core/panic/

unwind_safe.rs

use crate::async_iter::AsyncIterator;
use crate::cell::UnsafeCell;
use crate::fmt;
use crate::future::Future;
use crate::ops::{Deref, DerefMut};
use crate::pin::Pin;
use crate::ptr::{NonNull, Unique};
use crate::task::{Context, Poll};

/// 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.
///
/// [`catch_unwind`]: ../../std/panic/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 turn 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`.
#[stable(feature = "catch_unwind", since = "1.9.0")]
#[rustc_diagnostic_item = "unwind_safe_trait"]
#[diagnostic::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.
#[stable(feature = "catch_unwind", since = "1.9.0")]
#[rustc_diagnostic_item = "ref_unwind_safe_trait"]
#[diagnostic::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
/// might 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`]: ../../std/panic/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<T: ?Sized> !UnwindSafe for &mut T {}
#[stable(feature = "catch_unwind", since = "1.9.0")]
impl<T: RefUnwindSafe + ?Sized> UnwindSafe for &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 = "none")]
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> UnwindSafe for AssertUnwindSafe<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> {}

#[cfg(target_has_atomic_load_store = "ptr")]
#[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicIsize {}
#[cfg(target_has_atomic_load_store = "8")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicI8 {}
#[cfg(target_has_atomic_load_store = "16")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicI16 {}
#[cfg(target_has_atomic_load_store = "32")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicI32 {}
#[cfg(target_has_atomic_load_store = "64")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicI64 {}
#[cfg(target_has_atomic_load_store = "128")]
#[unstable(feature = "integer_atomics", issue = "99069")]
impl RefUnwindSafe for crate::sync::atomic::AtomicI128 {}

#[cfg(target_has_atomic_load_store = "ptr")]
#[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicUsize {}
#[cfg(target_has_atomic_load_store = "8")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicU8 {}
#[cfg(target_has_atomic_load_store = "16")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicU16 {}
#[cfg(target_has_atomic_load_store = "32")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicU32 {}
#[cfg(target_has_atomic_load_store = "64")]
#[stable(feature = "integer_atomics_stable", since = "1.34.0")]
impl RefUnwindSafe for crate::sync::atomic::AtomicU64 {}
#[cfg(target_has_atomic_load_store = "128")]
#[unstable(feature = "integer_atomics", issue = "99069")]
impl RefUnwindSafe for crate::sync::atomic::AtomicU128 {}

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

#[cfg(target_has_atomic_load_store = "ptr")]
#[stable(feature = "unwind_safe_atomic_refs", since = "1.14.0")]
impl<T> RefUnwindSafe for crate::sync::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;

    #[inline]
    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()
    }
}

#[stable(feature = "assertunwindsafe_default", since = "1.62.0")]
impl<T: Default> Default for AssertUnwindSafe<T> {
    fn default() -> Self {
        Self(Default::default())
    }
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl<F: Future> Future for AssertUnwindSafe<F> {
    type Output = F::Output;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // SAFETY: pin projection. AssertUnwindSafe follows structural pinning.
        let pinned_field = unsafe { Pin::map_unchecked_mut(self, |x| &mut x.0) };
        F::poll(pinned_field, cx)
    }
}

#[unstable(feature = "async_iterator", issue = "79024")]
impl<S: AsyncIterator> AsyncIterator for AssertUnwindSafe<S> {
    type Item = S::Item;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<S::Item>> {
        // SAFETY: pin projection. AssertUnwindSafe follows structural pinning.
        unsafe { self.map_unchecked_mut(|x| &mut x.0) }.poll_next(cx)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.0.size_hint()
    }
}