1 //! Implementation of various bits and pieces of the `panic!` macro and
2 //! associated runtime pieces.
4 //! Specifically, this module contains the implementation of:
7 //! * Executing a panic up to doing the actual implementation
8 //! * Shims around "try"
10 use core::panic::{BoxMeUp, Location, PanicInfo};
14 use crate::intrinsics;
15 use crate::mem::{self, ManuallyDrop};
18 use crate::sync::atomic::{AtomicBool, Ordering};
19 use crate::sys::stdio::panic_output;
20 use crate::sys_common::backtrace::{self, RustBacktrace};
21 use crate::sys_common::rwlock::RWLock;
22 use crate::sys_common::{thread_info, util};
26 use crate::io::set_panic;
27 // make sure to use the stderr output configured
28 // by libtest in the real copy of std
30 use realstd::io::set_panic;
32 // Binary interface to the panic runtime that the standard library depends on.
34 // The standard library is tagged with `#![needs_panic_runtime]` (introduced in
35 // RFC 1513) to indicate that it requires some other crate tagged with
36 // `#![panic_runtime]` to exist somewhere. Each panic runtime is intended to
37 // implement these symbols (with the same signatures) so we can get matched up
40 // One day this may look a little less ad-hoc with the compiler helping out to
41 // hook up these functions, but it is not this day!
42 #[allow(improper_ctypes)]
44 fn __rust_maybe_catch_panic(
48 vtable_ptr: *mut usize,
51 /// `payload` is actually a `*mut &mut dyn BoxMeUp` but that would cause FFI warnings.
52 /// It cannot be `Box<dyn BoxMeUp>` because the other end of this call does not depend
53 /// on liballoc, and thus cannot use `Box`.
55 fn __rust_start_panic(payload: usize) -> u32;
58 /// This function is called by the panic runtime if FFI code catches a Rust
59 /// panic but doesn't rethrow it. We don't support this case since it messes
60 /// with our panic count.
61 #[rustc_std_internal_symbol]
62 extern "C" fn __rust_drop_panic() -> ! {
63 rtabort!("Rust panics must be rethrown");
66 #[derive(Copy, Clone)]
69 Custom(*mut (dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send)),
72 static HOOK_LOCK: RWLock = RWLock::new();
73 static mut HOOK: Hook = Hook::Default;
75 /// Registers a custom panic hook, replacing any that was previously registered.
77 /// The panic hook is invoked when a thread panics, but before the panic runtime
78 /// is invoked. As such, the hook will run with both the aborting and unwinding
79 /// runtimes. The default hook prints a message to standard error and generates
80 /// a backtrace if requested, but this behavior can be customized with the
81 /// `set_hook` and [`take_hook`] functions.
83 /// [`take_hook`]: ./fn.take_hook.html
85 /// The hook is provided with a `PanicInfo` struct which contains information
86 /// about the origin of the panic, including the payload passed to `panic!` and
87 /// the source code location from which the panic originated.
89 /// The panic hook is a global resource.
93 /// Panics if called from a panicking thread.
97 /// The following will print "Custom panic hook":
102 /// panic::set_hook(Box::new(|_| {
103 /// println!("Custom panic hook");
106 /// panic!("Normal panic");
108 #[stable(feature = "panic_hooks", since = "1.10.0")]
109 pub fn set_hook(hook: Box<dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send>) {
110 if thread::panicking() {
111 panic!("cannot modify the panic hook from a panicking thread");
117 HOOK = Hook::Custom(Box::into_raw(hook));
118 HOOK_LOCK.write_unlock();
120 if let Hook::Custom(ptr) = old_hook {
121 #[allow(unused_must_use)]
129 /// Unregisters the current panic hook, returning it.
131 /// *See also the function [`set_hook`].*
133 /// [`set_hook`]: ./fn.set_hook.html
135 /// If no custom hook is registered, the default hook will be returned.
139 /// Panics if called from a panicking thread.
143 /// The following will print "Normal panic":
148 /// panic::set_hook(Box::new(|_| {
149 /// println!("Custom panic hook");
152 /// let _ = panic::take_hook();
154 /// panic!("Normal panic");
156 #[stable(feature = "panic_hooks", since = "1.10.0")]
157 pub fn take_hook() -> Box<dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send> {
158 if thread::panicking() {
159 panic!("cannot modify the panic hook from a panicking thread");
165 HOOK = Hook::Default;
166 HOOK_LOCK.write_unlock();
169 Hook::Default => Box::new(default_hook),
170 Hook::Custom(ptr) => Box::from_raw(ptr),
175 fn default_hook(info: &PanicInfo<'_>) {
176 // If this is a double panic, make sure that we print a backtrace
177 // for this panic. Otherwise only print it if logging is enabled.
178 let backtrace_env = if update_panic_count(0) >= 2 {
179 RustBacktrace::Print(backtrace_rs::PrintFmt::Full)
181 backtrace::rust_backtrace_env()
184 // The current implementation always returns `Some`.
185 let location = info.location().unwrap();
187 let msg = match info.payload().downcast_ref::<&'static str>() {
189 None => match info.payload().downcast_ref::<String>() {
194 let thread = thread_info::current_thread();
195 let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
197 let write = |err: &mut dyn crate::io::Write| {
198 let _ = writeln!(err, "thread '{}' panicked at '{}', {}", name, msg, location);
200 static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
202 match backtrace_env {
203 RustBacktrace::Print(format) => drop(backtrace::print(err, format)),
204 RustBacktrace::Disabled => {}
205 RustBacktrace::RuntimeDisabled => {
206 if FIRST_PANIC.swap(false, Ordering::SeqCst) {
209 "note: run with `RUST_BACKTRACE=1` \
210 environment variable to display a backtrace."
217 if let Some(mut local) = set_panic(None) {
218 // NB. In `cfg(test)` this uses the forwarding impl
219 // for `Box<dyn (::realstd::io::Write) + Send>`.
221 set_panic(Some(local));
222 } else if let Some(mut out) = panic_output() {
229 #[unstable(feature = "update_panic_count", issue = "none")]
230 pub fn update_panic_count(amt: isize) -> usize {
231 use crate::cell::Cell;
232 thread_local! { static PANIC_COUNT: Cell<usize> = Cell::new(0) }
234 PANIC_COUNT.with(|c| {
235 let next = (c.get() as isize + amt) as usize;
242 pub use realstd::rt::update_panic_count;
244 /// Invoke a closure, capturing the cause of an unwinding panic if one occurs.
245 pub unsafe fn r#try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<dyn Any + Send>> {
251 // We do some sketchy operations with ownership here for the sake of
252 // performance. We can only pass pointers down to
253 // `__rust_maybe_catch_panic` (can't pass objects by value), so we do all
254 // the ownership tracking here manually using a union.
256 // We go through a transition where:
258 // * First, we set the data to be the closure that we're going to call.
259 // * When we make the function call, the `do_call` function below, we take
260 // ownership of the function pointer. At this point the `Data` union is
261 // entirely uninitialized.
262 // * If the closure successfully returns, we write the return value into the
263 // data's return slot. Note that `ptr::write` is used as it's overwriting
264 // uninitialized data.
265 // * Finally, when we come back out of the `__rust_maybe_catch_panic` we're
266 // in one of two states:
268 // 1. The closure didn't panic, in which case the return value was
269 // filled in. We move it out of `data` and return it.
270 // 2. The closure panicked, in which case the return value wasn't
271 // filled in. In this case the entire `data` union is invalid, so
272 // there is no need to drop anything.
274 // Once we stack all that together we should have the "most efficient'
275 // method of calling a catch panic whilst juggling ownership.
276 let mut any_data = 0;
277 let mut any_vtable = 0;
278 let mut data = Data { f: ManuallyDrop::new(f) };
280 let r = __rust_maybe_catch_panic(
282 &mut data as *mut _ as *mut u8,
288 debug_assert!(update_panic_count(0) == 0);
289 Ok(ManuallyDrop::into_inner(data.r))
291 update_panic_count(-1);
292 debug_assert!(update_panic_count(0) == 0);
293 Err(mem::transmute(raw::TraitObject {
294 data: any_data as *mut _,
295 vtable: any_vtable as *mut _,
299 fn do_call<F: FnOnce() -> R, R>(data: *mut u8) {
301 let data = data as *mut Data<F, R>;
302 let data = &mut (*data);
303 let f = ManuallyDrop::take(&mut data.f);
304 data.r = ManuallyDrop::new(f());
309 /// Determines whether the current thread is unwinding because of panic.
310 pub fn panicking() -> bool {
311 update_panic_count(0) != 0
314 /// The entry point for panicking with a formatted message.
316 /// This is designed to reduce the amount of code required at the call
317 /// site as much as possible (so that `panic!()` has as low an impact
318 /// on (e.g.) the inlining of other functions as possible), by moving
319 /// the actual formatting into this shared place.
320 #[unstable(feature = "libstd_sys_internals", reason = "used by the panic! macro", issue = "none")]
322 // If panic_immediate_abort, inline the abort call,
323 // otherwise avoid inlining because of it is cold path.
324 #[cfg_attr(not(feature = "panic_immediate_abort"), track_caller)]
325 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
326 #[cfg_attr(feature = "panic_immediate_abort", inline)]
327 pub fn begin_panic_fmt(msg: &fmt::Arguments<'_>) -> ! {
328 if cfg!(feature = "panic_immediate_abort") {
329 unsafe { intrinsics::abort() }
332 let info = PanicInfo::internal_constructor(Some(msg), Location::caller());
333 begin_panic_handler(&info)
336 /// Entry point of panics from the libcore crate (`panic_impl` lang item).
337 #[cfg_attr(not(test), panic_handler)]
339 pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
340 struct PanicPayload<'a> {
341 inner: &'a fmt::Arguments<'a>,
342 string: Option<String>,
345 impl<'a> PanicPayload<'a> {
346 fn new(inner: &'a fmt::Arguments<'a>) -> PanicPayload<'a> {
347 PanicPayload { inner, string: None }
350 fn fill(&mut self) -> &mut String {
351 use crate::fmt::Write;
353 let inner = self.inner;
354 // Lazily, the first time this gets called, run the actual string formatting.
355 self.string.get_or_insert_with(|| {
356 let mut s = String::new();
357 drop(s.write_fmt(*inner));
363 unsafe impl<'a> BoxMeUp for PanicPayload<'a> {
364 fn take_box(&mut self) -> *mut (dyn Any + Send) {
365 // We do two allocations here, unfortunately. But (a) they're required with the current
366 // scheme, and (b) we don't handle panic + OOM properly anyway (see comment in
367 // begin_panic below).
368 let contents = mem::take(self.fill());
369 Box::into_raw(Box::new(contents))
372 fn get(&mut self) -> &(dyn Any + Send) {
377 let loc = info.location().unwrap(); // The current implementation always returns Some
378 let msg = info.message().unwrap(); // The current implementation always returns Some
379 rust_panic_with_hook(&mut PanicPayload::new(msg), info.message(), loc);
382 /// This is the entry point of panicking for the non-format-string variants of
383 /// panic!() and assert!(). In particular, this is the only entry point that supports
384 /// arbitrary payloads, not just format strings.
385 #[unstable(feature = "libstd_sys_internals", reason = "used by the panic! macro", issue = "none")]
386 #[cfg_attr(not(test), lang = "begin_panic")]
387 // lang item for CTFE panic support
388 // never inline unless panic_immediate_abort to avoid code
389 // bloat at the call sites as much as possible
390 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
393 pub fn begin_panic<M: Any + Send>(msg: M, #[cfg(bootstrap)] _: &(&str, u32, u32)) -> ! {
394 if cfg!(feature = "panic_immediate_abort") {
395 unsafe { intrinsics::abort() }
398 rust_panic_with_hook(&mut PanicPayload::new(msg), None, Location::caller());
400 struct PanicPayload<A> {
404 impl<A: Send + 'static> PanicPayload<A> {
405 fn new(inner: A) -> PanicPayload<A> {
406 PanicPayload { inner: Some(inner) }
410 unsafe impl<A: Send + 'static> BoxMeUp for PanicPayload<A> {
411 fn take_box(&mut self) -> *mut (dyn Any + Send) {
412 // Note that this should be the only allocation performed in this code path. Currently
413 // this means that panic!() on OOM will invoke this code path, but then again we're not
414 // really ready for panic on OOM anyway. If we do start doing this, then we should
415 // propagate this allocation to be performed in the parent of this thread instead of the
416 // thread that's panicking.
417 let data = match self.inner.take() {
418 Some(a) => Box::new(a) as Box<dyn Any + Send>,
419 None => process::abort(),
424 fn get(&mut self) -> &(dyn Any + Send) {
427 None => process::abort(),
433 /// Central point for dispatching panics.
435 /// Executes the primary logic for a panic, including checking for recursive
436 /// panics, panic hooks, and finally dispatching to the panic runtime to either
438 fn rust_panic_with_hook(
439 payload: &mut dyn BoxMeUp,
440 message: Option<&fmt::Arguments<'_>>,
441 location: &Location<'_>,
443 let panics = update_panic_count(1);
445 // If this is the third nested call (e.g., panics == 2, this is 0-indexed),
446 // the panic hook probably triggered the last panic, otherwise the
447 // double-panic check would have aborted the process. In this case abort the
448 // process real quickly as we don't want to try calling it again as it'll
449 // probably just panic again.
451 util::dumb_print(format_args!(
452 "thread panicked while processing \
455 unsafe { intrinsics::abort() }
459 let mut info = PanicInfo::internal_constructor(message, location);
462 // Some platforms (like wasm) know that printing to stderr won't ever actually
463 // print anything, and if that's the case we can skip the default
464 // hook. Since string formatting happens lazily when calling `payload`
465 // methods, this means we avoid formatting the string at all!
466 // (The panic runtime might still call `payload.take_box()` though and trigger
468 Hook::Default if panic_output().is_none() => {}
470 info.set_payload(payload.get());
473 Hook::Custom(ptr) => {
474 info.set_payload(payload.get());
478 HOOK_LOCK.read_unlock();
482 // If a thread panics while it's already unwinding then we
483 // have limited options. Currently our preference is to
484 // just abort. In the future we may consider resuming
485 // unwinding or otherwise exiting the thread cleanly.
486 util::dumb_print(format_args!(
487 "thread panicked while panicking. \
490 unsafe { intrinsics::abort() }
496 /// This is the entry point for `resume_unwind`.
497 /// It just forwards the payload to the panic runtime.
498 pub fn rust_panic_without_hook(payload: Box<dyn Any + Send>) -> ! {
499 update_panic_count(1);
501 struct RewrapBox(Box<dyn Any + Send>);
503 unsafe impl BoxMeUp for RewrapBox {
504 fn take_box(&mut self) -> *mut (dyn Any + Send) {
505 Box::into_raw(mem::replace(&mut self.0, Box::new(())))
508 fn get(&mut self) -> &(dyn Any + Send) {
513 rust_panic(&mut RewrapBox(payload))
516 /// An unmangled function (through `rustc_std_internal_symbol`) on which to slap
519 #[cfg_attr(not(test), rustc_std_internal_symbol)]
520 fn rust_panic(mut msg: &mut dyn BoxMeUp) -> ! {
522 let obj = &mut msg as *mut &mut dyn BoxMeUp;
523 __rust_start_panic(obj as usize)
525 rtabort!("failed to initiate panic, error {}", code)