Macro `assert`

macro_rules! assert {
    ($cond:expr $(,)?) => { ... };
    ($cond:expr, $($arg:tt)+) => { ... };
}

Asserts that a boolean expression is true at runtime.

This will invoke the [panic!] macro if the provided expression cannot be evaluated to true at runtime.

Uses

Assertions are always checked in both debug and release builds, and cannot be disabled. See [debug_assert!] for assertions that are not enabled in release builds by default.

Unsafe code may rely on assert! to enforce run-time invariants that, if violated could lead to unsafety.

Other use-cases of assert! include testing and enforcing run-time invariants in safe code (whose violation cannot result in unsafety).

Custom Messages

This macro has a second form, where a custom panic message can be provided with or without arguments for formatting. See std::fmt for syntax for this form. Expressions used as format arguments will only be evaluated if the assertion fails.

Examples

// the panic message for these assertions is the stringified value of the
// expression given.
assert!(true);

fn some_computation() -> bool {
    // Some expensive computation here
    true
}

assert!(some_computation());

// assert with a custom message
let x = true;
assert!(x, "x wasn't true!");

let a = 3; let b = 27;
assert!(a + b == 30, "a = {}, b = {}", a, b);