## Unrecoverable Errors with `panic!`

Sometimes, bad things happen, and there’s nothing that you can do about it. For
these cases, Rust has the `panic!` macro. When this macro executes, your
program will print a failure message, unwind and clean up the stack, and then
quit. The most common situation this occurs in is when a bug of some kind has
been detected and it’s not clear to the programmer how to handle the error.

> ### Unwinding the Stack Versus Aborting on Panic
>
> By default, when a `panic!` occurs, the program starts
> *unwinding*, which means Rust walks back up the stack and cleans up the data
> from each function it encounters, but this walking and cleanup is a lot of
> work. The alternative is to immediately *abort*, 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 on panic by adding `panic = 'abort'` to the appropriate `[profile]`
> sections in your *Cargo.toml*. For example, if you want to abort on panic in
> release mode:
>
> ```toml
> [profile.release]
> panic = 'abort'
> ```

Let’s try calling `panic!` with a simple program:

Filename: src/main.rs

```rust,should_panic
fn main() {
    panic!("crash and burn");
}
```

If you run it, you’ll see something like this:

```text
$ cargo run
   Compiling panic v0.1.0 (file:///projects/panic)
    Finished dev [unoptimized + debuginfo] target(s) in 0.25 secs
     Running `target/debug/panic`
thread 'main' panicked at 'crash and burn', src/main.rs:2
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
```

The last three lines contain the error message caused by the call to `panic!`.
The first line shows our panic message and the place in our source code where
the panic occurred: *src/main.rs:2* indicates that it’s the second line of our
*src/main.rs* file.

In this case, the line indicated is part of our code, and if we go to that line
we see the `panic!` macro call. In other cases, the `panic!` call might be in
code that our code calls. The filename and line number reported by the error
message will be someone else’s code where the `panic!` macro is called, not the
line of our code that eventually led to the `panic!`. We can use the backtrace
of the functions the `panic!` call came from to figure this out.

### Using a `panic!` Backtrace

Let’s look at another example to see what it’s like when a `panic!` call comes
from a library because of a bug in our code instead of from our code calling
the macro directly:

Filename: src/main.rs

```rust,should_panic
fn main() {
    let v = vec![1, 2, 3];

    v[100];
}
```

We’re attempting to access the hundredth element of our vector, but it only has
three elements. In this situation, Rust will panic. Using `[]` 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.

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 *buffer
overread*, and can lead to security vulnerabilities if an attacker can
manipulate the index in such a way as to read data they shouldn’t be allowed to
that is stored after the array.

In order 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:

```text
$ cargo run
   Compiling panic v0.1.0 (file:///projects/panic)
    Finished dev [unoptimized + debuginfo] target(s) in 0.27 secs
     Running `target/debug/panic`
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is
100', /stable-dist-rustc/build/src/libcollections/vec.rs:1362
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
```

This points at a file we didn’t write, *libcollections/vec.rs*. That’s the
implementation of `Vec<T>` in the standard library. The code that gets run when
we use `[]` on our vector `v` is in *libcollections/vec.rs*, and that is where
the `panic!` is actually happening.

The next note line tells us that we can set the `RUST_BACKTRACE` environment
variable to get a backtrace of exactly what happened to cause the error. Let’s
try that. Listing 9-1 shows the output:

```text
$ RUST_BACKTRACE=1 cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/panic`
thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 100', /stable-dist-rustc/build/src/libcollections/vec.rs:1392
stack backtrace:
   1:     0x560ed90ec04c - std::sys::imp::backtrace::tracing::imp::write::hf33ae72d0baa11ed
                        at /stable-dist-rustc/build/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:42
   2:     0x560ed90ee03e - std::panicking::default_hook::{{closure}}::h59672b733cc6a455
                        at /stable-dist-rustc/build/src/libstd/panicking.rs:351
   3:     0x560ed90edc44 - std::panicking::default_hook::h1670459d2f3f8843
                        at /stable-dist-rustc/build/src/libstd/panicking.rs:367
   4:     0x560ed90ee41b - std::panicking::rust_panic_with_hook::hcf0ddb069e7abcd7
                        at /stable-dist-rustc/build/src/libstd/panicking.rs:555
   5:     0x560ed90ee2b4 - std::panicking::begin_panic::hd6eb68e27bdf6140
                        at /stable-dist-rustc/build/src/libstd/panicking.rs:517
   6:     0x560ed90ee1d9 - std::panicking::begin_panic_fmt::abcd5965948b877f8
                        at /stable-dist-rustc/build/src/libstd/panicking.rs:501
   7:     0x560ed90ee167 - rust_begin_unwind
                        at /stable-dist-rustc/build/src/libstd/panicking.rs:477
   8:     0x560ed911401d - core::panicking::panic_fmt::hc0f6d7b2c300cdd9
                        at /stable-dist-rustc/build/src/libcore/panicking.rs:69
   9:     0x560ed9113fc8 - core::panicking::panic_bounds_check::h02a4af86d01b3e96
                        at /stable-dist-rustc/build/src/libcore/panicking.rs:56
  10:     0x560ed90e71c5 - <collections::vec::Vec<T> as core::ops::Index<usize>>::index::h98abcd4e2a74c41
                        at /stable-dist-rustc/build/src/libcollections/vec.rs:1392
  11:     0x560ed90e727a - panic::main::h5d6b77c20526bc35
                        at /home/you/projects/panic/src/main.rs:4
  12:     0x560ed90f5d6a - __rust_maybe_catch_panic
                        at /stable-dist-rustc/build/src/libpanic_unwind/lib.rs:98
  13:     0x560ed90ee926 - std::rt::lang_start::hd7c880a37a646e81
                        at /stable-dist-rustc/build/src/libstd/panicking.rs:436
                        at /stable-dist-rustc/build/src/libstd/panic.rs:361
                        at /stable-dist-rustc/build/src/libstd/rt.rs:57
  14:     0x560ed90e7302 - main
  15:     0x7f0d53f16400 - __libc_start_main
  16:     0x560ed90e6659 - _start
  17:                0x0 - <unknown>
```

Listing 9-1: The backtrace generated by a call to
`panic!` displayed when the environment variable `RUST_BACKTRACE` is set

That’s a lot of output! Line 11 of the backtrace points to the line in our
project causing the problem: *src/main.rs*, line four. A backtrace is a list of
all the functions that have been called to get to this point. Backtraces in
Rust work like 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.

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 order
to figure out how we got to this location with values that caused the panic. In
our example 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 try to
request an element at index 100 from a vector that only contains three items.
When your code panics in the future, you’ll need to figure out for your
particular case what action the code is taking with what values that causes the
panic and what the code should do instead.

We’ll come back to `panic!` and when we should and should not use these methods
later in the chapter. Next, we’ll now look at how to recover from an error with
`Result`.
