core/borrow.rs
1//! Utilities for working with borrowed data.
2
3#![stable(feature = "rust1", since = "1.0.0")]
4
5/// A trait for borrowing data.
6///
7/// In Rust, it is common to provide different representations of a type for
8/// different use cases. For instance, storage location and management for a
9/// value can be specifically chosen as appropriate for a particular use via
10/// pointer types such as [`Box<T>`] or [`Rc<T>`]. Beyond these generic
11/// wrappers that can be used with any type, some types provide optional
12/// facets providing potentially costly functionality. An example for such a
13/// type is [`String`] which adds the ability to extend a string to the basic
14/// [`str`]. This requires keeping additional information unnecessary for a
15/// simple, immutable string.
16///
17/// These types provide access to the underlying data through references
18/// to the type of that data. They are said to be ‘borrowed as’ that type.
19/// For instance, a [`Box<T>`] can be borrowed as `T` while a [`String`]
20/// can be borrowed as `str`.
21///
22/// Types express that they can be borrowed as some type `T` by implementing
23/// `Borrow<T>`, providing a reference to a `T` in the trait’s
24/// [`borrow`] method. A type is free to borrow as several different types.
25/// If it wishes to mutably borrow as the type, allowing the underlying data
26/// to be modified, it can additionally implement [`BorrowMut<T>`].
27///
28/// Further, when providing implementations for additional traits, it needs
29/// to be considered whether they should behave identically to those of the
30/// underlying type as a consequence of acting as a representation of that
31/// underlying type. Generic code typically uses `Borrow<T>` when it relies
32/// on the identical behavior of these additional trait implementations.
33/// These traits will likely appear as additional trait bounds.
34///
35/// In particular `Eq`, `Ord` and `Hash` must be equivalent for
36/// borrowed and owned values: `x.borrow() == y.borrow()` should give the
37/// same result as `x == y`.
38///
39/// If generic code merely needs to work for all types that can
40/// provide a reference to related type `T`, it is often better to use
41/// [`AsRef<T>`] as more types can safely implement it.
42///
43/// [`Box<T>`]: ../../std/boxed/struct.Box.html
44/// [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
45/// [`Rc<T>`]: ../../std/rc/struct.Rc.html
46/// [`String`]: ../../std/string/struct.String.html
47/// [`borrow`]: Borrow::borrow
48///
49/// # Examples
50///
51/// As a data collection, [`HashMap<K, V>`] owns both keys and values. If
52/// the key’s actual data is wrapped in a managing type of some kind, it
53/// should, however, still be possible to search for a value using a
54/// reference to the key’s data. For instance, if the key is a string, then
55/// it is likely stored with the hash map as a [`String`], while it should
56/// be possible to search using a [`&str`][`str`]. Thus, `insert` needs to
57/// operate on a `String` while `get` needs to be able to use a `&str`.
58///
59/// Slightly simplified, the relevant parts of `HashMap<K, V>` look like
60/// this:
61///
62/// ```
63/// use std::borrow::Borrow;
64/// use std::hash::Hash;
65///
66/// pub struct HashMap<K, V> {
67/// # marker: ::std::marker::PhantomData<(K, V)>,
68/// // fields omitted
69/// }
70///
71/// impl<K, V> HashMap<K, V> {
72/// pub fn insert(&self, key: K, value: V) -> Option<V>
73/// where K: Hash + Eq
74/// {
75/// # unimplemented!()
76/// // ...
77/// }
78///
79/// pub fn get<Q>(&self, k: &Q) -> Option<&V>
80/// where
81/// K: Borrow<Q>,
82/// Q: Hash + Eq + ?Sized
83/// {
84/// # unimplemented!()
85/// // ...
86/// }
87/// }
88/// ```
89///
90/// The entire hash map is generic over a key type `K`. Because these keys
91/// are stored with the hash map, this type has to own the key’s data.
92/// When inserting a key-value pair, the map is given such a `K` and needs
93/// to find the correct hash bucket and check if the key is already present
94/// based on that `K`. It therefore requires `K: Hash + Eq`.
95///
96/// When searching for a value in the map, however, having to provide a
97/// reference to a `K` as the key to search for would require to always
98/// create such an owned value. For string keys, this would mean a `String`
99/// value needs to be created just for the search for cases where only a
100/// `str` is available.
101///
102/// Instead, the `get` method is generic over the type of the underlying key
103/// data, called `Q` in the method signature above. It states that `K`
104/// borrows as a `Q` by requiring that `K: Borrow<Q>`. By additionally
105/// requiring `Q: Hash + Eq`, it signals the requirement that `K` and `Q`
106/// have implementations of the `Hash` and `Eq` traits that produce identical
107/// results.
108///
109/// The implementation of `get` relies in particular on identical
110/// implementations of `Hash` by determining the key’s hash bucket by calling
111/// `Hash::hash` on the `Q` value even though it inserted the key based on
112/// the hash value calculated from the `K` value.
113///
114/// As a consequence, the hash map breaks if a `K` wrapping a `Q` value
115/// produces a different hash than `Q`. For instance, imagine you have a
116/// type that wraps a string but compares ASCII letters ignoring their case:
117///
118/// ```
119/// pub struct CaseInsensitiveString(String);
120///
121/// impl PartialEq for CaseInsensitiveString {
122/// fn eq(&self, other: &Self) -> bool {
123/// self.0.eq_ignore_ascii_case(&other.0)
124/// }
125/// }
126///
127/// impl Eq for CaseInsensitiveString { }
128/// ```
129///
130/// Because two equal values need to produce the same hash value, the
131/// implementation of `Hash` needs to ignore ASCII case, too:
132///
133/// ```
134/// # use std::hash::{Hash, Hasher};
135/// # pub struct CaseInsensitiveString(String);
136/// impl Hash for CaseInsensitiveString {
137/// fn hash<H: Hasher>(&self, state: &mut H) {
138/// for c in self.0.as_bytes() {
139/// c.to_ascii_lowercase().hash(state)
140/// }
141/// }
142/// }
143/// ```
144///
145/// Can `CaseInsensitiveString` implement `Borrow<str>`? It certainly can
146/// provide a reference to a string slice via its contained owned string.
147/// But because its `Hash` implementation differs, it behaves differently
148/// from `str` and therefore must not, in fact, implement `Borrow<str>`.
149/// If it wants to allow others access to the underlying `str`, it can do
150/// that via `AsRef<str>` which doesn’t carry any extra requirements.
151///
152/// [`Hash`]: crate::hash::Hash
153/// [`HashMap<K, V>`]: ../../std/collections/struct.HashMap.html
154/// [`String`]: ../../std/string/struct.String.html
155#[stable(feature = "rust1", since = "1.0.0")]
156#[rustc_diagnostic_item = "Borrow"]
157pub trait Borrow<Borrowed: ?Sized> {
158 /// Immutably borrows from an owned value.
159 ///
160 /// # Examples
161 ///
162 /// ```
163 /// use std::borrow::Borrow;
164 ///
165 /// fn check<T: Borrow<str>>(s: T) {
166 /// assert_eq!("Hello", s.borrow());
167 /// }
168 ///
169 /// let s = "Hello".to_string();
170 ///
171 /// check(s);
172 ///
173 /// let s = "Hello";
174 ///
175 /// check(s);
176 /// ```
177 #[stable(feature = "rust1", since = "1.0.0")]
178 fn borrow(&self) -> &Borrowed;
179}
180
181/// A trait for mutably borrowing data.
182///
183/// As a companion to [`Borrow<T>`] this trait allows a type to borrow as
184/// an underlying type by providing a mutable reference. See [`Borrow<T>`]
185/// for more information on borrowing as another type.
186#[stable(feature = "rust1", since = "1.0.0")]
187#[rustc_diagnostic_item = "BorrowMut"]
188pub trait BorrowMut<Borrowed: ?Sized>: Borrow<Borrowed> {
189 /// Mutably borrows from an owned value.
190 ///
191 /// # Examples
192 ///
193 /// ```
194 /// use std::borrow::BorrowMut;
195 ///
196 /// fn check<T: BorrowMut<[i32]>>(mut v: T) {
197 /// assert_eq!(&mut [1, 2, 3], v.borrow_mut());
198 /// }
199 ///
200 /// let v = vec![1, 2, 3];
201 ///
202 /// check(v);
203 /// ```
204 #[stable(feature = "rust1", since = "1.0.0")]
205 fn borrow_mut(&mut self) -> &mut Borrowed;
206}
207
208#[stable(feature = "rust1", since = "1.0.0")]
209impl<T: ?Sized> Borrow<T> for T {
210 #[rustc_diagnostic_item = "noop_method_borrow"]
211 fn borrow(&self) -> &T {
212 self
213 }
214}
215
216#[stable(feature = "rust1", since = "1.0.0")]
217impl<T: ?Sized> BorrowMut<T> for T {
218 fn borrow_mut(&mut self) -> &mut T {
219 self
220 }
221}
222
223#[stable(feature = "rust1", since = "1.0.0")]
224impl<T: ?Sized> Borrow<T> for &T {
225 fn borrow(&self) -> &T {
226 &**self
227 }
228}
229
230#[stable(feature = "rust1", since = "1.0.0")]
231impl<T: ?Sized> Borrow<T> for &mut T {
232 fn borrow(&self) -> &T {
233 &**self
234 }
235}
236
237#[stable(feature = "rust1", since = "1.0.0")]
238impl<T: ?Sized> BorrowMut<T> for &mut T {
239 fn borrow_mut(&mut self) -> &mut T {
240 &mut **self
241 }
242}