src/libcore/panicking.rs

//! 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.

// ignore-tidy-undocumented-unsafe

#![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};

#[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") {
        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], &[]), Location::caller())
}

#[cold]
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
#[lang = "panic_bounds_check"] // needed by codegen for panic on OOB array/slice access
fn panic_bounds_check(location: &Location<'_>, index: usize, len: usize) -> ! {
    if cfg!(feature = "panic_immediate_abort") {
        unsafe { super::intrinsics::abort() }
    }

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

#[cold]
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
#[cfg_attr(feature = "panic_immediate_abort", inline)]
pub fn panic_fmt(fmt: fmt::Arguments<'_>, location: &Location<'_>) -> ! {
    if cfg!(feature = "panic_immediate_abort") {
        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);
    unsafe { panic_impl(&pi) }
}