Panicking is a core part of the Rust language. Built-in operations like indexing are runtime checked for memory safety. When out of bounds indexing is attempted this results in a panic.

In the standard library panicking has a defined behavior: it unwinds the stack of the panicking thread, unless the user opted for aborting the program on panics.

In programs without standard library, however, the panicking behavior is left undefined. A behavior can be chosen by declaring a #[panic_handler] function. This function must appear exactly once in the dependency graph of a program, and must have the following signature: fn(&PanicInfo) -> !, where PanicInfo is a struct containing information about the location of the panic.

Given that embedded systems range from user facing to safety critical (cannot crash) there's no one size fits all panicking behavior but there are plenty of commonly used behaviors. These common behaviors have been packaged into crates that define the #[panic_handler] function. Some examples include:

• panic-abort. A panic causes the abort instruction to be executed.
• panic-halt. A panic causes the program, or the current thread, to halt by entering an infinite loop.
• panic-itm. The panicking message is logged using the ITM, an ARM Cortex-M specific peripheral.
• panic-semihosting. The panicking message is logged to the host using the semihosting technique.

You may be able to find even more crates searching for the panic-handler keyword on crates.io.

A program can pick one of these behaviors simply by linking to the corresponding crate. The fact that the panicking behavior is expressed in the source of an application as a single line of code is not only useful as documentation but can also be used to change the panicking behavior according to the compilation profile. For example:

#![no_main]
#![no_std]

// dev profile: easier to debug panics; can put a breakpoint on `rust_begin_unwind`
#[cfg(debug_assertions)]
extern crate panic_halt;

// release profile: minimize the binary size of the application
#[cfg(not(debug_assertions))]
extern crate panic_abort;

// ..

In this example the crate links to the panic-halt crate when built with the dev profile (cargo build), but links to the panic-abort crate when built with the release profile (cargo build --release).

Here's an example that tries to index an array beyond its length. The operation results in a panic.

#![no_main]
#![no_std]

extern crate panic_semihosting;

use cortex_m_rt::entry;

#[entry]
fn main() -> ! {
    let xs = [0, 1, 2];
    let i = xs.len() + 1;
    let _y = xs[i]; // out of bounds access

    loop {}
}

This example chose the panic-semihosting behavior which prints the panic message to the host console using semihosting.

$ cargo run
     Running `qemu-system-arm -cpu cortex-m3 -machine lm3s6965evb (..)
panicked at 'index out of bounds: the len is 3 but the index is 4', src/main.rs:12:13

You can try changing the behavior to panic-halt and confirm that no message is printed in that case.