#[panic_handler] is used to define the behavior of panic! in #![no_std] applications. The #[panic_handler] attribute must be applied to a function with signature fn(&PanicInfo) -> ! and such function must appear once in the dependency graph of a binary / dylib / cdylib crate. The API of PanicInfo can be found in the API docs.

Given that #![no_std] applications have no standard output and that some #![no_std] applications, e.g. embedded applications, need different panicking behaviors for development and for release it can be helpful to have panic crates, crate that only contain a #[panic_handler]. This way applications can easily swap the panicking behavior by simply linking to a different panic crate.

Below is shown an example where an application has a different panicking behavior depending on whether is compiled using the dev profile (cargo build) or using the release profile (cargo build --release).

// crate: panic-semihosting -- log panic messages to the host stderr using semihosting

#![no_std]

use core::fmt::{Write, self};
use core::panic::PanicInfo;

struct HStderr {
    // ..
#     _0: (),
}
#
# impl HStderr {
#     fn new() -> HStderr { HStderr { _0: () } }
# }
#
# impl fmt::Write for HStderr {
#     fn write_str(&mut self, _: &str) -> fmt::Result { Ok(()) }
# }

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    let mut host_stderr = HStderr::new();

    // logs "panicked at '$reason', src/main.rs:27:4" to the host stderr
    writeln!(host_stderr, "{}", info).ok();

    loop {}
}

// crate: panic-halt -- halt the thread on panic; messages are discarded

#![no_std]

use core::panic::PanicInfo;

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    loop {}
}

// crate: app

#![no_std]

// dev profile
#[cfg(debug_assertions)]
extern crate panic_semihosting;

// release profile
#[cfg(not(debug_assertions))]
extern crate panic_halt;

// omitted: other `extern crate`s

fn main() {
    // ..
}