Auto merge of #84662 - dtolnay:unwindsafe, r=Amanieu

Move UnwindSafe, RefUnwindSafe, AssertUnwindSafe to core

They were previously only available in std::panic, not core::panic.

- https://doc.rust-lang.org/1.51.0/std/panic/trait.UnwindSafe.html
- https://doc.rust-lang.org/1.51.0/std/panic/trait.RefUnwindSafe.html
- https://doc.rust-lang.org/1.51.0/std/panic/struct.AssertUnwindSafe.html

Where this is relevant: trait objects! Inside a `#![no_std]` library it's otherwise impossible to have a struct holding a trait object, and at the same time can be used from downstream std crates in a way that doesn't interfere with catch_unwind.

```rust
// common library

#![no_std]

pub struct Thing {
    pub(crate) x: &'static (dyn SomeTrait + Send + Sync),
}

pub(crate) trait SomeTrait {...}
```

```rust
// downstream application

fn main() {
    let thing: library::Thing = ...;
    let _ = std::panic::catch_unwind(|| { let _ = thing; });  // does not work :(
}
```

See https://github.com/dtolnay/colorous/blob/a4131708e2f05d2377964981896ff62dbc9b027b/src/gradient.rs#L7-L15 for a real life example of needing to work around this problem. In particular that workaround would not even be viable if implementors of the trait were provided externally by a caller, as the `feature = "std"` would become non-additive in that case.

What happens without the UnwindSafe constraints:

```rust
fn main() {
    let gradient = colorous::VIRIDIS;
    let _ = std::panic::catch_unwind(|| { let _ = gradient; });
}
```

```console
error[E0277]: the type `(dyn colorous::gradient::EvalGradient + Send + Sync + 'static)` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
   --> src/main.rs:3:13
    |
3   |     let _ = std::panic::catch_unwind(|| { let _ = gradient; });
    |             ^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn colorous::gradient::EvalGradient + Send + Sync + 'static)` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    |
   ::: .rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:430:40
    |
430 | pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
    |                                        ---------- required by this bound in `catch_unwind`
    |
    = help: within `Gradient`, the trait `RefUnwindSafe` is not implemented for `(dyn colorous::gradient::EvalGradient + Send + Sync + 'static)`
    = note: required because it appears within the type `&'static (dyn colorous::gradient::EvalGradient + Send + Sync + 'static)`
    = note: required because it appears within the type `Gradient`
    = note: required because of the requirements on the impl of `UnwindSafe` for `&Gradient`
    = note: required because it appears within the type `[closure@src/main.rs:3:38: 3:62]`
```

Author: bors

library/alloc/src/rc.rs

@@ -262,6 +262,7 @@ use core::marker::{self, PhantomData, Unpin, Unsize};
 use core::mem::size_of_val;
 use core::mem::{self, align_of_val_raw, forget};
 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
+use core::panic::{RefUnwindSafe, UnwindSafe};
 #[cfg(not(no_global_oom_handling))]
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
@@ -314,6 +315,9 @@ impl<T: ?Sized> !marker::Send for Rc<T> {}
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> !marker::Sync for Rc<T> {}
 
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Rc<T> {}
+
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
 

library/alloc/src/sync.rs

@@ -19,6 +19,7 @@ use core::marker::{PhantomData, Unpin, Unsize};
 use core::mem::size_of_val;
 use core::mem::{self, align_of_val_raw};
 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
+use core::panic::{RefUnwindSafe, UnwindSafe};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
 #[cfg(not(no_global_oom_handling))]
@@ -240,6 +241,9 @@ unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
 
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
+
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
 

library/core/src/panic.rs

@@ -2,8 +2,18 @@
 
 #![stable(feature = "core_panic_info", since = "1.41.0")]
 
+mod location;
+mod panic_info;
+mod unwind_safe;
+
 use crate::any::Any;
-use crate::fmt;
+
+#[stable(feature = "panic_hooks", since = "1.10.0")]
+pub use self::location::Location;
+#[stable(feature = "panic_hooks", since = "1.10.0")]
+pub use self::panic_info::PanicInfo;
+#[stable(feature = "catch_unwind", since = "1.9.0")]
+pub use self::unwind_safe::{AssertUnwindSafe, RefUnwindSafe, UnwindSafe};
 
 #[doc(hidden)]
 #[unstable(feature = "edition_panic", issue = "none", reason = "use panic!() instead")]
@@ -39,334 +49,6 @@ pub macro panic_2021 {
     ),
 }
 
-/// A struct providing information about a panic.
-///
-/// `PanicInfo` structure is passed to a panic hook set by the [`set_hook`]
-/// function.
-///
-/// [`set_hook`]: ../../std/panic/fn.set_hook.html
-///
-/// # Examples
-///
-/// ```should_panic
-/// use std::panic;
-///
-/// panic::set_hook(Box::new(|panic_info| {
-///     if let Some(s) = panic_info.payload().downcast_ref::<&str>() {
-///         println!("panic occurred: {:?}", s);
-///     } else {
-///         println!("panic occurred");
-///     }
-/// }));
-///
-/// panic!("Normal panic");
-/// ```
-#[lang = "panic_info"]
-#[stable(feature = "panic_hooks", since = "1.10.0")]
-#[derive(Debug)]
-pub struct PanicInfo<'a> {
-    payload: &'a (dyn Any + Send),
-    message: Option<&'a fmt::Arguments<'a>>,
-    location: &'a Location<'a>,
-}
-
-impl<'a> PanicInfo<'a> {
-    #[unstable(
-        feature = "panic_internals",
-        reason = "internal details of the implementation of the `panic!` and related macros",
-        issue = "none"
-    )]
-    #[doc(hidden)]
-    #[inline]
-    pub fn internal_constructor(
-        message: Option<&'a fmt::Arguments<'a>>,
-        location: &'a Location<'a>,
-    ) -> Self {
-        struct NoPayload;
-        PanicInfo { location, message, payload: &NoPayload }
-    }
-
-    #[unstable(
-        feature = "panic_internals",
-        reason = "internal details of the implementation of the `panic!` and related macros",
-        issue = "none"
-    )]
-    #[doc(hidden)]
-    #[inline]
-    pub fn set_payload(&mut self, info: &'a (dyn Any + Send)) {
-        self.payload = info;
-    }
-
-    /// Returns the payload associated with the panic.
-    ///
-    /// This will commonly, but not always, be a `&'static str` or [`String`].
-    ///
-    /// [`String`]: ../../std/string/struct.String.html
-    ///
-    /// # Examples
-    ///
-    /// ```should_panic
-    /// use std::panic;
-    ///
-    /// panic::set_hook(Box::new(|panic_info| {
-    ///     if let Some(s) = panic_info.payload().downcast_ref::<&str>() {
-    ///         println!("panic occurred: {:?}", s);
-    ///     } else {
-    ///         println!("panic occurred");
-    ///     }
-    /// }));
-    ///
-    /// panic!("Normal panic");
-    /// ```
-    #[stable(feature = "panic_hooks", since = "1.10.0")]
-    pub fn payload(&self) -> &(dyn Any + Send) {
-        self.payload
-    }
-
-    /// If the `panic!` macro from the `core` crate (not from `std`)
-    /// was used with a formatting string and some additional arguments,
-    /// returns that message ready to be used for example with [`fmt::write`]
-    #[unstable(feature = "panic_info_message", issue = "66745")]
-    pub fn message(&self) -> Option<&fmt::Arguments<'_>> {
-        self.message
-    }
-
-    /// Returns information about the location from which the panic originated,
-    /// if available.
-    ///
-    /// This method will currently always return [`Some`], but this may change
-    /// in future versions.
-    ///
-    /// # Examples
-    ///
-    /// ```should_panic
-    /// use std::panic;
-    ///
-    /// panic::set_hook(Box::new(|panic_info| {
-    ///     if let Some(location) = panic_info.location() {
-    ///         println!("panic occurred in file '{}' at line {}",
-    ///             location.file(),
-    ///             location.line(),
-    ///         );
-    ///     } else {
-    ///         println!("panic occurred but can't get location information...");
-    ///     }
-    /// }));
-    ///
-    /// panic!("Normal panic");
-    /// ```
-    #[stable(feature = "panic_hooks", since = "1.10.0")]
-    pub fn location(&self) -> Option<&Location<'_>> {
-        // NOTE: If this is changed to sometimes return None,
-        // deal with that case in std::panicking::default_hook and std::panicking::begin_panic_fmt.
-        Some(&self.location)
-    }
-}
-
-#[stable(feature = "panic_hook_display", since = "1.26.0")]
-impl fmt::Display for PanicInfo<'_> {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        formatter.write_str("panicked at ")?;
-        if let Some(message) = self.message {
-            write!(formatter, "'{}', ", message)?
-        } else if let Some(payload) = self.payload.downcast_ref::<&'static str>() {
-            write!(formatter, "'{}', ", payload)?
-        }
-        // NOTE: we cannot use downcast_ref::<String>() here
-        // since String is not available in libcore!
-        // The payload is a String when `std::panic!` is called with multiple arguments,
-        // but in that case the message is also available.
-
-        self.location.fmt(formatter)
-    }
-}
-
-/// A struct containing information about the location of a panic.
-///
-/// This structure is created by [`PanicInfo::location()`].
-///
-/// # Examples
-///
-/// ```should_panic
-/// use std::panic;
-///
-/// panic::set_hook(Box::new(|panic_info| {
-///     if let Some(location) = panic_info.location() {
-///         println!("panic occurred in file '{}' at line {}", location.file(), location.line());
-///     } else {
-///         println!("panic occurred but can't get location information...");
-///     }
-/// }));
-///
-/// panic!("Normal panic");
-/// ```
-///
-/// # Comparisons
-///
-/// Comparisons for equality and ordering are made in file, line, then column priority.
-/// Files are compared as strings, not `Path`, which could be unexpected.
-/// See [`Location::file`]'s documentation for more discussion.
-#[lang = "panic_location"]
-#[derive(Copy, Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
-#[stable(feature = "panic_hooks", since = "1.10.0")]
-pub struct Location<'a> {
-    file: &'a str,
-    line: u32,
-    col: u32,
-}
-
-impl<'a> Location<'a> {
-    /// Returns the source location of the caller of this function. If that function's caller is
-    /// annotated then its call location will be returned, and so on up the stack to the first call
-    /// within a non-tracked function body.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::panic::Location;
-    ///
-    /// /// Returns the [`Location`] at which it is called.
-    /// #[track_caller]
-    /// fn get_caller_location() -> &'static Location<'static> {
-    ///     Location::caller()
-    /// }
-    ///
-    /// /// Returns a [`Location`] from within this function's definition.
-    /// fn get_just_one_location() -> &'static Location<'static> {
-    ///     get_caller_location()
-    /// }
-    ///
-    /// let fixed_location = get_just_one_location();
-    /// assert_eq!(fixed_location.file(), file!());
-    /// assert_eq!(fixed_location.line(), 14);
-    /// assert_eq!(fixed_location.column(), 5);
-    ///
-    /// // running the same untracked function in a different location gives us the same result
-    /// let second_fixed_location = get_just_one_location();
-    /// assert_eq!(fixed_location.file(), second_fixed_location.file());
-    /// assert_eq!(fixed_location.line(), second_fixed_location.line());
-    /// assert_eq!(fixed_location.column(), second_fixed_location.column());
-    ///
-    /// let this_location = get_caller_location();
-    /// assert_eq!(this_location.file(), file!());
-    /// assert_eq!(this_location.line(), 28);
-    /// assert_eq!(this_location.column(), 21);
-    ///
-    /// // running the tracked function in a different location produces a different value
-    /// let another_location = get_caller_location();
-    /// assert_eq!(this_location.file(), another_location.file());
-    /// assert_ne!(this_location.line(), another_location.line());
-    /// assert_ne!(this_location.column(), another_location.column());
-    /// ```
-    #[stable(feature = "track_caller", since = "1.46.0")]
-    #[rustc_const_unstable(feature = "const_caller_location", issue = "76156")]
-    #[track_caller]
-    pub const fn caller() -> &'static Location<'static> {
-        crate::intrinsics::caller_location()
-    }
-}
-
-impl<'a> Location<'a> {
-    #![unstable(
-        feature = "panic_internals",
-        reason = "internal details of the implementation of the `panic!` and related macros",
-        issue = "none"
-    )]
-    #[doc(hidden)]
-    pub const fn internal_constructor(file: &'a str, line: u32, col: u32) -> Self {
-        Location { file, line, col }
-    }
-
-    /// Returns the name of the source file from which the panic originated.
-    ///
-    /// # `&str`, not `&Path`
-    ///
-    /// The returned name refers to a source path on the compiling system, but it isn't valid to
-    /// represent this directly as a `&Path`. The compiled code may run on a different system with
-    /// a different `Path` implementation than the system providing the contents and this library
-    /// does not currently have a different "host path" type.
-    ///
-    /// The most surprising behavior occurs when "the same" file is reachable via multiple paths in
-    /// the module system (usually using the `#[path = "..."]` attribute or similar), which can
-    /// cause what appears to be identical code to return differing values from this function.
-    ///
-    /// # Cross-compilation
-    ///
-    /// This value is not suitable for passing to `Path::new` or similar constructors when the host
-    /// platform and target platform differ.
-    ///
-    /// # Examples
-    ///
-    /// ```should_panic
-    /// use std::panic;
-    ///
-    /// panic::set_hook(Box::new(|panic_info| {
-    ///     if let Some(location) = panic_info.location() {
-    ///         println!("panic occurred in file '{}'", location.file());
-    ///     } else {
-    ///         println!("panic occurred but can't get location information...");
-    ///     }
-    /// }));
-    ///
-    /// panic!("Normal panic");
-    /// ```
-    #[stable(feature = "panic_hooks", since = "1.10.0")]
-    pub fn file(&self) -> &str {
-        self.file
-    }
-
-    /// Returns the line number from which the panic originated.
-    ///
-    /// # Examples
-    ///
-    /// ```should_panic
-    /// use std::panic;
-    ///
-    /// panic::set_hook(Box::new(|panic_info| {
-    ///     if let Some(location) = panic_info.location() {
-    ///         println!("panic occurred at line {}", location.line());
-    ///     } else {
-    ///         println!("panic occurred but can't get location information...");
-    ///     }
-    /// }));
-    ///
-    /// panic!("Normal panic");
-    /// ```
-    #[stable(feature = "panic_hooks", since = "1.10.0")]
-    pub fn line(&self) -> u32 {
-        self.line
-    }
-
-    /// Returns the column from which the panic originated.
-    ///
-    /// # Examples
-    ///
-    /// ```should_panic
-    /// use std::panic;
-    ///
-    /// panic::set_hook(Box::new(|panic_info| {
-    ///     if let Some(location) = panic_info.location() {
-    ///         println!("panic occurred at column {}", location.column());
-    ///     } else {
-    ///         println!("panic occurred but can't get location information...");
-    ///     }
-    /// }));
-    ///
-    /// panic!("Normal panic");
-    /// ```
-    #[stable(feature = "panic_col", since = "1.25.0")]
-    pub fn column(&self) -> u32 {
-        self.col
-    }
-}
-
-#[stable(feature = "panic_hook_display", since = "1.26.0")]
-impl fmt::Display for Location<'_> {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(formatter, "{}:{}:{}", self.file, self.line, self.col)
-    }
-}
-
 /// An internal trait used by libstd to pass data from libstd to `panic_unwind`
 /// and other panic runtimes. Not intended to be stabilized any time soon, do
 /// not use.