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 { true } // a very simple function

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);