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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><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 class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><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 class="expanded "><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 class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><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 class="expanded "><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 class="expanded "><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 class="expanded "><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 class="expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li class="expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li class="expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Storing UTF-8 Encoded Text with Strings</a></li><li class="expanded "><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 class="expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li class="expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html" class="active"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li class="expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li class="expanded "><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 class="expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li class="expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li class="expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li class="expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li class="expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li class="expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li class="expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li class="expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li class="expanded "><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 class="expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li class="expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li class="expanded "><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 class="expanded "><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 class="expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li class="expanded "><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 class="expanded "><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 class="expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li class="expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li class="expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li class="expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li class="expanded "><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 class="expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li class="expanded "><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 class="expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li class="expanded "><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 class="expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li class="expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li class="expanded "><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 class="expanded "><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 class="expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> Running Code on Cleanup with the Drop Trait</a></li><li class="expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li class="expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li class="expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Reference Cycles Can Leak Memory</a></li></ol></li><li class="expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li class="expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li class="expanded "><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 class="expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li class="expanded "><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 class="expanded "><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 class="expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li class="expanded "><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 class="expanded "><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 class="expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li class="expanded "><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 class="expanded "><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li class="expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li class="expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li class="expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li class="expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> Advanced Traits</a></li><li class="expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> Advanced Types</a></li><li class="expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> Advanced Functions and Closures</a></li><li class="expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> Macros</a></li></ol></li><li class="expanded "><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 class="expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> Building a Single-Threaded Web Server</a></li><li class="expanded "><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 class="expanded "><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 class="expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li class="expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li class="expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li class="expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li class="expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li class="expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li class="expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li class="expanded "><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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<h2><a class="header" href="#unrecoverable-errors-with-panic" id="unrecoverable-errors-with-panic">Unrecoverable Errors with <code>panic!</code></a></h2>
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<p>Sometimes, bad things happen in your code, and there’s nothing you can do about
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it. In these cases, Rust has the <code>panic!</code> macro. When the <code>panic!</code> macro
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executes, your program will print a failure message, unwind and clean up the
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stack, and then quit. This most commonly occurs when a bug of some kind has
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been detected and it’s not clear to the programmer how to handle the error.</p>
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<blockquote>
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<h3><a class="header" href="#unwinding-the-stack-or-aborting-in-response-to-a-panic" id="unwinding-the-stack-or-aborting-in-response-to-a-panic">Unwinding the Stack or Aborting in Response to a Panic</a></h3>
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||
<p>By default, when a panic occurs, the program starts <em>unwinding</em>, which
|
||
means Rust walks back up the stack and cleans up the data from each function
|
||
it encounters. But this walking back and cleanup is a lot of work. The
|
||
alternative is to immediately <em>abort</em>, which ends the program without
|
||
cleaning up. Memory that the program was using will then need to be cleaned
|
||
up by the operating system. If in your project you need to make the resulting
|
||
binary as small as possible, you can switch from unwinding to aborting upon a
|
||
panic by adding <code>panic = 'abort'</code> to the appropriate <code>[profile]</code> sections in
|
||
your <em>Cargo.toml</em> file. For example, if you want to abort on panic in release
|
||
mode, add this:</p>
|
||
<pre><code class="language-toml">[profile.release]
|
||
panic = 'abort'
|
||
</code></pre>
|
||
</blockquote>
|
||
<p>Let’s try calling <code>panic!</code> in a simple program:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust should_panic panics">fn main() {
|
||
panic!("crash and burn");
|
||
}
|
||
</code></pre></pre>
|
||
<p>When you run the program, you’ll see something like this:</p>
|
||
<pre><code class="language-text">$ cargo run
|
||
Compiling panic v0.1.0 (file:///projects/panic)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.25s
|
||
Running `target/debug/panic`
|
||
thread 'main' panicked at 'crash and burn', src/main.rs:2:5
|
||
note: Run with `RUST_BACKTRACE=1` for a backtrace.
|
||
</code></pre>
|
||
<p>The call to <code>panic!</code> causes the error message contained in the last two lines.
|
||
The first line shows our panic message and the place in our source code where
|
||
the panic occurred: <em>src/main.rs:2:5</em> indicates that it’s the second line,
|
||
fifth character of our <em>src/main.rs</em> file.</p>
|
||
<p>In this case, the line indicated is part of our code, and if we go to that
|
||
line, we see the <code>panic!</code> macro call. In other cases, the <code>panic!</code> call might
|
||
be in code that our code calls, and the filename and line number reported by
|
||
the error message will be someone else’s code where the <code>panic!</code> macro is
|
||
called, not the line of our code that eventually led to the <code>panic!</code> call. We
|
||
can use the backtrace of the functions the <code>panic!</code> call came from to figure
|
||
out the part of our code that is causing the problem. We’ll discuss what a
|
||
backtrace is in more detail next.</p>
|
||
<h3><a class="header" href="#using-a-panic-backtrace" id="using-a-panic-backtrace">Using a <code>panic!</code> Backtrace</a></h3>
|
||
<p>Let’s look at another example to see what it’s like when a <code>panic!</code> call comes
|
||
from a library because of a bug in our code instead of from our code calling
|
||
the macro directly. Listing 9-1 has some code that attempts to access an
|
||
element by index in a vector.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust should_panic panics">fn main() {
|
||
let v = vec![1, 2, 3];
|
||
|
||
v[99];
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 9-1: Attempting to access an element beyond the
|
||
end of a vector, which will cause a call to <code>panic!</code></span></p>
|
||
<p>Here, we’re attempting to access the 100th element of our vector (which is at
|
||
index 99 because indexing starts at zero), but it has only 3 elements. In this
|
||
situation, Rust will panic. Using <code>[]</code> is supposed to return an element, but if
|
||
you pass an invalid index, there’s no element that Rust could return here that
|
||
would be correct.</p>
|
||
<p>Other languages, like C, will attempt to give you exactly what you asked for in
|
||
this situation, even though it isn’t what you want: you’ll get whatever is at
|
||
the location in memory that would correspond to that element in the vector,
|
||
even though the memory doesn’t belong to the vector. This is called a <em>buffer
|
||
overread</em> and can lead to security vulnerabilities if an attacker is able to
|
||
manipulate the index in such a way as to read data they shouldn’t be allowed to
|
||
that is stored after the array.</p>
|
||
<p>To protect your program from this sort of vulnerability, if you try to read an
|
||
element at an index that doesn’t exist, Rust will stop execution and refuse to
|
||
continue. Let’s try it and see:</p>
|
||
<pre><code class="language-text">$ cargo run
|
||
Compiling panic v0.1.0 (file:///projects/panic)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.27s
|
||
Running `target/debug/panic`
|
||
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', libcore/slice/mod.rs:2448:10
|
||
note: Run with `RUST_BACKTRACE=1` for a backtrace.
|
||
</code></pre>
|
||
<p>This error points at a file we didn’t write, <em>libcore/slice/mod.rs</em>. That’s the
|
||
implementation of <code>slice</code> in the Rust source code. The code that gets run when
|
||
we use <code>[]</code> on our vector <code>v</code> is in <em>libcore/slice/mod.rs</em>, and that is where
|
||
the <code>panic!</code> is actually happening.</p>
|
||
<p>The next note line tells us that we can set the <code>RUST_BACKTRACE</code> environment
|
||
variable to get a backtrace of exactly what happened to cause the error. A
|
||
<em>backtrace</em> is a list of all the functions that have been called to get to this
|
||
point. Backtraces in Rust work as they do in other languages: the key to
|
||
reading the backtrace is to start from the top and read until you see files you
|
||
wrote. That’s the spot where the problem originated. The lines above the lines
|
||
mentioning your files are code that your code called; the lines below are code
|
||
that called your code. These lines might include core Rust code, standard
|
||
library code, or crates that you’re using. Let’s try getting a backtrace by
|
||
setting the <code>RUST_BACKTRACE</code> environment variable to any value except 0.
|
||
Listing 9-2 shows output similar to what you’ll see.</p>
|
||
<pre><code class="language-text">$ RUST_BACKTRACE=1 cargo run
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.00s
|
||
Running `target/debug/panic`
|
||
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 99', libcore/slice/mod.rs:2448:10
|
||
stack backtrace:
|
||
0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
|
||
at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
|
||
1: std::sys_common::backtrace::print
|
||
at libstd/sys_common/backtrace.rs:71
|
||
at libstd/sys_common/backtrace.rs:59
|
||
2: std::panicking::default_hook::{{closure}}
|
||
at libstd/panicking.rs:211
|
||
3: std::panicking::default_hook
|
||
at libstd/panicking.rs:227
|
||
4: <std::panicking::begin_panic::PanicPayload<A> as core::panic::BoxMeUp>::get
|
||
at libstd/panicking.rs:476
|
||
5: std::panicking::continue_panic_fmt
|
||
at libstd/panicking.rs:390
|
||
6: std::panicking::try::do_call
|
||
at libstd/panicking.rs:325
|
||
7: core::ptr::drop_in_place
|
||
at libcore/panicking.rs:77
|
||
8: core::ptr::drop_in_place
|
||
at libcore/panicking.rs:59
|
||
9: <usize as core::slice::SliceIndex<[T]>>::index
|
||
at libcore/slice/mod.rs:2448
|
||
10: core::slice::<impl core::ops::index::Index<I> for [T]>::index
|
||
at libcore/slice/mod.rs:2316
|
||
11: <alloc::vec::Vec<T> as core::ops::index::Index<I>>::index
|
||
at liballoc/vec.rs:1653
|
||
12: panic::main
|
||
at src/main.rs:4
|
||
13: std::rt::lang_start::{{closure}}
|
||
at libstd/rt.rs:74
|
||
14: std::panicking::try::do_call
|
||
at libstd/rt.rs:59
|
||
at libstd/panicking.rs:310
|
||
15: macho_symbol_search
|
||
at libpanic_unwind/lib.rs:102
|
||
16: std::alloc::default_alloc_error_hook
|
||
at libstd/panicking.rs:289
|
||
at libstd/panic.rs:392
|
||
at libstd/rt.rs:58
|
||
17: std::rt::lang_start
|
||
at libstd/rt.rs:74
|
||
18: panic::main
|
||
</code></pre>
|
||
<p><span class="caption">Listing 9-2: The backtrace generated by a call to
|
||
<code>panic!</code> displayed when the environment variable <code>RUST_BACKTRACE</code> is set</span></p>
|
||
<p>That’s a lot of output! The exact output you see might be different depending
|
||
on your operating system and Rust version. In order to get backtraces with this
|
||
information, debug symbols must be enabled. Debug symbols are enabled by
|
||
default when using <code>cargo build</code> or <code>cargo run</code> without the <code>--release</code> flag,
|
||
as we have here.</p>
|
||
<p>In the output in Listing 9-2, line 12 of the backtrace points to the line in
|
||
our project that’s causing the problem: line 4 of <em>src/main.rs</em>. If we don’t
|
||
want our program to panic, the location pointed to by the first line mentioning
|
||
a file we wrote is where we should start investigating. In Listing 9-1, where
|
||
we deliberately wrote code that would panic in order to demonstrate how to use
|
||
backtraces, the way to fix the panic is to not request an element at index 99
|
||
from a vector that only contains 3 items. When your code panics in the future,
|
||
you’ll need to figure out what action the code is taking with what values to
|
||
cause the panic and what the code should do instead.</p>
|
||
<p>We’ll come back to <code>panic!</code> and when we should and should not use <code>panic!</code> to
|
||
handle error conditions in the <a href="ch09-03-to-panic-or-not-to-panic.html#to-panic-or-not-to-panic">“To <code>panic!</code> or Not to
|
||
<code>panic!</code>”</a><!-- ignore --> section later in this
|
||
chapter. Next, we’ll look at how to recover from an error using <code>Result</code>.</p>
|
||
|
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