Macro bail_cast_error

macro_rules! bail_cast_error {
    () => { ... };
    (@ source = $source:expr, $($fmt_arg:tt)*) => { ... };
    (source = $source:expr, $($fmt_arg:tt)*) => { ... };
    ($($fmt_arg:tt)*) => { ... };
}

Constructs a CastErrorInner and bails out.