## Unrecoverable Errors with `panic!`

Sometimes bad things happen in your code, and there’s nothing you can do about it. In these cases, Rust has the `panic!` macro. There are two ways to cause a panic in practice: by taking an action that causes our code to panic (such as accessing an array past the end) or by explicitly calling the `panic!` macro. In both cases, we cause a panic in our program. By default, these panics will print a failure message, unwind, clean up the stack, and quit. Via an environment variable, you can also have Rust display the call stack when a panic occurs to make it easier to track down the source of the panic.

### Unwinding the Stack or Aborting in Response to a 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. However, walking back and cleaning up is a lot of work. Rust therefore allows you to choose the alternative of immediately aborting, 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 resultant binary as small as possible, you can switch from unwinding to aborting upon a panic by adding `panic = 'abort'` to the appropriate `[profile]` sections in your Cargo.toml file. For example, if you want to abort on panic in release mode, add this:

    [profile.release]
    panic = 'abort'

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

Filename: src/main.rs

    fn main() {
        panic!("crash and burn");
    }

When you run the program, you’ll see something like this:

    $ cargo run
       Compiling panic v0.1.0 (file:///projects/panic)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.25s
         Running `target/debug/panic`

    thread 'main' panicked at src/main.rs:2:5:
    crash and burn
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The call to `panic!` 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: src/main.rs:2:5 indicates that it’s the second line, fifth character 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, and 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!` call.

We can use the backtrace of the functions the `panic!` call came from to figure out the part of our code that is causing the problem. To understand how to use a `panic!` backtrace, let’s look at another example and 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. Listing 9-1 has some code that attempts to access an index in a vector beyond the range of valid indexes.

Filename: src/main.rs

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

        v[99];
    }

Listing 9-1: Attempting to access an element beyond the end of a vector, which will cause a call to `panic!`

Here, we’re attempting to access the 100th element of our vector (which is at index 99 because indexing starts at zero), but the vector has only 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.

In C, attempting to read beyond the end of a data structure is undefined behavior. You might get whatever is at the location in memory that would correspond to that element in the data structure, even though the memory doesn’t belong to that structure. This is called a buffer overread 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 data structure.

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:

    $ cargo run
       Compiling panic v0.1.0 (file:///projects/panic)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.27s
         Running `target/debug/panic`

    thread 'main' panicked at src/main.rs:4:6:
    index out of bounds: the len is 3 but the index is 99
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This error points at line 4 of our main.rs where we attempt to access index 99 of the vector in `v`.

The `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. A backtrace 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 that spot are code that your code has called; the lines below are code that called your code. These before-and-after lines might include core Rust code, standard library code, or crates that you’re using. Let’s try to get a backtrace by setting the `RUST_BACKTRACE` environment variable to any value except `0`. Listing 9-2 shows output similar to what you’ll see.

    $ RUST_BACKTRACE=1 cargo run
    thread 'main' panicked at src/main.rs:4:6:
    index out of bounds: the len is 3 but the index is 99
    stack backtrace:
       0: rust_begin_unwind
                 at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/std/src/panicking.rs:692:5
       1: core::panicking::panic_fmt
                 at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/core/src/panicking.rs:75:14
       2: core::panicking::panic_bounds_check
                 at /rustc/4d91de4e48198da2e33413efdcd9cd2cc0c46688/library/core/src/panicking.rs:273:5
       3: <usize as core::slice::index::SliceIndex<[T]>>::index
                 at file:///home/.rustup/toolchains/1.85/lib/rustlib/src/rust/library/core/src/slice/index.rs:274:10
       4: core::slice::index::<impl core::ops::index::Index<I> for [T]>::index
                 at file:///home/.rustup/toolchains/1.85/lib/rustlib/src/rust/library/core/src/slice/index.rs:16:9
       5: <alloc::vec::Vec<T,A> as core::ops::index::Index<I>>::index
                 at file:///home/.rustup/toolchains/1.85/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:3361:9
       6: panic::main
                 at ./src/main.rs:4:6
       7: core::ops::function::FnOnce::call_once
                 at file:///home/.rustup/toolchains/1.85/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
    note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Listing 9-2: The backtrace generated by a call to `panic!` displayed when the environment variable `RUST_BACKTRACE` is set

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 `cargo build` or `cargo run` without the `--release` flag, as we have here.

In the output in Listing 9-2, line 6 of the backtrace points to the line in our project that’s causing the problem: line 4 of src/main.rs. If we don’t want our program to panic, we should start our investigation at the location pointed to by the first line mentioning a file we wrote. In Listing 9-1, where we deliberately wrote code that would panic, the way to fix the panic is to not request an element beyond the range of the vector indexes. 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.

We’ll come back to `panic!` and when we should and should not use `panic!` to handle error conditions in the “To `panic!` or Not to `panic!`” section later in this chapter. Next, we’ll look at how to recover from an error using `Result`.