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    //! 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:
    //!
    //! ```
    //! fn panic_impl(pi: &core::panic::PanicInfo<'_>) -> !
    //! # { loop {} }
    //! ```
    //!
    //! This definition allows for panicking with any general message, but it does not
    //! allow for failing with a `Box<Any>` value. (`PanicInfo` just contains a `&(dyn Any + Send)`,
    //! for which we fill in a dummy value in `PanicInfo::internal_constructor`.)
    //! 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. The actual symbol is declared through the `#[panic_handler]` attribute.

    #![allow(dead_code, missing_docs)]
    #![unstable(
        feature = "core_panic",
        reason = "internal details of the implementation of the `panic!` \
                  and related macros",
        issue = "none"
    )]

    use crate::fmt;
    use crate::panic::{Location, PanicInfo};

    /// The underlying implementation of libcore's `panic!` macro when no formatting is used.
    #[cold]
    // never inline unless panic_immediate_abort to avoid code
    // bloat at the call sites as much as possible
    #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
    #[track_caller]
    #[lang = "panic"] // needed by codegen for panic on overflow and other `Assert` MIR terminators
    pub fn panic(expr: &str) -> ! {
        if cfg!(feature = "panic_immediate_abort") {
            // remove `unsafe` (and safety comment) on bootstrap bump
            #[cfg_attr(not(bootstrap), allow(unused_unsafe))]
            // SAFETY: the `abort` intrinsic has no requirements to be called.
            unsafe {
                super::intrinsics::abort()
            }
        }

        // 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.
        panic_fmt(fmt::Arguments::new_v1(&[expr], &[]));
    }

    #[cold]
    #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
    #[track_caller]
    #[lang = "panic_bounds_check"] // needed by codegen for panic on OOB array/slice access
    fn panic_bounds_check(index: usize, len: usize) -> ! {
        if cfg!(feature = "panic_immediate_abort") {
            // remove `unsafe` (and safety comment) on bootstrap bump
            #[cfg_attr(not(bootstrap), allow(unused_unsafe))]
            // SAFETY: the `abort` intrinsic has no requirements to be called.
            unsafe {
                super::intrinsics::abort()
            }
        }

        panic!("index out of bounds: the len is {} but the index is {}", len, index)
    }

    /// The underlying implementation of libcore's `panic!` macro when formatting is used.
    #[cold]
    #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
    #[cfg_attr(feature = "panic_immediate_abort", inline)]
    #[track_caller]
    pub fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
        if cfg!(feature = "panic_immediate_abort") {
            // remove `unsafe` (and safety comment) on bootstrap bump
            #[cfg_attr(not(bootstrap), allow(unused_unsafe))]
            // SAFETY: the `abort` intrinsic has no requirements to be called.
            unsafe {
                super::intrinsics::abort()
            }
        }

        // NOTE This function never crosses the FFI boundary; it's a Rust-to-Rust call
        // that gets resolved to the `#[panic_handler]` function.
        extern "Rust" {
            #[lang = "panic_impl"]
            fn panic_impl(pi: &PanicInfo<'_>) -> !;
        }

        let pi = PanicInfo::internal_constructor(Some(&fmt), Location::caller());

        // SAFETY: `panic_impl` is defined in safe Rust code and thus is safe to call.
        unsafe { panic_impl(&pi) }
    }