Macro ensure 

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

Return early with an error if a condition is not satisfied.