Macro format_err

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

Construct an ad-hoc error from a string.

This evaluates to an Error. It can take either just a string, or a format string with arguments. It also can take any custom type which implements Debug and Display.