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</pre><pre class="rust ">
<span class="comment">// Copyright 2014 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="doccomment">//! A module for working with borrowed data.</span>
<span class="attribute">#![<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="doccomment">/// A trait for borrowing data.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// In Rust, it is common to provide different representations of a type for</span>
<span class="doccomment">/// different use cases. For instance, storage location and management for a</span>
<span class="doccomment">/// value can be specifically chosen as appropriate for a particular use via</span>
<span class="doccomment">/// pointer types such as [`Box<T>`] or [`Rc<T>`]. Beyond these generic</span>
<span class="doccomment">/// wrappers that can be used with any type, some types provide optional</span>
<span class="doccomment">/// facets providing potentially costly functionality. An example for such a</span>
<span class="doccomment">/// type is [`String`] which adds the ability to extend a string to the basic</span>
<span class="doccomment">/// [`str`]. This requires keeping additional information unnecessary for a</span>
<span class="doccomment">/// simple, immutable string.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// These types provide access to the underlying data through references</span>
<span class="doccomment">/// to the type of that data. They are said to be ‘borrowed as’ that type.</span>
<span class="doccomment">/// For instance, a [`Box<T>`] can be borrowed as `T` while a [`String`]</span>
<span class="doccomment">/// can be borrowed as `str`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Types express that they can be borrowed as some type `T` by implementing</span>
<span class="doccomment">/// `Borrow<T>`, providing a reference to a `T` in the trait’s</span>
<span class="doccomment">/// [`borrow`] method. A type is free to borrow as several different types.</span>
<span class="doccomment">/// If it wishes to mutably borrow as the type – allowing the underlying data</span>
<span class="doccomment">/// to be modified, it can additionally implement [`BorrowMut<T>`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Further, when providing implementations for additional traits, it needs</span>
<span class="doccomment">/// to be considered whether they should behave identical to those of the</span>
<span class="doccomment">/// underlying type as a consequence of acting as a representation of that</span>
<span class="doccomment">/// underlying type. Generic code typically uses `Borrow<T>` when it relies</span>
<span class="doccomment">/// on the identical behavior of these additional trait implementations.</span>
<span class="doccomment">/// These traits will likely appear as additional trait bounds.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If generic code merely needs to work for all types that can</span>
<span class="doccomment">/// provide a reference to related type `T`, it is often better to use</span>
<span class="doccomment">/// [`AsRef<T>`] as more types can safely implement it.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`AsRef<T>`]: ../../std/convert/trait.AsRef.html</span>
<span class="doccomment">/// [`BorrowMut<T>`]: trait.BorrowMut.html</span>
<span class="doccomment">/// [`Box<T>`]: ../../std/boxed/struct.Box.html</span>
<span class="doccomment">/// [`Mutex<T>`]: ../../std/sync/struct.Mutex.html</span>
<span class="doccomment">/// [`Rc<T>`]: ../../std/rc/struct.Rc.html</span>
<span class="doccomment">/// [`str`]: ../../std/primitive.str.html</span>
<span class="doccomment">/// [`String`]: ../../std/string/struct.String.html</span>
<span class="doccomment">/// [`borrow`]: #tymethod.borrow</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// As a data collection, [`HashMap<K, V>`] owns both keys and values. If</span>
<span class="doccomment">/// the key’s actual data is wrapped in a managing type of some kind, it</span>
<span class="doccomment">/// should, however, still be possible to search for a value using a</span>
<span class="doccomment">/// reference to the key’s data. For instance, if the key is a string, then</span>
<span class="doccomment">/// it is likely stored with the hash map as a [`String`], while it should</span>
<span class="doccomment">/// be possible to search using a [`&str`][`str`]. Thus, `insert` needs to</span>
<span class="doccomment">/// operate on a `String` while `get` needs to be able to use a `&str`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Slightly simplified, the relevant parts of `HashMap<K, V>` look like</span>
<span class="doccomment">/// this:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::borrow::Borrow;</span>
<span class="doccomment">/// use std::hash::Hash;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// pub struct HashMap<K, V> {</span>
<span class="doccomment">/// # marker: ::std::marker::PhantomData<(K, V)>,</span>
<span class="doccomment">/// // fields omitted</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// impl<K, V> HashMap<K, V> {</span>
<span class="doccomment">/// pub fn insert(&self, key: K, value: V) -> Option<V></span>
<span class="doccomment">/// where K: Hash + Eq</span>
<span class="doccomment">/// {</span>
<span class="doccomment">/// # unimplemented!()</span>
<span class="doccomment">/// // ...</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// pub fn get<Q>(&self, k: &Q) -> Option<&V></span>
<span class="doccomment">/// where</span>
<span class="doccomment">/// K: Borrow<Q>,</span>
<span class="doccomment">/// Q: Hash + Eq + ?Sized</span>
<span class="doccomment">/// {</span>
<span class="doccomment">/// # unimplemented!()</span>
<span class="doccomment">/// // ...</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The entire hash map is generic over a key type `K`. Because these keys</span>
<span class="doccomment">/// are stored with the hash map, this type has to own the key’s data.</span>
<span class="doccomment">/// When inserting a key-value pair, the map is given such a `K` and needs</span>
<span class="doccomment">/// to find the correct hash bucket and check if the key is already present</span>
<span class="doccomment">/// based on that `K`. It therefore requires `K: Hash + Eq`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// When searching for a value in the map, however, having to provide a</span>
<span class="doccomment">/// reference to a `K` as the key to search for would require to always</span>
<span class="doccomment">/// create such an owned value. For string keys, this would mean a `String`</span>
<span class="doccomment">/// value needs to be created just for the search for cases where only a</span>
<span class="doccomment">/// `str` is available.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Instead, the `get` method is generic over the type of the underlying key</span>
<span class="doccomment">/// data, called `Q` in the method signature above. It states that `K`</span>
<span class="doccomment">/// borrows as a `Q` by requiring that `K: Borrow<Q>`. By additionally</span>
<span class="doccomment">/// requiring `Q: Hash + Eq`, it signals the requirement that `K` and `Q`</span>
<span class="doccomment">/// have implementations of the `Hash` and `Eq` traits that produce identical</span>
<span class="doccomment">/// results.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The implementation of `get` relies in particular on identical</span>
<span class="doccomment">/// implementations of `Hash` by determining the key’s hash bucket by calling</span>
<span class="doccomment">/// `Hash::hash` on the `Q` value even though it inserted the key based on</span>
<span class="doccomment">/// the hash value calculated from the `K` value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// As a consequence, the hash map breaks if a `K` wrapping a `Q` value</span>
<span class="doccomment">/// produces a different hash than `Q`. For instance, imagine you have a</span>
<span class="doccomment">/// type that wraps a string but compares ASCII letters ignoring their case:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// pub struct CaseInsensitiveString(String);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// impl PartialEq for CaseInsensitiveString {</span>
<span class="doccomment">/// fn eq(&self, other: &Self) -> bool {</span>
<span class="doccomment">/// self.0.eq_ignore_ascii_case(&other.0)</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// impl Eq for CaseInsensitiveString { }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Because two equal values need to produce the same hash value, the</span>
<span class="doccomment">/// implementation of `Hash` needs to ignore ASCII case, too:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # use std::hash::{Hash, Hasher};</span>
<span class="doccomment">/// # pub struct CaseInsensitiveString(String);</span>
<span class="doccomment">/// impl Hash for CaseInsensitiveString {</span>
<span class="doccomment">/// fn hash<H: Hasher>(&self, state: &mut H) {</span>
<span class="doccomment">/// for c in self.0.as_bytes() {</span>
<span class="doccomment">/// c.to_ascii_lowercase().hash(state)</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Can `CaseInsensitiveString` implement `Borrow<str>`? It certainly can</span>
<span class="doccomment">/// provide a reference to a string slice via its contained owned string.</span>
<span class="doccomment">/// But because its `Hash` implementation differs, it behaves differently</span>
<span class="doccomment">/// from `str` and therefore must not, in fact, implement `Borrow<str>`.</span>
<span class="doccomment">/// If it wants to allow others access to the underlying `str`, it can do</span>
<span class="doccomment">/// that via `AsRef<str>` which doesn’t carry any extra requirements.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Hash`]: ../../std/hash/trait.Hash.html</span>
<span class="doccomment">/// [`HashMap<K, V>`]: ../../std/collections/struct.HashMap.html</span>
<span class="doccomment">/// [`String`]: ../../std/string/struct.String.html</span>
<span class="doccomment">/// [`str`]: ../../std/primitive.str.html</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Borrow</span><span class="op"><</span><span class="ident">Borrowed</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> {
<span class="doccomment">/// Immutably borrows from an owned value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::borrow::Borrow;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn check<T: Borrow<str>>(s: T) {</span>
<span class="doccomment">/// assert_eq!("Hello", s.borrow());</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let s = "Hello".to_string();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// check(s);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let s = "Hello";</span>
<span class="doccomment">///</span>
<span class="doccomment">/// check(s);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">fn</span> <span class="ident">borrow</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">Borrowed</span>;
}
<span class="doccomment">/// A trait for mutably borrowing data.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// As a companion to [`Borrow<T>`] this trait allows a type to borrow as</span>
<span class="doccomment">/// an underlying type by providing a mutable reference. See [`Borrow<T>`]</span>
<span class="doccomment">/// for more information on borrowing as another type.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Borrow<T>`]: trait.Borrow.html</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">BorrowMut</span><span class="op"><</span><span class="ident">Borrowed</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> : <span class="ident">Borrow</span><span class="op"><</span><span class="ident">Borrowed</span><span class="op">></span> {
<span class="doccomment">/// Mutably borrows from an owned value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::borrow::BorrowMut;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn check<T: BorrowMut<[i32]>>(mut v: T) {</span>
<span class="doccomment">/// assert_eq!(&mut [1, 2, 3], v.borrow_mut());</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let v = vec![1, 2, 3];</span>
<span class="doccomment">///</span>
<span class="doccomment">/// check(v);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">fn</span> <span class="ident">borrow_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">Borrowed</span>;
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Borrow</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="kw">for</span> <span class="ident">T</span> {
<span class="kw">fn</span> <span class="ident">borrow</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">T</span> { <span class="self">self</span> }
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">BorrowMut</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="kw">for</span> <span class="ident">T</span> {
<span class="kw">fn</span> <span class="ident">borrow_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span> { <span class="self">self</span> }
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Borrow</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">T</span> {
<span class="kw">fn</span> <span class="ident">borrow</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">T</span> { <span class="kw-2">&</span><span class="kw-2">*</span><span class="kw-2">*</span><span class="self">self</span> }
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Borrow</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">T</span> {
<span class="kw">fn</span> <span class="ident">borrow</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">T</span> { <span class="kw-2">&</span><span class="kw-2">*</span><span class="kw-2">*</span><span class="self">self</span> }
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">BorrowMut</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">T</span> {
<span class="kw">fn</span> <span class="ident">borrow_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span> { <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="kw-2">*</span><span class="kw-2">*</span><span class="self">self</span> }
}
</pre>
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