core/

borrow.rs

//! Utilities 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 identically 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. /// /// In particular `Eq`, `Ord` and `Hash` must be equivalent for /// borrowed and owned values: `x.borrow() == y.borrow()` should give the /// same result as `x == y`. /// /// 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. /// /// [`Box<T>`]: ../../std/boxed/struct.Box.html /// [`Mutex<T>`]: ../../std/sync/struct.Mutex.html /// [`Rc<T>`]: ../../std/rc/struct.Rc.html /// [`String`]: ../../std/string/struct.String.html /// [`borrow`]: Borrow::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`]: crate::hash::Hash /// [`HashMap<K, V>`]: ../../std/collections/struct.HashMap.html /// [`String`]: ../../std/string/struct.String.html #[stable(feature = "rust1", since = "1.0.0")] #[rustc_diagnostic_item = "Borrow"] 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. #[stable(feature = "rust1", since = "1.0.0")] #[rustc_diagnostic_item = "BorrowMut"] 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 {     #[rustc_diagnostic_item = "noop_method_borrow"]     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 {         self     } }