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.
62 #[rustc_std_internal_symbol]
63 extern "C" fn __rust_drop_panic() -> ! {
64 rtabort!("Rust panics must be rethrown");
67 #[derive(Copy, Clone)]
70 Custom(*mut (dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send)),
73 static HOOK_LOCK: RWLock = RWLock::new();
74 static mut HOOK: Hook = Hook::Default;
76 /// Registers a custom panic hook, replacing any that was previously registered.
78 /// The panic hook is invoked when a thread panics, but before the panic runtime
79 /// is invoked. As such, the hook will run with both the aborting and unwinding
80 /// runtimes. The default hook prints a message to standard error and generates
81 /// a backtrace if requested, but this behavior can be customized with the
82 /// `set_hook` and [`take_hook`] functions.
84 /// [`take_hook`]: ./fn.take_hook.html
86 /// The hook is provided with a `PanicInfo` struct which contains information
87 /// about the origin of the panic, including the payload passed to `panic!` and
88 /// the source code location from which the panic originated.
90 /// The panic hook is a global resource.
94 /// Panics if called from a panicking thread.
98 /// The following will print "Custom panic hook":
103 /// panic::set_hook(Box::new(|_| {
104 /// println!("Custom panic hook");
107 /// panic!("Normal panic");
109 #[stable(feature = "panic_hooks", since = "1.10.0")]
110 pub fn set_hook(hook: Box<dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send>) {
111 if thread::panicking() {
112 panic!("cannot modify the panic hook from a panicking thread");
118 HOOK = Hook::Custom(Box::into_raw(hook));
119 HOOK_LOCK.write_unlock();
121 if let Hook::Custom(ptr) = old_hook {
122 #[allow(unused_must_use)]
130 /// Unregisters the current panic hook, returning it.
132 /// *See also the function [`set_hook`].*
134 /// [`set_hook`]: ./fn.set_hook.html
136 /// If no custom hook is registered, the default hook will be returned.
140 /// Panics if called from a panicking thread.
144 /// The following will print "Normal panic":
149 /// panic::set_hook(Box::new(|_| {
150 /// println!("Custom panic hook");
153 /// let _ = panic::take_hook();
155 /// panic!("Normal panic");
157 #[stable(feature = "panic_hooks", since = "1.10.0")]
158 pub fn take_hook() -> Box<dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send> {
159 if thread::panicking() {
160 panic!("cannot modify the panic hook from a panicking thread");
166 HOOK = Hook::Default;
167 HOOK_LOCK.write_unlock();
170 Hook::Default => Box::new(default_hook),
171 Hook::Custom(ptr) => Box::from_raw(ptr),
176 fn default_hook(info: &PanicInfo<'_>) {
177 // If this is a double panic, make sure that we print a backtrace
178 // for this panic. Otherwise only print it if logging is enabled.
179 let backtrace_env = if update_panic_count(0) >= 2 {
180 RustBacktrace::Print(backtrace_rs::PrintFmt::Full)
182 backtrace::rust_backtrace_env()
185 // The current implementation always returns `Some`.
186 let location = info.location().unwrap();
188 let msg = match info.payload().downcast_ref::<&'static str>() {
190 None => match info.payload().downcast_ref::<String>() {
195 let thread = thread_info::current_thread();
196 let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
198 let write = |err: &mut dyn crate::io::Write| {
199 let _ = writeln!(err, "thread '{}' panicked at '{}', {}", name, msg, location);
201 static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
203 match backtrace_env {
204 RustBacktrace::Print(format) => drop(backtrace::print(err, format)),
205 RustBacktrace::Disabled => {}
206 RustBacktrace::RuntimeDisabled => {
207 if FIRST_PANIC.swap(false, Ordering::SeqCst) {
210 "note: run with `RUST_BACKTRACE=1` \
211 environment variable to display a backtrace"
218 if let Some(mut local) = set_panic(None) {
219 // NB. In `cfg(test)` this uses the forwarding impl
220 // for `Box<dyn (::realstd::io::Write) + Send>`.
222 set_panic(Some(local));
223 } else if let Some(mut out) = panic_output() {
230 #[unstable(feature = "update_panic_count", issue = "none")]
231 pub fn update_panic_count(amt: isize) -> usize {
232 use crate::cell::Cell;
233 thread_local! { static PANIC_COUNT: Cell<usize> = Cell::new(0) }
235 PANIC_COUNT.with(|c| {
236 let next = (c.get() as isize + amt) as usize;
243 pub use realstd::rt::update_panic_count;
245 /// Invoke a closure, capturing the cause of an unwinding panic if one occurs.
246 pub unsafe fn r#try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<dyn Any + Send>> {
252 // We do some sketchy operations with ownership here for the sake of
253 // performance. We can only pass pointers down to
254 // `__rust_maybe_catch_panic` (can't pass objects by value), so we do all
255 // the ownership tracking here manually using a union.
257 // We go through a transition where:
259 // * First, we set the data to be the closure that we're going to call.
260 // * When we make the function call, the `do_call` function below, we take
261 // ownership of the function pointer. At this point the `Data` union is
262 // entirely uninitialized.
263 // * If the closure successfully returns, we write the return value into the
264 // data's return slot. Note that `ptr::write` is used as it's overwriting
265 // uninitialized data.
266 // * Finally, when we come back out of the `__rust_maybe_catch_panic` we're
267 // in one of two states:
269 // 1. The closure didn't panic, in which case the return value was
270 // filled in. We move it out of `data` and return it.
271 // 2. The closure panicked, in which case the return value wasn't
272 // filled in. In this case the entire `data` union is invalid, so
273 // there is no need to drop anything.
275 // Once we stack all that together we should have the "most efficient'
276 // method of calling a catch panic whilst juggling ownership.
277 let mut any_data = 0;
278 let mut any_vtable = 0;
279 let mut data = Data { f: ManuallyDrop::new(f) };
281 let r = __rust_maybe_catch_panic(
283 &mut data as *mut _ as *mut u8,
289 debug_assert!(update_panic_count(0) == 0);
290 Ok(ManuallyDrop::into_inner(data.r))
292 update_panic_count(-1);
293 debug_assert!(update_panic_count(0) == 0);
294 Err(mem::transmute(raw::TraitObject {
295 data: any_data as *mut _,
296 vtable: any_vtable as *mut _,
300 fn do_call<F: FnOnce() -> R, R>(data: *mut u8) {
302 let data = data as *mut Data<F, R>;
303 let data = &mut (*data);
304 let f = ManuallyDrop::take(&mut data.f);
305 data.r = ManuallyDrop::new(f());
310 /// Determines whether the current thread is unwinding because of panic.
311 pub fn panicking() -> bool {
312 update_panic_count(0) != 0
315 /// The entry point for panicking with a formatted message.
317 /// This is designed to reduce the amount of code required at the call
318 /// site as much as possible (so that `panic!()` has as low an impact
319 /// on (e.g.) the inlining of other functions as possible), by moving
320 /// the actual formatting into this shared place.
321 #[unstable(feature = "libstd_sys_internals", reason = "used by the panic! macro", issue = "none")]
323 // If panic_immediate_abort, inline the abort call,
324 // otherwise avoid inlining because of it is cold path.
325 #[cfg_attr(not(feature = "panic_immediate_abort"), track_caller)]
326 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
327 #[cfg_attr(feature = "panic_immediate_abort", inline)]
328 pub fn begin_panic_fmt(msg: &fmt::Arguments<'_>) -> ! {
329 if cfg!(feature = "panic_immediate_abort") {
330 unsafe { intrinsics::abort() }
333 let info = PanicInfo::internal_constructor(Some(msg), Location::caller());
334 begin_panic_handler(&info)
337 /// Entry point of panics from the libcore crate (`panic_impl` lang item).
338 #[cfg_attr(not(test), panic_handler)]
340 pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
341 struct PanicPayload<'a> {
342 inner: &'a fmt::Arguments<'a>,
343 string: Option<String>,
346 impl<'a> PanicPayload<'a> {
347 fn new(inner: &'a fmt::Arguments<'a>) -> PanicPayload<'a> {
348 PanicPayload { inner, string: None }
351 fn fill(&mut self) -> &mut String {
352 use crate::fmt::Write;
354 let inner = self.inner;
355 // Lazily, the first time this gets called, run the actual string formatting.
356 self.string.get_or_insert_with(|| {
357 let mut s = String::new();
358 drop(s.write_fmt(*inner));
364 unsafe impl<'a> BoxMeUp for PanicPayload<'a> {
365 fn take_box(&mut self) -> *mut (dyn Any + Send) {
366 // We do two allocations here, unfortunately. But (a) they're required with the current
367 // scheme, and (b) we don't handle panic + OOM properly anyway (see comment in
368 // begin_panic below).
369 let contents = mem::take(self.fill());
370 Box::into_raw(Box::new(contents))
373 fn get(&mut self) -> &(dyn Any + Send) {
378 let loc = info.location().unwrap(); // The current implementation always returns Some
379 let msg = info.message().unwrap(); // The current implementation always returns Some
380 rust_panic_with_hook(&mut PanicPayload::new(msg), info.message(), loc);
383 /// This is the entry point of panicking for the non-format-string variants of
384 /// panic!() and assert!(). In particular, this is the only entry point that supports
385 /// arbitrary payloads, not just format strings.
386 #[unstable(feature = "libstd_sys_internals", reason = "used by the panic! macro", issue = "none")]
387 #[cfg_attr(not(test), lang = "begin_panic")]
388 // lang item for CTFE panic support
389 // never inline unless panic_immediate_abort to avoid code
390 // bloat at the call sites as much as possible
391 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
394 pub fn begin_panic<M: Any + Send>(msg: M, #[cfg(bootstrap)] _: &(&str, u32, u32)) -> ! {
395 if cfg!(feature = "panic_immediate_abort") {
396 unsafe { intrinsics::abort() }
399 rust_panic_with_hook(&mut PanicPayload::new(msg), None, Location::caller());
401 struct PanicPayload<A> {
405 impl<A: Send + 'static> PanicPayload<A> {
406 fn new(inner: A) -> PanicPayload<A> {
407 PanicPayload { inner: Some(inner) }
411 unsafe impl<A: Send + 'static> BoxMeUp for PanicPayload<A> {
412 fn take_box(&mut self) -> *mut (dyn Any + Send) {
413 // Note that this should be the only allocation performed in this code path. Currently
414 // this means that panic!() on OOM will invoke this code path, but then again we're not
415 // really ready for panic on OOM anyway. If we do start doing this, then we should
416 // propagate this allocation to be performed in the parent of this thread instead of the
417 // thread that's panicking.
418 let data = match self.inner.take() {
419 Some(a) => Box::new(a) as Box<dyn Any + Send>,
420 None => process::abort(),
425 fn get(&mut self) -> &(dyn Any + Send) {
428 None => process::abort(),
434 /// Central point for dispatching panics.
436 /// Executes the primary logic for a panic, including checking for recursive
437 /// panics, panic hooks, and finally dispatching to the panic runtime to either
439 fn rust_panic_with_hook(
440 payload: &mut dyn BoxMeUp,
441 message: Option<&fmt::Arguments<'_>>,
442 location: &Location<'_>,
444 let panics = update_panic_count(1);
446 // If this is the third nested call (e.g., panics == 2, this is 0-indexed),
447 // the panic hook probably triggered the last panic, otherwise the
448 // double-panic check would have aborted the process. In this case abort the
449 // process real quickly as we don't want to try calling it again as it'll
450 // probably just panic again.
452 util::dumb_print(format_args!(
453 "thread panicked while processing \
456 unsafe { intrinsics::abort() }
460 let mut info = PanicInfo::internal_constructor(message, location);
463 // Some platforms (like wasm) know that printing to stderr won't ever actually
464 // print anything, and if that's the case we can skip the default
465 // hook. Since string formatting happens lazily when calling `payload`
466 // methods, this means we avoid formatting the string at all!
467 // (The panic runtime might still call `payload.take_box()` though and trigger
469 Hook::Default if panic_output().is_none() => {}
471 info.set_payload(payload.get());
474 Hook::Custom(ptr) => {
475 info.set_payload(payload.get());
479 HOOK_LOCK.read_unlock();
483 // If a thread panics while it's already unwinding then we
484 // have limited options. Currently our preference is to
485 // just abort. In the future we may consider resuming
486 // unwinding or otherwise exiting the thread cleanly.
487 util::dumb_print(format_args!(
488 "thread panicked while panicking. \
491 unsafe { intrinsics::abort() }
497 /// This is the entry point for `resume_unwind`.
498 /// It just forwards the payload to the panic runtime.
499 pub fn rust_panic_without_hook(payload: Box<dyn Any + Send>) -> ! {
500 update_panic_count(1);
502 struct RewrapBox(Box<dyn Any + Send>);
504 unsafe impl BoxMeUp for RewrapBox {
505 fn take_box(&mut self) -> *mut (dyn Any + Send) {
506 Box::into_raw(mem::replace(&mut self.0, Box::new(())))
509 fn get(&mut self) -> &(dyn Any + Send) {
514 rust_panic(&mut RewrapBox(payload))
517 /// An unmangled function (through `rustc_std_internal_symbol`) on which to slap
520 #[cfg_attr(not(test), rustc_std_internal_symbol)]
521 fn rust_panic(mut msg: &mut dyn BoxMeUp) -> ! {
523 let obj = &mut msg as *mut &mut dyn BoxMeUp;
524 __rust_start_panic(obj as usize)
526 rtabort!("failed to initiate panic, error {}", code)