# Macro assert

1.6.0 · Source

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

Expand description

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);