mirror of
https://github.com/rust-lang/rust.git
synced 2024-12-01 19:23:50 +00:00
242 lines
8.5 KiB
Rust
242 lines
8.5 KiB
Rust
//! 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")]
|
||
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 {
|
||
&mut **self
|
||
}
|
||
}
|