core::panic!

macro_rules! panic {
    () => (
        panic!("explicit panic")
    );
    ($msg:expr) => ({
        static _MSG_FILE_LINE: (&'static str, &'static str, u32) = ($msg, file!(), line!());
        ::core::panicking::panic(&_MSG_FILE_LINE)
    });
    ($fmt:expr, $($arg:tt)*) => ({
        // The leading _'s are to avoid dead code warnings if this is
        // used inside a dead function. Just `#[allow(dead_code)]` is
        // insufficient, since the user may have
        // `#[forbid(dead_code)]` and which cannot be overridden.
        static _FILE_LINE: (&'static str, u32) = (file!(), line!());
        ::core::panicking::panic_fmt(format_args!($fmt, $($arg)*), &_FILE_LINE)
    });
}

Entry point of thread panic, for details, see std::macros