# Macro debug_assert

1.6.0 · Source

    macro_rules! debug_assert {
        ($($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.

Like `assert!`, this macro also has a second version, where a custom panic message can be provided.

## §Uses

Unlike `assert!`, `debug_assert!` statements are only enabled in non optimized builds by default. An optimized build will not execute `debug_assert!` statements unless `-C debug-assertions` is passed to the compiler. This makes `debug_assert!` useful for checks that are too expensive to be present in a release build but may be helpful during development. The result of expanding `debug_assert!` is always type checked.

An unchecked assertion allows a program in an inconsistent state to keep running, which might have unexpected consequences but does not introduce unsafety as long as this only happens in safe code. The performance cost of assertions, however, is not measurable in general. Replacing `assert!` with `debug_assert!` is thus only encouraged after thorough profiling, and more importantly, only in safe code!

## §Examples

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

    fn some_expensive_computation() -> bool {
        // Some expensive computation here
        true
    }
    debug_assert!(some_expensive_computation());

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

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