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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Panic support for libcore
    //!
    //! The core library cannot define panicking, but it does *declare* panicking. This
    //! means that the functions inside of libcore are allowed to panic, but to be
    //! useful an upstream crate must define panicking for libcore to use. The current
    //! interface for panicking is:
    //!
    //! ```ignore
    //! fn panic_impl(fmt: fmt::Arguments, &(&'static str, u32)) -> !;
    //! ```
    //!
    //! This definition allows for panicking with any general message, but it does not
    //! allow for failing with a `Box<Any>` value. The reason for this is that libcore
    //! is not allowed to allocate.
    //!
    //! This module contains a few other panicking functions, but these are just the
    //! necessary lang items for the compiler. All panics are funneled through this
    //! one function. Currently, the actual symbol is declared in the standard
    //! library, but the location of this may change over time.

    #![allow(dead_code, missing_docs)]
    #![unstable(feature = "core_panic",
                reason = "internal details of the implementation of the `panic!` \
                          and related macros",
                issue = "0")]

    use fmt;

    #[cold] #[inline(never)] // this is the slow path, always
    #[lang = "panic"]
    pub fn panic(expr_file_line: &(&'static str, &'static str, u32)) -> ! {
        // Use Arguments::new_v1 instead of format_args!("{}", expr) to potentially
        // reduce size overhead. The format_args! macro uses str's Display trait to
        // write expr, which calls Formatter::pad, which must accommodate string
        // truncation and padding (even though none is used here). Using
        // Arguments::new_v1 may allow the compiler to omit Formatter::pad from the
        // output binary, saving up to a few kilobytes.
        let (expr, file, line) = *expr_file_line;
        panic_fmt(fmt::Arguments::new_v1(&[expr], &[]), &(file, line))
    }

    #[cold] #[inline(never)]
    #[lang = "panic_bounds_check"]
    fn panic_bounds_check(file_line: &(&'static str, u32),
                         index: usize, len: usize) -> ! {
        panic_fmt(format_args!("index out of bounds: the len is {} but the index is {}",
                               len, index), file_line)
    }

    #[cold] #[inline(never)]
    pub fn panic_fmt(fmt: fmt::Arguments, file_line: &(&'static str, u32)) -> ! {
        #[allow(improper_ctypes)]
        extern {
            #[lang = "panic_fmt"]
            #[unwind]
            fn panic_impl(fmt: fmt::Arguments, file: &'static str, line: u32) -> !;
        }
        let (file, line) = *file_line;
        unsafe { panic_impl(fmt, file, line) }
    }

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