Macro failure::ensure

macro_rules! ensure {
    ($cond : expr) => { ... };
    ($cond : expr, $e : expr) => { ... };
    ($cond : expr, $fmt : expr, $($arg : tt) *) => { ... };
}

Exits a function early with an Error if the condition is not satisfied.

Similar to assert!, ensure! takes a condition and exits the function if the condition fails. Unlike assert!, ensure! returns an Error, it does not panic.