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<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong aria-hidden="true">2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong aria-hidden="true">3.</strong> Syntax and Semantics</a></li><li><ol class="section"><li><a href="variable-bindings.html"><strong aria-hidden="true">3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong aria-hidden="true">3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong aria-hidden="true">3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="if.html"><strong aria-hidden="true">3.5.</strong> if</a></li><li><a href="loops.html"><strong aria-hidden="true">3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong aria-hidden="true">3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong aria-hidden="true">3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong aria-hidden="true">3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.10.</strong> Lifetimes</a></li><li><a href="mutability.html" class="active"><strong aria-hidden="true">3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong aria-hidden="true">3.12.</strong> Structs</a></li><li><a href="enums.html"><strong aria-hidden="true">3.13.</strong> Enums</a></li><li><a href="match.html"><strong aria-hidden="true">3.14.</strong> Match</a></li><li><a href="patterns.html"><strong aria-hidden="true">3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong aria-hidden="true">3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong aria-hidden="true">3.17.</strong> Strings</a></li><li><a href="generics.html"><strong aria-hidden="true">3.18.</strong> Generics</a></li><li><a href="traits.html"><strong aria-hidden="true">3.19.</strong> Traits</a></li><li><a href="drop.html"><strong aria-hidden="true">3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong aria-hidden="true">3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong aria-hidden="true">3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong aria-hidden="true">3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong aria-hidden="true">3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong aria-hidden="true">3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong aria-hidden="true">3.26.</strong> const and static</a></li><li><a href="attributes.html"><strong aria-hidden="true">3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong aria-hidden="true">3.28.</strong> type aliases</a></li><li><a href="casting-between-types.html"><strong aria-hidden="true">3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong aria-hidden="true">3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong aria-hidden="true">3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong aria-hidden="true">3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong aria-hidden="true">3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong aria-hidden="true">3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong aria-hidden="true">3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong aria-hidden="true">3.36.</strong> unsafe</a></li></ol></li><li><a href="effective-rust.html"><strong aria-hidden="true">4.</strong> Effective Rust</a></li><li><ol class="section"><li><a href="the-stack-and-the-heap.html"><strong aria-hidden="true">4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong aria-hidden="true">4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html"><strong aria-hidden="true">4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong aria-hidden="true">4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong aria-hidden="true">4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong aria-hidden="true">4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong aria-hidden="true">4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong aria-hidden="true">4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong aria-hidden="true">4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong aria-hidden="true">4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong aria-hidden="true">4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong aria-hidden="true">4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">4.13.</strong> Procedural Macros (and custom derive)</a></li></ol></li><li><a href="glossary.html"><strong aria-hidden="true">5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong aria-hidden="true">6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong aria-hidden="true">7.</strong> Bibliography</a></li></ol>
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<h1 class="menu-title">The Rust Programming Language</h1>
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<a class="header" href="mutability.html#mutability" id="mutability"><h1>Mutability</h1></a>
<p>Mutability, the ability to change something, works a bit differently in Rust
than in other languages. The first aspect of mutability is its non-default
status:</p>
<pre><code class="language-rust ignore">let x = 5;
x = 6; // Error!
</code></pre>
<p>We can introduce mutability with the <code>mut</code> keyword:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let mut x = 5;
x = 6; // No problem!
#}</code></pre></pre>
<p>This is a mutable <a href="variable-bindings.html">variable binding</a>. When a binding is mutable, it means
you’re allowed to change what the binding points to. So in the above example,
it’s not so much that the value at <code>x</code> is changing, but that the binding
changed from one <code>i32</code> to another.</p>
<p>You can also create a <a href="references-and-borrowing.html">reference</a> to it, using <code>&x</code>, but if you want to use the reference to change it, you will need a mutable reference:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let mut x = 5;
let y = &mut x;
#}</code></pre></pre>
<p><code>y</code> is an immutable binding to a mutable reference, which means that you can’t bind 'y' to something else (<code>y = &mut z</code>), but <code>y</code> can be used to bind <code>x</code> to something else (<code>*y = 5</code>). A subtle distinction.</p>
<p>Of course, if you need both:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let mut x = 5;
let mut y = &mut x;
#}</code></pre></pre>
<p>Now <code>y</code> can be bound to another value, and the value it’s referencing can be
changed.</p>
<p>It’s important to note that <code>mut</code> is part of a <a href="patterns.html">pattern</a>, so you
can do things like this:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let (mut x, y) = (5, 6);
fn foo(mut x: i32) {
# }
#}</code></pre></pre>
<p>Note that here, the <code>x</code> is mutable, but not the <code>y</code>.</p>
<a class="header" href="mutability.html#interior-vs-exterior-mutability" id="interior-vs-exterior-mutability"><h1>Interior vs. Exterior Mutability</h1></a>
<p>However, when we say something is ‘immutable’ in Rust, that doesn’t mean that
it’s not able to be changed: we are referring to its ‘exterior mutability’ that
in this case is immutable. Consider, for example, <a href="../../std/sync/struct.Arc.html"><code>Arc<T></code></a>:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
use std::sync::Arc;
let x = Arc::new(5);
let y = x.clone();
#}</code></pre></pre>
<p>When we call <code>clone()</code>, the <code>Arc<T></code> needs to update the reference count. Yet
we’ve not used any <code>mut</code>s here, <code>x</code> is an immutable binding, and we didn’t take
<code>&mut 5</code> or anything. So what gives?</p>
<p>To understand this, we have to go back to the core of Rust’s guiding
philosophy, memory safety, and the mechanism by which Rust guarantees it, the
<a href="ownership.html">ownership</a> system, and more specifically, <a href="references-and-borrowing.html#borrowing">borrowing</a>:</p>
<blockquote>
<p>You may have one or the other of these two kinds of borrows, but not both at
the same time:</p>
<ul>
<li>one or more references (<code>&T</code>) to a resource,</li>
<li>exactly one mutable reference (<code>&mut T</code>).</li>
</ul>
</blockquote>
<p>So, that’s the real definition of ‘immutability’: is this safe to have two
pointers to? In <code>Arc<T></code>’s case, yes: the mutation is entirely contained inside
the structure itself. It’s not user facing. For this reason, it hands out <code>&T</code>
with <code>clone()</code>. If it handed out <code>&mut T</code>s, though, that would be a problem.</p>
<p>Other types, like the ones in the <a href="../../std/cell/index.html"><code>std::cell</code></a> module, have the
opposite: interior mutability. For example:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
use std::cell::RefCell;
let x = RefCell::new(42);
let y = x.borrow_mut();
#}</code></pre></pre>
<p>RefCell hands out <code>&mut</code> references to what’s inside of it with the
<code>borrow_mut()</code> method. Isn’t that dangerous? What if we do:</p>
<pre><code class="language-rust ignore">use std::cell::RefCell;
let x = RefCell::new(42);
let y = x.borrow_mut();
let z = x.borrow_mut();
# (y, z);
</code></pre>
<p>This will in fact panic, at runtime. This is what <code>RefCell</code> does: it enforces
Rust’s borrowing rules at runtime, and <code>panic!</code>s if they’re violated. This
allows us to get around another aspect of Rust’s mutability rules. Let’s talk
about it first.</p>
<a class="header" href="mutability.html#field-level-mutability" id="field-level-mutability"><h2>Field-level mutability</h2></a>
<p>Mutability is a property of either a borrow (<code>&mut</code>) or a binding (<code>let mut</code>).
This means that, for example, you cannot have a <a href="structs.html"><code>struct</code></a> with
some fields mutable and some immutable:</p>
<pre><code class="language-rust ignore">struct Point {
x: i32,
mut y: i32, // Nope.
}
</code></pre>
<p>The mutability of a struct is in its binding:</p>
<pre><code class="language-rust ignore">struct Point {
x: i32,
y: i32,
}
let mut a = Point { x: 5, y: 6 };
a.x = 10;
let b = Point { x: 5, y: 6 };
b.x = 10; // Error: cannot assign to immutable field `b.x`.
</code></pre>
<p>However, by using <a href="../../std/cell/struct.Cell.html"><code>Cell<T></code></a>, you can emulate field-level mutability:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
use std::cell::Cell;
struct Point {
x: i32,
y: Cell<i32>,
}
let point = Point { x: 5, y: Cell::new(6) };
point.y.set(7);
println!("y: {:?}", point.y);
#}</code></pre></pre>
<p>This will print <code>y: Cell { value: 7 }</code>. We’ve successfully updated <code>y</code>.</p>
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