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<ol class="chapter"><li class="affix"><a href="title-page.html">The Rust Programming Language</a></li><li class="affix"><a href="foreword.html">Foreword</a></li><li class="affix"><a href="ch00-00-introduction.html">Introduction</a></li><li><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> Paths for Referring to an Item in the Module Tree</a></li><li><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> Bringing Paths Into Scope with the use Keyword</a></li><li><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> Separating Modules into Different Files</a></li></ol></li><li><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Storing UTF-8 Encoded Text with Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Storing Keys with Associated Values in Hash Maps</a></li></ol></li><li><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Using Box<T> to Point to Data on the Heap</a></li><li><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> Treating Smart Pointers Like Regular References with the Deref Trait</a></li><li><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> Running Code on Cleanup with the Drop Trait</a></li><li><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Reference Cycles Can Leak Memory</a></li></ol></li><li><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Using Message Passing to Transfer Data Between Threads</a></li><li><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency with the Sync and Send Traits</a></li></ol></li><li><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects That Allow for Values of Different Types</a></li><li><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns Can Be Used</a></li><li><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html" class="active"><strong aria-hidden="true">19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.5.</strong> Advanced Functions and Closures</a></li><li><a href="ch19-06-macros.html"><strong aria-hidden="true">19.6.</strong> Macros</a></li></ol></li><li><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> Building a Single-Threaded Web Server</a></li><li><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning Our Single-Threaded Server into a Multithreaded Server</a></li><li><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<a class="header" href="ch19-02-advanced-lifetimes.html#advanced-lifetimes" id="advanced-lifetimes"><h2>Advanced Lifetimes</h2></a>
<p>In Chapter 10 in the <a href="ch10-03-lifetime-syntax.html#validating-references-with-lifetimes">“Validating References with Lifetimes”</a><!-- ignore --> section, you learned how
to annotate references with lifetime parameters to tell Rust how lifetimes of
different references relate. You saw how every reference has a lifetime, but
most of the time, Rust will let you elide lifetimes. Now we’ll look at three
advanced features of lifetimes that we haven’t covered yet:</p>
<ul>
<li>Lifetime subtyping: ensures that one lifetime outlives another lifetime</li>
<li>Lifetime bounds: specifies a lifetime for a reference to a generic type</li>
<li>Inference of trait object lifetimes: allows the compiler to infer trait
object lifetimes and when they need to be specified</li>
</ul>
<a class="header" href="ch19-02-advanced-lifetimes.html#ensuring-one-lifetime-outlives-another-with-lifetime-subtyping" id="ensuring-one-lifetime-outlives-another-with-lifetime-subtyping"><h3>Ensuring One Lifetime Outlives Another with Lifetime Subtyping</h3></a>
<p><em>Lifetime subtyping</em> specifies that one lifetime should outlive another
lifetime. To explore lifetime subtyping, imagine we want to write a parser.
We’ll use a structure called <code>Context</code> that holds a reference to the string
we’re parsing. We’ll write a parser that will parse this string and return
success or failure. The parser will need to borrow the <code>Context</code> to do the
parsing. Listing 19-12 implements this parser code, except the code doesn’t
have the required lifetime annotations, so it won’t compile.</p>
<p><span class="filename">Filename: src/lib.rs</span></p>
<pre><code class="language-rust ignore does_not_compile">struct Context(&str);
struct Parser {
context: &Context,
}
impl Parser {
fn parse(&self) -> Result<(), &str> {
Err(&self.context.0[1..])
}
}
</code></pre>
<p><span class="caption">Listing 19-12: Defining a parser without lifetime
annotations</span></p>
<p>Compiling the code results in errors because Rust expects lifetime parameters
on the string slice in <code>Context</code> and the reference to a <code>Context</code> in <code>Parser</code>.</p>
<p>For simplicity’s sake, the <code>parse</code> function returns <code>Result<(), &str></code>. That
is, the function will do nothing on success and, on failure, will return the
part of the string slice that didn’t parse correctly. A real implementation
would provide more error information and would return a structured data type
when parsing succeeds. We won’t be discussing those details because they aren’t
relevant to the lifetimes part of this example.</p>
<p>To keep this code simple, we won’t write any parsing logic. However, it’s very
likely that somewhere in the parsing logic we would handle invalid input by
returning an error that references the part of the input that is invalid; this
reference is what makes the code example interesting in regard to lifetimes.
Let’s pretend that the logic of our parser is that the input is invalid after
the first byte. Note that this code might panic if the first byte is not on a
valid character boundary; again, we’re simplifying the example to focus on the
lifetimes involved.</p>
<p>To get this code to compile, we need to fill in the lifetime parameters for the
string slice in <code>Context</code> and the reference to the <code>Context</code> in <code>Parser</code>. The
most straightforward way to do this is to use the same lifetime name
everywhere, as shown in Listing 19-13. Recall from the <a href="ch10-03-lifetime-syntax.html#lifetime-annotations-in-struct-definitions">“Lifetime Annotations
in Struct Definitions”</a><!-- ignore
--> section in Chapter 10 that each of <code>struct Context<'a></code>, <code>struct Parser<'a></code>, and <code>impl<'a></code> is declaring a new lifetime parameter. While their
names happen to all be the same, the three lifetime parameters declared in this
example aren’t related.</p>
<p><span class="filename">Filename: src/lib.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct Context<'a>(&'a str);
struct Parser<'a> {
context: &'a Context<'a>,
}
impl<'a> Parser<'a> {
fn parse(&self) -> Result<(), &str> {
Err(&self.context.0[1..])
}
}
#}</code></pre></pre>
<p><span class="caption">Listing 19-13: Annotating all references in <code>Context</code> and
<code>Parser</code> with lifetime parameters</span></p>
<p>This code compiles just fine. It tells Rust that a <code>Parser</code> holds a reference
to a <code>Context</code> with lifetime <code>'a</code> and that <code>Context</code> holds a string slice that
also lives as long as the reference to the <code>Context</code> in <code>Parser</code>. Rust’s
compiler error message stated that lifetime parameters were required for these
references, and we’ve now added lifetime parameters.</p>
<p>Next, in Listing 19-14, we’ll add a function that takes an instance of
<code>Context</code>, uses a <code>Parser</code> to parse that context, and returns what <code>parse</code>
returns. This code doesn’t quite work.</p>
<p><span class="filename">Filename: src/lib.rs</span></p>
<pre><code class="language-rust ignore does_not_compile">fn parse_context(context: Context) -> Result<(), &str> {
Parser { context: &context }.parse()
}
</code></pre>
<p><span class="caption">Listing 19-14: An attempt to add a <code>parse_context</code>
function that takes a <code>Context</code> and uses a <code>Parser</code></span></p>
<p>We get two verbose errors when we try to compile the code with the addition of
the <code>parse_context</code> function:</p>
<pre><code class="language-text">error[E0597]: borrowed value does not live long enough
--> src/lib.rs:14:5
|
14 | Parser { context: &context }.parse()
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ does not live long enough
15 | }
| - temporary value only lives until here
|
note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 13:1...
--> src/lib.rs:13:1
|
13 | / fn parse_context(context: Context) -> Result<(), &str> {
14 | | Parser { context: &context }.parse()
15 | | }
| |_^
error[E0597]: `context` does not live long enough
--> src/lib.rs:14:24
|
14 | Parser { context: &context }.parse()
| ^^^^^^^ does not live long enough
15 | }
| - borrowed value only lives until here
|
note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 13:1...
--> src/lib.rs:13:1
|
13 | / fn parse_context(context: Context) -> Result<(), &str> {
14 | | Parser { context: &context }.parse()
15 | | }
| |_^
</code></pre>
<p>These errors state that the <code>Parser</code> instance that is created and the <code>context</code>
parameter live only until the end of the <code>parse_context</code> function. But they
both need to live for the entire lifetime of the function.</p>
<p>In other words, <code>Parser</code> and <code>context</code> need to <em>outlive</em> the entire function
and be valid before the function starts as well as after it ends for all the
references in this code to always be valid. The <code>Parser</code> we’re creating and the
<code>context</code> parameter go out of scope at the end of the function, because
<code>parse_context</code> takes ownership of <code>context</code>.</p>
<p>To figure out why these errors occur, let’s look at the definitions in Listing
19-13 again, specifically the references in the signature of the <code>parse</code> method:</p>
<pre><code class="language-rust ignore">fn parse(&self) -> Result<(), &str> {
</code></pre>
<p>Remember the elision rules? If we annotate the lifetimes of the references
rather than eliding, the signature would be as follows:</p>
<pre><code class="language-rust ignore">fn parse<'a>(&'a self) -> Result<(), &'a str> {
</code></pre>
<p>That is, the error part of the return value of <code>parse</code> has a lifetime that is
tied to the lifetime of the <code>Parser</code> instance (that of <code>&self</code> in the <code>parse</code>
method signature). That makes sense: the returned string slice references the
string slice in the <code>Context</code> instance held by the <code>Parser</code>, and the definition
of the <code>Parser</code> struct specifies that the lifetime of the reference to
<code>Context</code> and the lifetime of the string slice that <code>Context</code> holds should be
the same.</p>
<p>The problem is that the <code>parse_context</code> function returns the value returned
from <code>parse</code>, so the lifetime of the return value of <code>parse_context</code> is tied to
the lifetime of the <code>Parser</code> as well. But the <code>Parser</code> instance created in the
<code>parse_context</code> function won’t live past the end of the function (it’s
temporary), and <code>context</code> will go out of scope at the end of the function
(<code>parse_context</code> takes ownership of it).</p>
<p>Rust thinks we’re trying to return a reference to a value that goes out of
scope at the end of the function, because we annotated all the lifetimes with
the same lifetime parameter. The annotations told Rust the lifetime of the
string slice that <code>Context</code> holds is the same as that of the lifetime of the
reference to <code>Context</code> that <code>Parser</code> holds.</p>
<p>The <code>parse_context</code> function can’t see that within the <code>parse</code> function, the
string slice returned will outlive <code>Context</code> and <code>Parser</code> and that the
reference <code>parse_context</code> returns refers to the string slice, not to <code>Context</code>
or <code>Parser</code>.</p>
<p>By knowing what the implementation of <code>parse</code> does, we know that the only
reason the return value of <code>parse</code> is tied to the <code>Parser</code> instance is that
it’s referencing the <code>Parser</code> instance’s <code>Context</code>, which is referencing the
string slice. So, it’s really the lifetime of the string slice that
<code>parse_context</code> needs to care about. We need a way to tell Rust that the string
slice in <code>Context</code> and the reference to the <code>Context</code> in <code>Parser</code> have
different lifetimes and that the return value of <code>parse_context</code> is tied to the
lifetime of the string slice in <code>Context</code>.</p>
<p>First, we’ll try giving <code>Parser</code> and <code>Context</code> different lifetime parameters,
as shown in Listing 19-15. We’ll use <code>'s</code> and <code>'c</code> as lifetime parameter names
to clarify which lifetime goes with the string slice in <code>Context</code> and which
goes with the reference to <code>Context</code> in <code>Parser</code>. Note that this solution won’t
completely fix the problem, but it’s a start. We’ll look at why this fix isn’t
sufficient when we try to compile.</p>
<p><span class="filename">Filename: src/lib.rs</span></p>
<pre><code class="language-rust ignore does_not_compile">struct Context<'s>(&'s str);
struct Parser<'c, 's> {
context: &'c Context<'s>,
}
impl<'c, 's> Parser<'c, 's> {
fn parse(&self) -> Result<(), &'s str> {
Err(&self.context.0[1..])
}
}
fn parse_context(context: Context) -> Result<(), &str> {
Parser { context: &context }.parse()
}
</code></pre>
<p><span class="caption">Listing 19-15: Specifying different lifetime parameters
for the references to the string slice and to <code>Context</code></span></p>
<p>We’ve annotated the lifetimes of the references in all the same places that we
annotated them in Listing 19-13. But this time we used different parameters
depending on whether the reference goes with the string slice or with
<code>Context</code>. We’ve also added an annotation to the string slice part of the
return value of <code>parse</code> to indicate that it goes with the lifetime of the
string slice in <code>Context</code>.</p>
<p>When we try to compile now, we get the following error:</p>
<pre><code class="language-text">error[E0491]: in type `&'c Context<'s>`, reference has a longer lifetime than the data it references
--> src/lib.rs:4:5
|
4 | context: &'c Context<'s>,
| ^^^^^^^^^^^^^^^^^^^^^^^^
|
note: the pointer is valid for the lifetime 'c as defined on the struct at 3:1
--> src/lib.rs:3:1
|
3 | / struct Parser<'c, 's> {
4 | | context: &'c Context<'s>,
5 | | }
| |_^
note: but the referenced data is only valid for the lifetime 's as defined on the struct at 3:1
--> src/lib.rs:3:1
|
3 | / struct Parser<'c, 's> {
4 | | context: &'c Context<'s>,
5 | | }
| |_^
</code></pre>
<p>Rust doesn’t know of any relationship between <code>'c</code> and <code>'s</code>. To be valid, the
referenced data in <code>Context</code> with lifetime <code>'s</code> needs to be constrained to
guarantee that it lives longer than the reference with lifetime <code>'c</code>. If <code>'s</code>
is not longer than <code>'c</code>, the reference to <code>Context</code> might not be valid.</p>
<p>Now we get to the point of this section: the Rust feature <em>lifetime
subtyping</em> specifies that one lifetime parameter lives at least as long as
another one. In the angle brackets where we declare lifetime parameters, we can
declare a lifetime <code>'a</code> as usual and declare a lifetime <code>'b</code> that lives at
least as long as <code>'a</code> by declaring <code>'b</code> using the syntax <code>'b: 'a</code>.</p>
<p>In our definition of <code>Parser</code>, to say that <code>'s</code> (the lifetime of the string
slice) is guaranteed to live at least as long as <code>'c</code> (the lifetime of the
reference to <code>Context</code>), we change the lifetime declarations to look like this:</p>
<p><span class="filename">Filename: src/lib.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# struct Context<'a>(&'a str);
#
struct Parser<'c, 's: 'c> {
context: &'c Context<'s>,
}
#}</code></pre></pre>
<p>Now the reference to <code>Context</code> in the <code>Parser</code> and the reference to the string
slice in the <code>Context</code> have different lifetimes; we’ve ensured that the
lifetime of the string slice is longer than the reference to the <code>Context</code>.</p>
<p>That was a very long-winded example, but as we mentioned at the start of this
chapter, Rust’s advanced features are very specific. You won’t often need the
syntax we described in this example, but in such situations, you’ll know how to
refer to something and give it the necessary lifetime.</p>
<a class="header" href="ch19-02-advanced-lifetimes.html#lifetime-bounds-on-references-to-generic-types" id="lifetime-bounds-on-references-to-generic-types"><h3>Lifetime Bounds on References to Generic Types</h3></a>
<p>In the <a href="ch10-02-traits.html#traits-as-parameters">“Traits as Parameters”</a><!-- ignore --> section in
Chapter 10, we discussed using trait bounds on generic types. We can also add
lifetime parameters as constraints on generic types; these are called <em>lifetime
bounds</em>. Lifetime bounds help Rust verify that references in generic types
won’t outlive the data they’re referencing.</p>
<p>As an example, consider a type that is a wrapper over references. Recall the
<code>RefCell<T></code> type from the <a href="ch15-05-interior-mutability.html#refcellt-and-the-interior-mutability-pattern">“<code>RefCell<T></code> and the Interior Mutability Pattern”</a><!-- ignore --> section in
Chapter 15: its <code>borrow</code> and <code>borrow_mut</code> methods return the types <code>Ref</code> and
<code>RefMut</code>, respectively. These types are wrappers over references that keep
track of the borrowing rules at runtime. The definition of the <code>Ref</code> struct is
shown in Listing 19-16, without lifetime bounds for now.</p>
<p><span class="filename">Filename: src/lib.rs</span></p>
<pre><code class="language-rust ignore does_not_compile">struct Ref<'a, T>(&'a T);
</code></pre>
<p><span class="caption">Listing 19-16: Defining a struct to wrap a reference to a
generic type, without lifetime bounds</span></p>
<p>Without explicitly constraining the lifetime <code>'a</code> in relation to the generic
parameter <code>T</code>, Rust will error because it doesn’t know how long the generic
type <code>T</code> will live:</p>
<pre><code class="language-text">error[E0309]: the parameter type `T` may not live long enough
--> src/lib.rs:1:19
|
1 | struct Ref<'a, T>(&'a T);
| ^^^^^^
|
= help: consider adding an explicit lifetime bound `T: 'a`...
note: ...so that the reference type `&'a T` does not outlive the data it points at
--> src/lib.rs:1:19
|
1 | struct Ref<'a, T>(&'a T);
| ^^^^^^
</code></pre>
<p>Because <code>T</code> can be any type, <code>T</code> could be a reference or a type that holds one
or more references, each of which could have their own lifetimes. Rust can’t be
sure <code>T</code> will live as long as <code>'a</code>.</p>
<p>Fortunately, the error provides helpful advice on how to specify the lifetime
bound in this case:</p>
<pre><code class="language-text">consider adding an explicit lifetime bound `T: 'a` so that the reference type
`&'a T` does not outlive the data it points at
</code></pre>
<p>Listing 19-17 shows how to apply this advice by specifying the lifetime bound
when we declare the generic type <code>T</code>.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct Ref<'a, T: 'a>(&'a T);
#}</code></pre></pre>
<p><span class="caption">Listing 19-17: Adding lifetime bounds on <code>T</code> to specify
that any references in <code>T</code> live at least as long as <code>'a</code></span></p>
<p>This code now compiles because the <code>T: 'a</code> syntax specifies that <code>T</code> can be any
type, but if it contains any references, the references must live at least as
long as <code>'a</code>.</p>
<p>We could solve this problem in a different way, as shown in the definition of a
<code>StaticRef</code> struct in Listing 19-18, by adding the <code>'static</code> lifetime bound on
<code>T</code>. This means if <code>T</code> contains any references, they must have the <code>'static</code>
lifetime.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct StaticRef<T: 'static>(&'static T);
#}</code></pre></pre>
<p><span class="caption">Listing 19-18: Adding a <code>'static</code> lifetime bound to <code>T</code>
to constrain <code>T</code> to types that have only <code>'static</code> references or no
references</span></p>
<p>Because <code>'static</code> means the reference must live as long as the entire program,
a type that contains no references meets the criteria of all references living
as long as the entire program (because there are no references). For the borrow
checker concerned about references living long enough, there is no real
distinction between a type that has no references and a type that has
references that live forever: both are the same for determining whether or not
a reference has a shorter lifetime than what it refers to.</p>
<a class="header" href="ch19-02-advanced-lifetimes.html#inference-of-trait-object-lifetimes" id="inference-of-trait-object-lifetimes"><h3>Inference of Trait Object Lifetimes</h3></a>
<p>In Chapter 17 in the <a href="ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types">“Using Trait Objects That Allow for Values of Different
Types”</a><!--
ignore --> section, we discussed trait objects, consisting of a trait behind a
reference, that allow us to use dynamic dispatch. We haven’t yet discussed what
happens if the type implementing the trait in the trait object has a lifetime
of its own. Consider Listing 19-19 where we have a trait <code>Red</code> and a struct
<code>Ball</code>. The <code>Ball</code> struct holds a reference (and thus has a lifetime parameter)
and also implements trait <code>Red</code>. We want to use an instance of <code>Ball</code> as the
trait object <code>Box<dyn Red></code>.</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">trait Red { }
struct Ball<'a> {
diameter: &'a i32,
}
impl<'a> Red for Ball<'a> { }
fn main() {
let num = 5;
let obj = Box::new(Ball { diameter: &num }) as Box<dyn Red>;
}
</code></pre></pre>
<p><span class="caption">Listing 19-19: Using a type that has a lifetime parameter
with a trait object</span></p>
<p>This code compiles without any errors, even though we haven’t explicitly
annotated the lifetimes involved in <code>obj</code>. This code works because there are
rules for working with lifetimes and trait objects:</p>
<ul>
<li>The default lifetime of a trait object is <code>'static</code>.</li>
<li>With <code>&'a Trait</code> or <code>&'a mut Trait</code>, the default lifetime of the trait object
is <code>'a</code>.</li>
<li>With a single <code>T: 'a</code> clause, the default lifetime of the trait object is
<code>'a</code>.</li>
<li>With multiple clauses like <code>T: 'a</code>, there is no default lifetime; we must be
explicit.</li>
</ul>
<p>When we must be explicit, we can add a lifetime bound on a trait object like
<code>Box<dyn Red></code> using the syntax <code>Box<dyn Red + 'static></code> or <code>Box<dyn Red + 'a></code>, depending on whether the reference lives for the entire program or not.
As with the other bounds, the syntax adding a lifetime bound means that any
implementor of the <code>Red</code> trait that has references inside the type must have
the same lifetime specified in the trait object bounds as those references.</p>
<p>Next, let’s look at some other advanced features that manage traits.</p>
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