std: Rename thread::catch_panic to panic::recover

This commit is an implementation of [RFC 1236] and [RFC 1323] which
rename the `thread::catch_panic` function to `panic::recover` while also
replacing the `Send + 'static` bounds with a new `PanicSafe` bound.

[RFC 1236]: https://github.com/rust-lang/rfcs/pull/1236
[RFC 1323]: https://github.com/rust-lang/rfcs/pull/1323

cc #27719

1 files changed, 255 insertions, 0 deletions

diff --git a/src/libstd/panic.rs b/src/libstd/panic.rs
new file mode 100644
index 00000000000..6e4ba337b08
--- /dev/null
+++ b/src/libstd/panic.rs
@@ -0,0 +1,255 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Panic support in the standard library
+
+#![unstable(feature = "std_panic", reason = "awaiting feedback",
+            issue = "27719")]
+
+use cell::UnsafeCell;
+use ops::{Deref, DerefMut};
+use ptr::{Unique, Shared};
+use rc::Rc;
+use sync::{Arc, Mutex, RwLock};
+use sys_common::unwind;
+use thread::Result;
+
+/// 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 `recover`
+/// boundary with no fear of panic safety.
+///
+/// ## What is panic 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 cricial 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 `recover` function in this
+/// module. Additionally, even if an invariant is witness, it typically isn't a
+/// problem in Rust because there's 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 panic 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 panic safety, but for more information
+/// about panic 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 `RecoverSafe`?
+///
+/// Now that we've got an idea of what panic safety is in Rust, it's also
+/// important to understand that this trait represents. As mentioned above, one
+/// way to witness broken invariants is through the `recover` function in this
+/// module as it allows catching a panic and then re-using the environment of
+/// the closure.
+///
+/// Simply but, a type `T` implements `RecoverSafe` if it cannot easily allow
+/// witnessing a broken invariant through the use of `recover` (catching a
+/// panic). This trait is a marker trait, so it is automatically implemented for
+/// many types, and it is also structurally composed (e.g. a struct is recover
+/// safe if all of its components are recover 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 `recover` that broken invariants may be
+/// witnessed and may need to be accounted for.
+///
+/// ## Who implements `RecoverSafe`?
+///
+/// Types such as `&mut T` and `&RefCell<T>` are examples which are **not**
+/// recover safe. The general idea is that any mutable state which can be shared
+/// across `recover` is not recover safe by default. This is because it is very
+/// easy to witness a broken invariant outside of `recover` as the data is
+/// simply accesed as usual.
+///
+/// Types like `&Mutex<T>`, however, are recover 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 `RecoverSafe` be used?
+///
+/// Is not intended that most types or functions need to worry about this trait.
+/// It is only used as a bound on the `recover` function and as mentioned above,
+/// the lack of `unsafe` means it is mostly an advisory. The `AssertRecoverSafe`
+/// wrapper struct in this module can be used to force this trait to be
+/// implemented for any closed over variables passed to the `recover` function
+/// (more on this below).
+#[unstable(feature = "recover", reason = "awaiting feedback", issue = "27719")]
+#[rustc_on_unimplemented = "the type {Self} may not be safely transferred \
+                            across a recover boundary"]
+pub trait RecoverSafe {}
+
+/// A marker trait representing types which do not contain an `UnsafeCell` by
+/// value internally.
+///
+/// This is a "helper marker trait" used to provide impl blocks for the
+/// `RecoverSafe` trait, for more information see that documentation.
+#[unstable(feature = "recover", reason = "awaiting feedback", issue = "27719")]
+#[rustc_on_unimplemented = "the type {Self} contains interior mutability \
+                            and a reference may not be safely transferrable \
+                            across a recover boundary"]
+pub trait NoUnsafeCell {}
+
+/// A simple wrapper around a type to assert that it is panic safe.
+///
+/// When using `recover` it may be the case that some of the closed over
+/// variables are not panic safe. For example if `&mut T` is captured the
+/// compiler will generate a warning indicating that it is not panic safe. It
+/// may not be the case, however, that this is actually a problem due to the
+/// specific usage of `recover` if panic safety is specifically taken into
+/// account. This wrapper struct is useful for a quick and lightweight
+/// annotation that a variable is indeed panic safe.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(recover, std_panic)]
+///
+/// use std::panic::{self, AssertRecoverSafe};
+///
+/// let mut variable = 4;
+///
+/// // This code will not compile becuause the closure captures `&mut variable`
+/// // which is not considered panic safe by default.
+///
+/// // panic::recover(|| {
+/// //     variable += 3;
+/// // });
+///
+/// // This, however, will compile due to the `AssertRecoverSafe` wrapper
+/// let result = {
+///     let mut wrapper = AssertRecoverSafe::new(&mut variable);
+///     panic::recover(move || {
+///         **wrapper += 3;
+///     })
+/// };
+/// // ...
+/// ```
+#[unstable(feature = "recover", reason = "awaiting feedback", issue = "27719")]
+pub struct AssertRecoverSafe<T>(T);
+
+// Implementations of the `RecoverSafe` trait:
+//
+// * By default everything is recover safe
+// * pointers T contains mutability of some form are not recover safe
+// * Unique, an owning pointer, lifts an implementation
+// * Types like Mutex/RwLock which are explicilty poisoned are recover safe
+// * Our custom AssertRecoverSafe wrapper is indeed recover safe
+impl RecoverSafe for .. {}
+impl<'a, T: ?Sized> !RecoverSafe for &'a mut T {}
+impl<'a, T: NoUnsafeCell + ?Sized> RecoverSafe for &'a T {}
+impl<T: NoUnsafeCell + ?Sized> RecoverSafe for *const T {}
+impl<T: NoUnsafeCell + ?Sized> RecoverSafe for *mut T {}
+impl<T: RecoverSafe> RecoverSafe for Unique<T> {}
+impl<T: NoUnsafeCell + ?Sized> RecoverSafe for Shared<T> {}
+impl<T: ?Sized> RecoverSafe for Mutex<T> {}
+impl<T: ?Sized> RecoverSafe for RwLock<T> {}
+impl<T> RecoverSafe for AssertRecoverSafe<T> {}
+
+// not covered via the Shared impl above b/c the inner contents use
+// Cell/AtomicUsize, but the usage here is recover safe so we can lift the
+// impl up one level to Arc/Rc itself
+impl<T: NoUnsafeCell + ?Sized> RecoverSafe for Rc<T> {}
+impl<T: NoUnsafeCell + ?Sized> RecoverSafe for Arc<T> {}
+
+// Pretty simple implementations for the `NoUnsafeCell` marker trait, basically
+// just saying that this is a marker trait and `UnsafeCell` is the only thing
+// which doesn't implement it (which then transitively applies to everything
+// else.
+impl NoUnsafeCell for .. {}
+impl<T: ?Sized> !NoUnsafeCell for UnsafeCell<T> {}
+
+impl<T> AssertRecoverSafe<T> {
+    /// Creates a new `AssertRecoverSafe` wrapper around the provided type.
+    #[unstable(feature = "recover", reason = "awaiting feedback", issue = "27719")]
+    pub fn new(t: T) -> AssertRecoverSafe<T> {
+        AssertRecoverSafe(t)
+    }
+}
+
+impl<T> Deref for AssertRecoverSafe<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}
+
+impl<T> DerefMut for AssertRecoverSafe<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.0
+    }
+}
+
+/// Invokes a closure, capturing the cause of 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.
+///
+/// The closure provided is required to adhere to the `RecoverSafe` to ensure
+/// that all captured variables are safe to cross this recover 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 panic safe without `unsafe` code. If it
+/// becomes a problem the associated `AssertRecoverSafe` wrapper type in this
+/// module can be used to quickly assert that the usage here is indeed exception
+/// safe.
+///
+/// [rfc]: https://github.com/rust-lang/rfcs/blob/master/text/1236-stabilize-catch-panic.md
+///
+/// # Examples
+///
+/// ```
+/// #![feature(recover, std_panic)]
+///
+/// use std::panic;
+///
+/// let result = panic::recover(|| {
+///     println!("hello!");
+/// });
+/// assert!(result.is_ok());
+///
+/// let result = panic::recover(|| {
+///     panic!("oh no!");
+/// });
+/// assert!(result.is_err());
+/// ```
+#[unstable(feature = "recover", reason = "awaiting feedback", issue = "27719")]
+pub fn recover<F: FnOnce() -> R + RecoverSafe, R>(f: F) -> Result<R> {
+    let mut result = None;
+    unsafe {
+        let result = &mut result;
+        try!(unwind::try(move || *result = Some(f())))
+    }
+    Ok(result.unwrap())
+}