Function std::rt::unwind::begin_unwind_fmt
pub fn begin_unwind_fmt(msg: &Arguments, file: &'static str, line: uint) -> !
The entry point for unwinding with a formatted message.
This is designed to reduce the amount of code required at the call
site as much as possible (so that fail!() has as low an impact
on (e.g.) the inlining of other functions as possible), by moving
the actual formatting into this shared place.