Macro postcondition 

macro_rules! postcondition {
    ($condition:expr) => { ... };
    ($condition:expr, $message:literal) => { ... };
    ($condition:expr, $($arg:tt)*) => { ... };
}

Equivalent to a no op when used with an unmodified Rust compiler. When compiled with MIRAI, this causes MIRAI to verify the condition at the point where it appears in a function, but to also add it a postcondition that can be assumed by the caller of the function.