Macro runtime_error 

macro_rules! runtime_error {
    ($error:literal) => { ... };
    ($error:expr) => { ... };
    ($error:literal, $($y:expr),+ $(,)?) => { ... };
}

Creates a crate::Error from a message (with format-like behaviour), wrapped in Err

Wrapping the result in Err is a convenience for functions that need to return immediately when an error has occurred.