Rust handles fatal errors with a “panic”.

Rust will trigger a panic if a fatal error happens at runtime:

fn main() {
    let v = vec![10, 20, 30];
    println!("v[100]: {}", v[100]);
  • Panics are for unrecoverable and unexpected errors.
    • Panics are symptoms of bugs in the program.
    • Runtime failures like failed bounds checks can panic
    • Assertions (such as assert!) panic on failure
    • Purpose-specific panics can use the panic! macro.
  • A panic will “unwind” the stack, dropping values just as if the functions had returned.
  • Use non-panicking APIs (such as Vec::get) if crashing is not acceptable.
This slide should take about 3 minutes.

By default, a panic will cause the stack to unwind. The unwinding can be caught:

use std::panic;

fn main() {
    let result = panic::catch_unwind(|| "No problem here!");

    let result = panic::catch_unwind(|| {
        panic!("oh no!");
  • Catching is unusual; do not attempt to implement exceptions with catch_unwind!
  • This can be useful in servers which should keep running even if a single request crashes.
  • This does not work if panic = 'abort' is set in your Cargo.toml.