15.4 Unrecoverable Errors in Rust

While the Result type is suitable for recoverable errors, some problems make continuing execution infeasible or unsafe. In such cases, Rust uses the panic! macro.

15.4.1 The panic! Macro

Calling panic! stops execution, optionally printing an error message and unwinding the stack (unless configured to abort):

fn main() {
    panic!("A critical unrecoverable error occurred!");
}

Certain actions induce a panic implicitly, such as accessing an out-of-bounds array index:

fn main() {
    let arr = [10, 20, 30];
    println!("Out of bounds element: {}", arr[99]); // Panics
}
  • assert!: Panics if a condition is false.
  • assert_eq! / assert_ne!: Compare two values for equality or inequality, panicking if the condition fails.

These macros are used primarily for testing or verifying assumptions during development.

15.4.3 Catching Panics

While catching panics is not typical in Rust, you can do so with std::panic::catch_unwind:

use std::panic;

fn main() {
    let result = panic::catch_unwind(|| {
        let array = [1, 2, 3];
        println!("{}", array[99]); // This will panic
    });

    match result {
        Ok(_) => println!("Code executed without panic."),
        Err(e) => println!("Caught a panic: {:?}", e),
    }
}

Key observations:

  • Limited Use Cases: Typically utilized in tests or FFI boundaries.
  • Not Control Flow: Panics signal grave errors, not standard branching.
  • Performance Overhead: Stack unwinding is not free.

15.4.4 Customizing Panic Behavior

You can configure panic behavior through the Cargo.toml or environment variables:

  • Panic Strategy: Specify in Cargo.toml:

    [profile.release]
panic = "abort"
    • unwind (default): Rust unwinds the stack and runs destructors.
    • abort: Immediate termination without unwinding.

  • Backtraces: Enable a backtrace by setting RUST_BACKTRACE=1:

    RUST_BACKTRACE=1 cargo run

Stack Unwinding vs. Aborting

  • Stack Unwinding: Cleans up resources by calling destructors before terminating. Helpful for debugging, but can increase binary size.
  • Immediate Termination: Terminates right away without cleanup. Reduces binary size but can complicate debugging and leak resources.