panic!

Learn unrecoverable errors with panic!.

panic!

panic! reports that the program has entered an unrecoverable state: the message is printed, the stack is unwound and the thread ends. Some operations panic on their own:

let v = vec![1, 2, 3];
// v[10];                 // index out of bounds
// let x: u8 = 255; x + 1; // overflow in a debug build
panic!("yapılandırma bozuk: {}", "port eksik");

RUST_BACKTRACE=1 cargo run shows the call chain at the moment of the panic.

unwind and abort

By default a panic unwinds the stack and runs each value's drop. For a smaller binary, terminating directly can be chosen:

# Cargo.toml
[profile.release]
panic = "abort"

unreachable!(), todo!() and assert!(kosul) are also macros that panic depending on a condition; assert_eq!(a, b) is common in tests.

Exercise Precondition

Write fn ikiye_katla(n: i32) -> i32: if n < 0, panic!("negatif değer: {}", n), otherwise return n * 2. Print the result of ikiye_katla(5).

Show example solution

fn ikiye_katla(n: i32) -> i32 {
    if n < 0 {
        panic!("negatif değer: {}", n);
    }
    n * 2
}

fn main() {
    println!("{}", ikiye_katla(5));
}

Test yourself

Which is a suitable case for a panic?
- The user entering a wrong password
- The network connection dropping
- A file not being found
- A logically impossible situation actually happening
Show answer

A logically impossible situation actually happening — Expected errors caused by the user or the environment should be handled with Result; panic is for bugs.

Conditional panic

____(n >= 0, "n negatif olamaz: {}", n);

Show answer

assert!(n >= 0, "n negatif olamaz: {}", n);

Macro and where to use it

panic!(..)

Unrecoverable error
assert!(..)

Condition check
todo!()

Not written yet
unreachable!()

An arm that should never run

Next lesson: ? operator

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