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    //! Panic support in the standard library.

    #![stable(feature = "std_panic", since = "1.9.0")]

    use any::Any;
    use cell::UnsafeCell;
    use fmt;
    use future::Future;
    use pin::Pin;
    use ops::{Deref, DerefMut};
    use panicking;
    use ptr::{Unique, NonNull};
    use rc::Rc;
    use sync::{Arc, Mutex, RwLock, atomic};
    use task::{LocalWaker, Poll};
    use thread::Result;

    #[stable(feature = "panic_hooks", since = "1.10.0")]
    pub use panicking::{take_hook, set_hook};

    #[stable(feature = "panic_hooks", since = "1.10.0")]
    pub use core::panic::{PanicInfo, Location};

    /// A marker trait which represents "panic safe" types in Rust.
    ///
    /// This trait is implemented by default for many types and behaves similarly in
    /// terms of inference of implementation to the [`Send`] and [`Sync`] traits. The
    /// purpose of this trait is to encode what types are safe to cross a [`catch_unwind`]
    /// boundary with no fear of unwind safety.
    ///
    /// [`Send`]: ../marker/trait.Send.html
    /// [`Sync`]: ../marker/trait.Sync.html
    /// [`catch_unwind`]: ./fn.catch_unwind.html
    ///
    /// ## What is unwind safety?
    ///
    /// In Rust a function can "return" early if it either panics or calls a
    /// function which transitively panics. This sort of control flow is not always
    /// anticipated, and has the possibility of causing subtle bugs through a
    /// combination of two critical components:
    ///
    /// 1. A data structure is in a temporarily invalid state when the thread
    ///    panics.
    /// 2. This broken invariant is then later observed.
    ///
    /// Typically in Rust, it is difficult to perform step (2) because catching a
    /// panic involves either spawning a thread (which in turns makes it difficult
    /// to later witness broken invariants) or using the `catch_unwind` function in this
    /// module. Additionally, even if an invariant is witnessed, it typically isn't a
    /// problem in Rust because there are no uninitialized values (like in C or C++).
    ///
    /// It is possible, however, for **logical** invariants to be broken in Rust,
    /// which can end up causing behavioral bugs. Another key aspect of unwind safety
    /// in Rust is that, in the absence of `unsafe` code, a panic cannot lead to
    /// memory unsafety.
    ///
    /// That was a bit of a whirlwind tour of unwind safety, but for more information
    /// about unwind safety and how it applies to Rust, see an [associated RFC][rfc].
    ///
    /// [rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1236-stabilize-catch-panic.md
    ///
    /// ## What is `UnwindSafe`?
    ///
    /// Now that we've got an idea of what unwind safety is in Rust, it's also
    /// important to understand what this trait represents. As mentioned above, one
    /// way to witness broken invariants is through the `catch_unwind` function in this
    /// module as it allows catching a panic and then re-using the environment of
    /// the closure.
    ///
    /// Simply put, a type `T` implements `UnwindSafe` if it cannot easily allow
    /// witnessing a broken invariant through the use of `catch_unwind` (catching a
    /// panic). This trait is an auto trait, so it is automatically implemented for
    /// many types, and it is also structurally composed (e.g., a struct is unwind
    /// safe if all of its components are unwind safe).
    ///
    /// Note, however, that this is not an unsafe trait, so there is not a succinct
    /// contract that this trait is providing. Instead it is intended as more of a
    /// "speed bump" to alert users of `catch_unwind` that broken invariants may be
    /// witnessed and may need to be accounted for.
    ///
    /// ## Who implements `UnwindSafe`?
    ///
    /// Types such as `&mut T` and `&RefCell<T>` are examples which are **not**
    /// unwind safe. The general idea is that any mutable state which can be shared
    /// across `catch_unwind` is not unwind safe by default. This is because it is very
    /// easy to witness a broken invariant outside of `catch_unwind` as the data is
    /// simply accessed as usual.
    ///
    /// Types like `&Mutex<T>`, however, are unwind safe because they implement
    /// poisoning by default. They still allow witnessing a broken invariant, but
    /// they already provide their own "speed bumps" to do so.
    ///
    /// ## When should `UnwindSafe` be used?
    ///
    /// It is not intended that most types or functions need to worry about this trait.
    /// It is only used as a bound on the `catch_unwind` function and as mentioned
    /// above, the lack of `unsafe` means it is mostly an advisory. The
    /// [`AssertUnwindSafe`] wrapper struct can be used to force this trait to be
    /// implemented for any closed over variables passed to `catch_unwind`.
    ///
    /// [`AssertUnwindSafe`]: ./struct.AssertUnwindSafe.html
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    #[rustc_on_unimplemented(
        message="the type `{Self}` may not be safely transferred across an unwind boundary",
        label="`{Self}` may not be safely transferred across an unwind boundary",
    )]
    pub auto trait UnwindSafe {}

    /// A marker trait representing types where a shared reference is considered
    /// unwind safe.
    ///
    /// This trait is namely not implemented by [`UnsafeCell`], the root of all
    /// interior mutability.
    ///
    /// This is a "helper marker trait" used to provide impl blocks for the
    /// [`UnwindSafe`] trait, for more information see that documentation.
    ///
    /// [`UnsafeCell`]: ../cell/struct.UnsafeCell.html
    /// [`UnwindSafe`]: ./trait.UnwindSafe.html
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    #[rustc_on_unimplemented(
        message="the type `{Self}` may contain interior mutability and a reference may not be safely \
                 transferrable across a catch_unwind boundary",
        label="`{Self}` may contain interior mutability and a reference may not be safely \
               transferrable across a catch_unwind boundary",
    )]
    pub auto trait RefUnwindSafe {}

    /// A simple wrapper around a type to assert that it is unwind safe.
    ///
    /// When using [`catch_unwind`] it may be the case that some of the closed over
    /// variables are not unwind safe. For example if `&mut T` is captured the
    /// compiler will generate a warning indicating that it is not unwind safe. It
    /// may not be the case, however, that this is actually a problem due to the
    /// specific usage of [`catch_unwind`] if unwind safety is specifically taken into
    /// account. This wrapper struct is useful for a quick and lightweight
    /// annotation that a variable is indeed unwind safe.
    ///
    /// [`catch_unwind`]: ./fn.catch_unwind.html
    /// # Examples
    ///
    /// One way to use `AssertUnwindSafe` is to assert that the entire closure
    /// itself is unwind safe, bypassing all checks for all variables:
    ///
    /// ```
    /// use std::panic::{self, AssertUnwindSafe};
    ///
    /// let mut variable = 4;
    ///
    /// // This code will not compile because the closure captures `&mut variable`
    /// // which is not considered unwind safe by default.
    ///
    /// // panic::catch_unwind(|| {
    /// //     variable += 3;
    /// // });
    ///
    /// // This, however, will compile due to the `AssertUnwindSafe` wrapper
    /// let result = panic::catch_unwind(AssertUnwindSafe(|| {
    ///     variable += 3;
    /// }));
    /// // ...
    /// ```
    ///
    /// Wrapping the entire closure amounts to a blanket assertion that all captured
    /// variables are unwind safe. This has the downside that if new captures are
    /// added in the future, they will also be considered unwind safe. Therefore,
    /// you may prefer to just wrap individual captures, as shown below. This is
    /// more annotation, but it ensures that if a new capture is added which is not
    /// unwind safe, you will get a compilation error at that time, which will
    /// allow you to consider whether that new capture in fact represent a bug or
    /// not.
    ///
    /// ```
    /// use std::panic::{self, AssertUnwindSafe};
    ///
    /// let mut variable = 4;
    /// let other_capture = 3;
    ///
    /// let result = {
    ///     let mut wrapper = AssertUnwindSafe(&mut variable);
    ///     panic::catch_unwind(move || {
    ///         **wrapper += other_capture;
    ///     })
    /// };
    /// // ...
    /// ```
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    pub struct AssertUnwindSafe<T>(
        #[stable(feature = "catch_unwind", since = "1.9.0")]
        pub T
    );

    // Implementations of the `UnwindSafe` trait:
    //
    // * By default everything is unwind safe
    // * pointers T contains mutability of some form are not unwind safe
    // * Unique, an owning pointer, lifts an implementation
    // * Types like Mutex/RwLock which are explicitly poisoned are unwind safe
    // * Our custom AssertUnwindSafe wrapper is indeed unwind safe

    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<'a, T: ?Sized> !UnwindSafe for &'a mut T {}
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<'a, T: RefUnwindSafe + ?Sized> UnwindSafe for &'a T {}
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T: RefUnwindSafe + ?Sized> UnwindSafe for *const T {}
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T: RefUnwindSafe + ?Sized> UnwindSafe for *mut T {}
    #[unstable(feature = "ptr_internals", issue = "0")]
    impl<T: UnwindSafe + ?Sized> UnwindSafe for Unique<T> {}
    #[stable(feature = "nonnull", since = "1.25.0")]
    impl<T: RefUnwindSafe + ?Sized> UnwindSafe for NonNull<T> {}
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T: ?Sized> UnwindSafe for Mutex<T> {}
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T: ?Sized> UnwindSafe for RwLock<T> {}
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T> UnwindSafe for AssertUnwindSafe<T> {}

    // not covered via the Shared impl above b/c the inner contents use
    // Cell/AtomicUsize, but the usage here is unwind safe so we can lift the
    // impl up one level to Arc/Rc itself
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Rc<T> {}
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}

    // Pretty simple implementations for the `RefUnwindSafe` marker trait,
    // basically just saying that `UnsafeCell` is the
    // only thing which doesn't implement it (which then transitively applies to
    // everything else).
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T: ?Sized> !RefUnwindSafe for UnsafeCell<T> {}
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T> RefUnwindSafe for AssertUnwindSafe<T> {}

    #[stable(feature = "unwind_safe_lock_refs", since = "1.12.0")]
    impl<T: ?Sized> RefUnwindSafe for Mutex<T> {}
    #[stable(feature = "unwind_safe_lock_refs", since = "1.12.0")]
    impl<T: ?Sized> RefUnwindSafe for RwLock<T> {}

    #[cfg(target_has_atomic = "ptr")]
    #[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
    impl RefUnwindSafe for atomic::AtomicIsize {}
    #[cfg(target_has_atomic = "8")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicI8 {}
    #[cfg(target_has_atomic = "16")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicI16 {}
    #[cfg(target_has_atomic = "32")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicI32 {}
    #[cfg(target_has_atomic = "64")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicI64 {}
    #[cfg(target_has_atomic = "128")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicI128 {}

    #[cfg(target_has_atomic = "ptr")]
    #[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
    impl RefUnwindSafe for atomic::AtomicUsize {}
    #[cfg(target_has_atomic = "8")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicU8 {}
    #[cfg(target_has_atomic = "16")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicU16 {}
    #[cfg(target_has_atomic = "32")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicU32 {}
    #[cfg(target_has_atomic = "64")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicU64 {}
    #[cfg(target_has_atomic = "128")]
    #[unstable(feature = "integer_atomics", issue = "32976")]
    impl RefUnwindSafe for atomic::AtomicU128 {}

    #[cfg(target_has_atomic = "8")]
    #[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
    impl RefUnwindSafe for atomic::AtomicBool {}

    #[cfg(target_has_atomic = "ptr")]
    #[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
    impl<T> RefUnwindSafe for atomic::AtomicPtr<T> {}

    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T> Deref for AssertUnwindSafe<T> {
        type Target = T;

        fn deref(&self) -> &T {
            &self.0
        }
    }

    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<T> DerefMut for AssertUnwindSafe<T> {
        fn deref_mut(&mut self) -> &mut T {
            &mut self.0
        }
    }

    #[stable(feature = "catch_unwind", since = "1.9.0")]
    impl<R, F: FnOnce() -> R> FnOnce<()> for AssertUnwindSafe<F> {
        type Output = R;

        extern "rust-call" fn call_once(self, _args: ()) -> R {
            (self.0)()
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<T: fmt::Debug> fmt::Debug for AssertUnwindSafe<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("AssertUnwindSafe")
                .field(&self.0)
                .finish()
        }
    }

    #[unstable(feature = "futures_api", issue = "50547")]
    impl<'a, F: Future> Future for AssertUnwindSafe<F> {
        type Output = F::Output;

        fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output> {
            let pinned_field = unsafe { Pin::map_unchecked_mut(self, |x| &mut x.0) };
            F::poll(pinned_field, lw)
        }
    }

    /// Invokes a closure, capturing the cause of an unwinding panic if one occurs.
    ///
    /// This function will return `Ok` with the closure's result if the closure
    /// does not panic, and will return `Err(cause)` if the closure panics. The
    /// `cause` returned is the object with which panic was originally invoked.
    ///
    /// It is currently undefined behavior to unwind from Rust code into foreign
    /// code, so this function is particularly useful when Rust is called from
    /// another language (normally C). This can run arbitrary Rust code, capturing a
    /// panic and allowing a graceful handling of the error.
    ///
    /// It is **not** recommended to use this function for a general try/catch
    /// mechanism. The [`Result`] type is more appropriate to use for functions that
    /// can fail on a regular basis. Additionally, this function is not guaranteed
    /// to catch all panics, see the "Notes" section below.
    ///
    /// [`Result`]: ../result/enum.Result.html
    ///
    /// The closure provided is required to adhere to the [`UnwindSafe`] trait to ensure
    /// that all captured variables are safe to cross this boundary. The purpose of
    /// this bound is to encode the concept of [exception safety][rfc] in the type
    /// system. Most usage of this function should not need to worry about this
    /// bound as programs are naturally unwind safe without `unsafe` code. If it
    /// becomes a problem the [`AssertUnwindSafe`] wrapper struct can be used to quickly
    /// assert that the usage here is indeed unwind safe.
    ///
    /// [`AssertUnwindSafe`]: ./struct.AssertUnwindSafe.html
    /// [`UnwindSafe`]: ./trait.UnwindSafe.html
    ///
    /// [rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1236-stabilize-catch-panic.md
    ///
    /// # Notes
    ///
    /// Note that this function **may not catch all panics** in Rust. A panic in
    /// Rust is not always implemented via unwinding, but can be implemented by
    /// aborting the process as well. This function *only* catches unwinding panics,
    /// not those that abort the process.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::panic;
    ///
    /// let result = panic::catch_unwind(|| {
    ///     println!("hello!");
    /// });
    /// assert!(result.is_ok());
    ///
    /// let result = panic::catch_unwind(|| {
    ///     panic!("oh no!");
    /// });
    /// assert!(result.is_err());
    /// ```
    #[stable(feature = "catch_unwind", since = "1.9.0")]
    pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
        unsafe {
            panicking::try(f)
        }
    }

    /// Triggers a panic without invoking the panic hook.
    ///
    /// This is designed to be used in conjunction with [`catch_unwind`] to, for
    /// example, carry a panic across a layer of C code.
    ///
    /// [`catch_unwind`]: ./fn.catch_unwind.html
    ///
    /// # Notes
    ///
    /// Note that panics in Rust are not always implemented via unwinding, but they
    /// may be implemented by aborting the process. If this function is called when
    /// panics are implemented this way then this function will abort the process,
    /// not trigger an unwind.
    ///
    /// # Examples
    ///
    /// ```should_panic
    /// use std::panic;
    ///
    /// let result = panic::catch_unwind(|| {
    ///     panic!("oh no!");
    /// });
    ///
    /// if let Err(err) = result {
    ///     panic::resume_unwind(err);
    /// }
    /// ```
    #[stable(feature = "resume_unwind", since = "1.9.0")]
    pub fn resume_unwind(payload: Box<dyn Any + Send>) -> ! {
        panicking::update_count_then_panic(payload)
    }

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