by determining the key’s hash bucket by calling
    /// `Hash::hash` on the `Q` value even though it inserted the key based on
    /// the hash value calculated from the `K` value.
    ///
    /// As a consequence, the hash map breaks if a `K` wrapping a `Q` value
    /// produces a different hash than `Q`. For instance, imagine you have a
    /// type that wraps a string but compares ASCII letters ignoring their case:
    ///
    /// ```
    /// pub struct CaseInsensitiveString(String);
    ///
    /// impl PartialEq for CaseInsensitiveString {
    ///     fn eq(&self, other: &Self) -> bool {
    ///         self.0.eq_ignore_ascii_case(&other.0)
    ///     }
    /// }
    ///
    /// impl Eq for CaseInsensitiveString { }
    /// ```
    ///
    /// Because two equal values need to produce the same hash value, the
    /// implementation of `Hash` needs to ignore ASCII case, too:
    ///
    /// ```
    /// # use std::hash::{Hash, Hasher};
    /// # pub struct CaseInsensitiveString(String);
    /// impl Hash for CaseInsensitiveString {
    ///     fn hash<H: Hasher>(&self, state: &mut H) {
    ///         for c in self.0.as_bytes() {
    ///             c.to_ascii_lowercase().hash(state)
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// Can `CaseInsensitiveString` implement `Borrow<str>`? It certainly can
    /// provide a reference to a string slice via its contained owned string.
    /// But because its `Hash` implementation differs, it behaves differently
    /// from `str` and therefore must not, in fact, implement `Borrow<str>`.
    /// If it wants to allow others access to the underlying `str`, it can do
    /// that via `AsRef<str>` which doesn’t carry any extra requirements.
    ///
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    /// [`HashMap<K, V>`]: ../../std/collections/struct.HashMap.html
    /// [`String`]: ../../std/string/struct.String.html
    /// [`str`]: ../../std/primitive.str.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Borrow<Borrowed: ?Sized> {
        /// Immutably borrows from an owned value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::borrow::Borrow;
        ///
        /// fn check<T: Borrow<str>>(s: T) {
        ///     assert_eq!("Hello", s.borrow());
        /// }
        ///
        /// let s = "Hello".to_string();
        ///
        /// check(s);
        ///
        /// let s = "Hello";
        ///
        /// check(s);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn borrow(&self) -> &Borrowed;
    }

    /// A trait for mutably borrowing data.
    ///
    /// As a companion to [`Borrow<T>`] this trait allows a type to borrow as
    /// an underlying type by providing a mutable reference. See [`Borrow<T>`]
    /// for more information on borrowing as another type.
    ///
    /// [`Borrow<T>`]: trait.Borrow.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait BorrowMut<Borrowed: ?Sized> : Borrow<Borrowed> {
        /// Mutably borrows from an owned value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::borrow::BorrowMut;
        ///
        /// fn check<T: BorrowMut<[i32]>>(mut v: T) {
        ///     assert_eq!(&mut [1, 2, 3], v.borrow_mut());
        /// }
        ///
        /// let v = vec![1, 2, 3];
        ///
        /// check(v);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn borrow_mut(&mut self) -> &mut Borrowed;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Borrow<T> for T {
        fn borrow(&self) -> &T { self }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> BorrowMut<T> for T {
        fn borrow_mut(&mut self) -> &mut T { self }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Borrow<T> for &T {
        fn borrow(&self) -> &T { &**self }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Borrow<T> for &mut T {
        fn borrow(&self) -> &T { &**self }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> BorrowMut<T> for &mut T {
        fn borrow_mut(&mut self) -> &mut T { &mut **self }
    }

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