249 lines
8.9 KiB
Rust
249 lines
8.9 KiB
Rust
//! Utilities for working with borrowed data.
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#![stable(feature = "rust1", since = "1.0.0")]
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/// A trait for borrowing data.
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///
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/// In Rust, it is common to provide different representations of a type for
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/// different use cases. For instance, storage location and management for a
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/// value can be specifically chosen as appropriate for a particular use via
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/// pointer types such as [`Box<T>`] or [`Rc<T>`]. Beyond these generic
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/// wrappers that can be used with any type, some types provide optional
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/// facets providing potentially costly functionality. An example for such a
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/// type is [`String`] which adds the ability to extend a string to the basic
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/// [`str`]. This requires keeping additional information unnecessary for a
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/// simple, immutable string.
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///
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/// These types provide access to the underlying data through references
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/// to the type of that data. They are said to be ‘borrowed as’ that type.
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/// For instance, a [`Box<T>`] can be borrowed as `T` while a [`String`]
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/// can be borrowed as `str`.
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///
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/// Types express that they can be borrowed as some type `T` by implementing
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/// `Borrow<T>`, providing a reference to a `T` in the trait’s
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/// [`borrow`] method. A type is free to borrow as several different types.
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/// If it wishes to mutably borrow as the type – allowing the underlying data
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/// to be modified, it can additionally implement [`BorrowMut<T>`].
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///
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/// Further, when providing implementations for additional traits, it needs
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/// to be considered whether they should behave identically to those of the
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/// underlying type as a consequence of acting as a representation of that
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/// underlying type. Generic code typically uses `Borrow<T>` when it relies
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/// on the identical behavior of these additional trait implementations.
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/// These traits will likely appear as additional trait bounds.
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///
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/// In particular `Eq`, `Ord` and `Hash` must be equivalent for
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/// borrowed and owned values: `x.borrow() == y.borrow()` should give the
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/// same result as `x == y`.
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///
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/// If generic code merely needs to work for all types that can
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/// provide a reference to related type `T`, it is often better to use
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/// [`AsRef<T>`] as more types can safely implement it.
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///
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/// [`Box<T>`]: ../../std/boxed/struct.Box.html
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/// [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
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/// [`Rc<T>`]: ../../std/rc/struct.Rc.html
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/// [`String`]: ../../std/string/struct.String.html
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/// [`borrow`]: Borrow::borrow
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///
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/// # Examples
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///
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/// As a data collection, [`HashMap<K, V>`] owns both keys and values. If
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/// the key’s actual data is wrapped in a managing type of some kind, it
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/// should, however, still be possible to search for a value using a
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/// reference to the key’s data. For instance, if the key is a string, then
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/// it is likely stored with the hash map as a [`String`], while it should
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/// be possible to search using a [`&str`][`str`]. Thus, `insert` needs to
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/// operate on a `String` while `get` needs to be able to use a `&str`.
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///
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/// Slightly simplified, the relevant parts of `HashMap<K, V>` look like
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/// this:
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///
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/// ```
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/// use std::borrow::Borrow;
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/// use std::hash::Hash;
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///
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/// pub struct HashMap<K, V> {
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/// # marker: ::std::marker::PhantomData<(K, V)>,
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/// // fields omitted
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/// }
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///
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/// impl<K, V> HashMap<K, V> {
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/// pub fn insert(&self, key: K, value: V) -> Option<V>
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/// where K: Hash + Eq
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/// {
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/// # unimplemented!()
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/// // ...
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/// }
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///
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/// pub fn get<Q>(&self, k: &Q) -> Option<&V>
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/// where
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/// K: Borrow<Q>,
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/// Q: Hash + Eq + ?Sized
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/// {
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/// # unimplemented!()
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/// // ...
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/// }
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/// }
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/// ```
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///
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/// The entire hash map is generic over a key type `K`. Because these keys
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/// are stored with the hash map, this type has to own the key’s data.
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/// When inserting a key-value pair, the map is given such a `K` and needs
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/// to find the correct hash bucket and check if the key is already present
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/// based on that `K`. It therefore requires `K: Hash + Eq`.
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///
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/// When searching for a value in the map, however, having to provide a
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/// reference to a `K` as the key to search for would require to always
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/// create such an owned value. For string keys, this would mean a `String`
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/// value needs to be created just for the search for cases where only a
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/// `str` is available.
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///
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/// Instead, the `get` method is generic over the type of the underlying key
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/// data, called `Q` in the method signature above. It states that `K`
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/// borrows as a `Q` by requiring that `K: Borrow<Q>`. By additionally
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/// requiring `Q: Hash + Eq`, it signals the requirement that `K` and `Q`
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/// have implementations of the `Hash` and `Eq` traits that produce identical
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/// results.
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///
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/// The implementation of `get` relies in particular on identical
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/// implementations of `Hash` by determining the key’s hash bucket by calling
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/// `Hash::hash` on the `Q` value even though it inserted the key based on
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/// the hash value calculated from the `K` value.
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///
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/// As a consequence, the hash map breaks if a `K` wrapping a `Q` value
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/// produces a different hash than `Q`. For instance, imagine you have a
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/// type that wraps a string but compares ASCII letters ignoring their case:
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///
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/// ```
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/// pub struct CaseInsensitiveString(String);
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///
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/// impl PartialEq for CaseInsensitiveString {
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/// fn eq(&self, other: &Self) -> bool {
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/// self.0.eq_ignore_ascii_case(&other.0)
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/// }
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/// }
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///
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/// impl Eq for CaseInsensitiveString { }
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/// ```
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///
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/// Because two equal values need to produce the same hash value, the
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/// implementation of `Hash` needs to ignore ASCII case, too:
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///
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/// ```
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/// # use std::hash::{Hash, Hasher};
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/// # pub struct CaseInsensitiveString(String);
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/// impl Hash for CaseInsensitiveString {
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/// fn hash<H: Hasher>(&self, state: &mut H) {
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/// for c in self.0.as_bytes() {
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/// c.to_ascii_lowercase().hash(state)
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/// }
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/// }
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/// }
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/// ```
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///
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/// Can `CaseInsensitiveString` implement `Borrow<str>`? It certainly can
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/// provide a reference to a string slice via its contained owned string.
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/// But because its `Hash` implementation differs, it behaves differently
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/// from `str` and therefore must not, in fact, implement `Borrow<str>`.
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/// If it wants to allow others access to the underlying `str`, it can do
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/// that via `AsRef<str>` which doesn’t carry any extra requirements.
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///
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/// [`Hash`]: crate::hash::Hash
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/// [`HashMap<K, V>`]: ../../std/collections/struct.HashMap.html
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/// [`String`]: ../../std/string/struct.String.html
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#[stable(feature = "rust1", since = "1.0.0")]
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#[rustc_diagnostic_item = "Borrow"]
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#[const_trait]
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pub trait Borrow<Borrowed: ?Sized> {
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/// Immutably borrows from an owned value.
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///
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/// # Examples
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///
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/// ```
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/// use std::borrow::Borrow;
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///
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/// fn check<T: Borrow<str>>(s: T) {
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/// assert_eq!("Hello", s.borrow());
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/// }
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///
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/// let s = "Hello".to_string();
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///
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/// check(s);
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///
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/// let s = "Hello";
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///
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/// check(s);
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/// ```
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#[stable(feature = "rust1", since = "1.0.0")]
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fn borrow(&self) -> &Borrowed;
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}
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/// A trait for mutably borrowing data.
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///
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/// As a companion to [`Borrow<T>`] this trait allows a type to borrow as
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/// an underlying type by providing a mutable reference. See [`Borrow<T>`]
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/// for more information on borrowing as another type.
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#[stable(feature = "rust1", since = "1.0.0")]
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#[const_trait]
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pub trait BorrowMut<Borrowed: ?Sized>: Borrow<Borrowed> {
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/// Mutably borrows from an owned value.
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///
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/// # Examples
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///
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/// ```
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/// use std::borrow::BorrowMut;
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///
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/// fn check<T: BorrowMut<[i32]>>(mut v: T) {
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/// assert_eq!(&mut [1, 2, 3], v.borrow_mut());
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/// }
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///
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/// let v = vec![1, 2, 3];
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///
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/// check(v);
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/// ```
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#[stable(feature = "rust1", since = "1.0.0")]
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fn borrow_mut(&mut self) -> &mut Borrowed;
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}
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#[stable(feature = "rust1", since = "1.0.0")]
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#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
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impl<T: ?Sized> const Borrow<T> for T {
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#[rustc_diagnostic_item = "noop_method_borrow"]
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fn borrow(&self) -> &T {
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self
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}
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}
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#[stable(feature = "rust1", since = "1.0.0")]
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#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
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impl<T: ?Sized> const BorrowMut<T> for T {
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fn borrow_mut(&mut self) -> &mut T {
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self
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}
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}
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#[stable(feature = "rust1", since = "1.0.0")]
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#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
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impl<T: ?Sized> const Borrow<T> for &T {
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fn borrow(&self) -> &T {
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&**self
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}
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}
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#[stable(feature = "rust1", since = "1.0.0")]
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#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
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impl<T: ?Sized> const Borrow<T> for &mut T {
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fn borrow(&self) -> &T {
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&**self
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}
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}
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#[stable(feature = "rust1", since = "1.0.0")]
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#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
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impl<T: ?Sized> const BorrowMut<T> for &mut T {
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fn borrow_mut(&mut self) -> &mut T {
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&mut **self
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}
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}
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