library/core/src/panicking.rs

//! Panic support for core
//!
//! In core, panicking is always done with a message, resulting in a `core::panic::PanicInfo`
//! containing a `fmt::Arguments`. In std, however, panicking can be done with panic_any, which
//! throws a `Box<dyn Any>` containing any type of value. Because of this,
//! `std::panic::PanicHookInfo` is a different type, which contains a `&dyn Any` instead of a
//! `fmt::Arguments`. std's panic handler will convert the `fmt::Arguments` to a `&dyn Any`
//! containing either a `&'static str` or `String` containing the formatted message.
//!
//! The core library cannot define any panic handler, but it can invoke it.
//! This means that the functions inside of core are allowed to panic, but to be
//! useful an upstream crate must define panicking for core to use. The current
//! interface for panicking is:
//!
//! ```
//! fn panic_impl(pi: &core::panic::PanicInfo<'_>) -> !
//! # { loop {} }
//! ```
//!
//! 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 = "panic_internals",
    reason = "internal details of the implementation of the `panic!` and related macros",
    issue = "none"
)]

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

#[cfg(feature = "panic_immediate_abort")]
compile_error!(
    "panic_immediate_abort is now a real panic strategy! \
    Enable it with `panic = \"immediate-abort\"` in Cargo.toml, \
    or with the compiler flags `-Zunstable-options -Cpanic=immediate-abort`. \
    In both cases, you still need to build core, e.g. with `-Zbuild-std`"
);

// First we define the two main entry points that all panics go through.
// In the end both are just convenience wrappers around `panic_impl`.

/// The entry point for panicking with a formatted message.
///
/// This is designed to reduce the amount of code required at the call
/// site as much as possible (so that `panic!()` has as low an impact
/// on (e.g.) the inlining of other functions as possible), by moving
/// the actual formatting into this shared place.
// If panic=immediate-abort, inline the abort call,
// otherwise avoid inlining because of it is cold path.
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold)]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
#[lang = "panic_fmt"] // needed for const-evaluated panics
#[rustc_do_not_const_check] // hooked by const-eval
#[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable
pub const fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
    if cfg!(panic = "immediate-abort") {
        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.
    unsafe extern "Rust" {
        #[lang = "panic_impl"]
        fn panic_impl(pi: &PanicInfo<'_>) -> !;
    }

    let pi = PanicInfo::new(
        &fmt,
        Location::caller(),
        /* can_unwind */ true,
        /* force_no_backtrace */ false,
    );

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

/// Like `panic_fmt`, but for non-unwinding panics.
///
/// Has to be a separate function so that it can carry the `rustc_nounwind` attribute.
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold)]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
// This attribute has the key side-effect that if the panic handler ignores `can_unwind`
// and unwinds anyway, we will hit the "unwinding out of nounwind function" guard,
// which causes a "panic in a function that cannot unwind".
#[rustc_nounwind]
#[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable
#[rustc_allow_const_fn_unstable(const_eval_select)]
pub const fn panic_nounwind_fmt(fmt: fmt::Arguments<'_>, force_no_backtrace: bool) -> ! {
    const_eval_select!(
        @capture { fmt: fmt::Arguments<'_>, force_no_backtrace: bool } -> !:
        if const #[track_caller] {
            // We don't unwind anyway at compile-time so we can call the regular `panic_fmt`.
            panic_fmt(fmt)
        } else #[track_caller] {
            if cfg!(panic = "immediate-abort") {
                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.
            unsafe extern "Rust" {
                #[lang = "panic_impl"]
                fn panic_impl(pi: &PanicInfo<'_>) -> !;
            }

            // PanicInfo with the `can_unwind` flag set to false forces an abort.
            let pi = PanicInfo::new(
                &fmt,
                Location::caller(),
                /* can_unwind */ false,
                force_no_backtrace,
            );

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

// Next we define a bunch of higher-level wrappers that all bottom out in the two core functions
// above.

/// The underlying implementation of core's `panic!` macro when no formatting is used.
// Never inline unless panic=immediate-abort to avoid code
// bloat at the call sites as much as possible.
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold)]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
#[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable
#[lang = "panic"] // used by lints and miri for panics
pub const fn panic(expr: &'static str) -> ! {
    // Use Arguments::from_str 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::from_str may allow the compiler to omit Formatter::pad from the
    // output binary, saving up to a few kilobytes.
    // However, this optimization only works for `'static` strings: `from_str` also makes this
    // message return `Some` from `Arguments::as_str`, which means it can become part of the panic
    // payload without any allocation or copying. Shorter-lived strings would become invalid as
    // stack frames get popped during unwinding, and couldn't be directly referenced from the
    // payload.
    panic_fmt(fmt::Arguments::from_str(expr));
}

// We generate functions for usage by compiler-generated assertions.
//
// Placing these functions in libcore means that all Rust programs can generate a jump into this
// code rather than expanding to panic("...") above, which adds extra bloat to call sites (for the
// constant string argument's pointer and length).
//
// This is especially important when this code is called often (e.g., with -Coverflow-checks) for
// reducing binary size impact.
macro_rules! panic_const {
    ($($lang:ident = $message:expr,)+) => {
        $(
            /// This is a panic called with a message that's a result of a MIR-produced Assert.
            //
            // never inline unless panic=immediate-abort to avoid code
            // bloat at the call sites as much as possible
            #[cfg_attr(not(panic = "immediate-abort"), inline(never), cold)]
            #[cfg_attr(panic = "immediate-abort", inline)]
            #[track_caller]
            #[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable
            #[lang = stringify!($lang)]
            pub const fn $lang() -> ! {
                // See the comment in `panic(&'static str)` for why we use `Arguments::from_str` here.
                panic_fmt(fmt::Arguments::from_str($message));
            }
        )+
    }
}

// Unfortunately this set of strings is replicated here and in a few places in the compiler in
// slightly different forms. It's not clear if there's a good way to deduplicate without adding
// special cases to the compiler (e.g., a const generic function wouldn't have a single definition
// shared across crates, which is exactly what we want here).
pub mod panic_const {
    use super::*;
    panic_const! {
        panic_const_add_overflow = "attempt to add with overflow",
        panic_const_sub_overflow = "attempt to subtract with overflow",
        panic_const_mul_overflow = "attempt to multiply with overflow",
        panic_const_div_overflow = "attempt to divide with overflow",
        panic_const_rem_overflow = "attempt to calculate the remainder with overflow",
        panic_const_neg_overflow = "attempt to negate with overflow",
        panic_const_shr_overflow = "attempt to shift right with overflow",
        panic_const_shl_overflow = "attempt to shift left with overflow",
        panic_const_div_by_zero = "attempt to divide by zero",
        panic_const_rem_by_zero = "attempt to calculate the remainder with a divisor of zero",
        panic_const_coroutine_resumed = "coroutine resumed after completion",
        panic_const_async_fn_resumed = "`async fn` resumed after completion",
        panic_const_async_gen_fn_resumed = "`async gen fn` resumed after completion",
        panic_const_gen_fn_none = "`gen fn` should just keep returning `None` after completion",
        panic_const_coroutine_resumed_panic = "coroutine resumed after panicking",
        panic_const_async_fn_resumed_panic = "`async fn` resumed after panicking",
        panic_const_async_gen_fn_resumed_panic = "`async gen fn` resumed after panicking",
        panic_const_gen_fn_none_panic = "`gen fn` should just keep returning `None` after panicking",
    }
    // Separated panic constants list for async drop feature
    // (May be joined when the corresponding lang items will be in the bootstrap)
    panic_const! {
        panic_const_coroutine_resumed_drop = "coroutine resumed after async drop",
        panic_const_async_fn_resumed_drop = "`async fn` resumed after async drop",
        panic_const_async_gen_fn_resumed_drop = "`async gen fn` resumed after async drop",
        panic_const_gen_fn_none_drop = "`gen fn` resumed after async drop",
    }
}

/// Like `panic`, but without unwinding and track_caller to reduce the impact on codesize on the caller.
/// If you want `#[track_caller]` for nicer errors, call `panic_nounwind_fmt` directly.
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold)]
#[cfg_attr(panic = "immediate-abort", inline)]
#[lang = "panic_nounwind"] // needed by codegen for non-unwinding panics
#[rustc_nounwind]
#[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable
pub const fn panic_nounwind(expr: &'static str) -> ! {
    panic_nounwind_fmt(fmt::Arguments::from_str(expr), /* force_no_backtrace */ false);
}

/// Like `panic_nounwind`, but also inhibits showing a backtrace.
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold)]
#[cfg_attr(panic = "immediate-abort", inline)]
#[rustc_nounwind]
pub fn panic_nounwind_nobacktrace(expr: &'static str) -> ! {
    panic_nounwind_fmt(fmt::Arguments::from_str(expr), /* force_no_backtrace */ true);
}

#[inline]
#[track_caller]
#[rustc_diagnostic_item = "unreachable_display"] // needed for `non-fmt-panics` lint
pub fn unreachable_display<T: fmt::Display>(x: &T) -> ! {
    panic_fmt(format_args!("internal error: entered unreachable code: {}", *x));
}

/// This exists solely for the 2015 edition `panic!` macro to trigger
/// a lint on `panic!(my_str_variable);`.
#[inline]
#[track_caller]
#[rustc_diagnostic_item = "panic_str_2015"]
#[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable
pub const fn panic_str_2015(expr: &str) -> ! {
    panic_display(&expr);
}

#[inline]
#[track_caller]
#[lang = "panic_display"] // needed for const-evaluated panics
#[rustc_do_not_const_check] // hooked by const-eval
#[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable
pub const fn panic_display<T: fmt::Display>(x: &T) -> ! {
    panic_fmt(format_args!("{}", *x));
}

#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[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!(panic = "immediate-abort") {
        super::intrinsics::abort()
    }

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

#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
#[lang = "panic_misaligned_pointer_dereference"] // needed by codegen for panic on misaligned pointer deref
#[rustc_nounwind] // `CheckAlignment` MIR pass requires this function to never unwind
fn panic_misaligned_pointer_dereference(required: usize, found: usize) -> ! {
    if cfg!(panic = "immediate-abort") {
        super::intrinsics::abort()
    }

    panic_nounwind_fmt(
        format_args!(
            "misaligned pointer dereference: address must be a multiple of {required:#x} but is {found:#x}"
        ),
        /* force_no_backtrace */ false,
    )
}

#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
#[lang = "panic_null_pointer_dereference"] // needed by codegen for panic on null pointer deref
#[rustc_nounwind] // `CheckNull` MIR pass requires this function to never unwind
fn panic_null_pointer_dereference() -> ! {
    if cfg!(panic = "immediate-abort") {
        super::intrinsics::abort()
    }

    panic_nounwind_fmt(
        format_args!("null pointer dereference occurred"),
        /* force_no_backtrace */ false,
    )
}

#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
#[lang = "panic_invalid_enum_construction"] // needed by codegen for panic on invalid enum construction.
#[rustc_nounwind] // `CheckEnums` MIR pass requires this function to never unwind
fn panic_invalid_enum_construction(source: u128) -> ! {
    if cfg!(panic = "immediate-abort") {
        super::intrinsics::abort()
    }

    panic_nounwind_fmt(
        format_args!("trying to construct an enum from an invalid value {source:#x}"),
        /* force_no_backtrace */ false,
    )
}

/// Panics because we cannot unwind out of a function.
///
/// This is a separate function to avoid the codesize impact of each crate containing the string to
/// pass to `panic_nounwind`.
///
/// This function is called directly by the codegen backend, and must not have
/// any extra arguments (including those synthesized by track_caller).
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[lang = "panic_cannot_unwind"] // needed by codegen for panic in nounwind function
#[rustc_nounwind]
fn panic_cannot_unwind() -> ! {
    // Keep the text in sync with `UnwindTerminateReason::as_str` in `rustc_middle`.
    panic_nounwind("panic in a function that cannot unwind")
}

/// Panics because we are unwinding out of a destructor during cleanup.
///
/// This is a separate function to avoid the codesize impact of each crate containing the string to
/// pass to `panic_nounwind`.
///
/// This function is called directly by the codegen backend, and must not have
/// any extra arguments (including those synthesized by track_caller).
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[lang = "panic_in_cleanup"] // needed by codegen for panic in nounwind function
#[rustc_nounwind]
fn panic_in_cleanup() -> ! {
    // Keep the text in sync with `UnwindTerminateReason::as_str` in `rustc_middle`.
    panic_nounwind_nobacktrace("panic in a destructor during cleanup")
}

/// This function is used instead of panic_fmt in const eval.
#[lang = "const_panic_fmt"] // needed by const-eval machine to replace calls to `panic_fmt` lang item
#[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable
pub const fn const_panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
    if let Some(msg) = fmt.as_str() {
        // The panic_display function is hooked by const eval.
        panic_display(&msg);
    } else {
        // SAFETY: This is only evaluated at compile time, which reliably
        // handles this UB (in case this branch turns out to be reachable
        // somehow).
        unsafe { crate::hint::unreachable_unchecked() };
    }
}

#[derive(Debug)]
#[doc(hidden)]
pub enum AssertKind {
    Eq,
    Ne,
    Match,
}

/// Internal function for `assert_eq!` and `assert_ne!` macros
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
#[doc(hidden)]
pub fn assert_failed<T, U>(
    kind: AssertKind,
    left: &T,
    right: &U,
    args: Option<fmt::Arguments<'_>>,
) -> !
where
    T: fmt::Debug + ?Sized,
    U: fmt::Debug + ?Sized,
{
    assert_failed_inner(kind, &left, &right, args)
}

/// Internal function for `assert_match!`
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
#[doc(hidden)]
pub fn assert_matches_failed<T: fmt::Debug + ?Sized>(
    left: &T,
    right: &str,
    args: Option<fmt::Arguments<'_>>,
) -> ! {
    // The pattern is a string so it can be displayed directly.
    struct Pattern<'a>(&'a str);
    impl fmt::Debug for Pattern<'_> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str(self.0)
        }
    }
    assert_failed_inner(AssertKind::Match, &left, &Pattern(right), args);
}

/// Non-generic version of the above functions, to avoid code bloat.
#[cfg_attr(not(panic = "immediate-abort"), inline(never), cold, optimize(size))]
#[cfg_attr(panic = "immediate-abort", inline)]
#[track_caller]
fn assert_failed_inner(
    kind: AssertKind,
    left: &dyn fmt::Debug,
    right: &dyn fmt::Debug,
    args: Option<fmt::Arguments<'_>>,
) -> ! {
    let op = match kind {
        AssertKind::Eq => "==",
        AssertKind::Ne => "!=",
        AssertKind::Match => "matches",
    };

    match args {
        Some(args) => panic!(
            r#"assertion `left {op} right` failed: {args}
  left: {left:?}
 right: {right:?}"#
        ),
        None => panic!(
            r#"assertion `left {op} right` failed
  left: {left:?}
 right: {right:?}"#
        ),
    }
}