mv std libs to library/

1 files changed, 1787 insertions, 0 deletions

diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
new file mode 100644
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--- /dev/null
+++ b/library/std/src/thread/mod.rs
@@ -0,0 +1,1787 @@
+//! Native threads.
+//!
+//! ## The threading model
+//!
+//! An executing Rust program consists of a collection of native OS threads,
+//! each with their own stack and local state. Threads can be named, and
+//! provide some built-in support for low-level synchronization.
+//!
+//! Communication between threads can be done through
+//! [channels], Rust's message-passing types, along with [other forms of thread
+//! synchronization](../../std/sync/index.html) and shared-memory data
+//! structures. In particular, types that are guaranteed to be
+//! threadsafe are easily shared between threads using the
+//! atomically-reference-counted container, [`Arc`].
+//!
+//! Fatal logic errors in Rust cause *thread panic*, during which
+//! a thread will unwind the stack, running destructors and freeing
+//! owned resources. While not meant as a 'try/catch' mechanism, panics
+//! in Rust can nonetheless be caught (unless compiling with `panic=abort`) with
+//! [`catch_unwind`](../../std/panic/fn.catch_unwind.html) and recovered
+//! from, or alternatively be resumed with
+//! [`resume_unwind`](../../std/panic/fn.resume_unwind.html). If the panic
+//! is not caught the thread will exit, but the panic may optionally be
+//! detected from a different thread with [`join`]. If the main thread panics
+//! without the panic being caught, the application will exit with a
+//! non-zero exit code.
+//!
+//! When the main thread of a Rust program terminates, the entire program shuts
+//! down, even if other threads are still running. However, this module provides
+//! convenient facilities for automatically waiting for the termination of a
+//! child thread (i.e., join).
+//!
+//! ## Spawning a thread
+//!
+//! A new thread can be spawned using the [`thread::spawn`][`spawn`] function:
+//!
+//! ```rust
+//! use std::thread;
+//!
+//! thread::spawn(move || {
+//!     // some work here
+//! });
+//! ```
+//!
+//! In this example, the spawned thread is "detached" from the current
+//! thread. This means that it can outlive its parent (the thread that spawned
+//! it), unless this parent is the main thread.
+//!
+//! The parent thread can also wait on the completion of the child
+//! thread; a call to [`spawn`] produces a [`JoinHandle`], which provides
+//! a `join` method for waiting:
+//!
+//! ```rust
+//! use std::thread;
+//!
+//! let child = thread::spawn(move || {
+//!     // some work here
+//! });
+//! // some work here
+//! let res = child.join();
+//! ```
+//!
+//! The [`join`] method returns a [`thread::Result`] containing [`Ok`] of the final
+//! value produced by the child thread, or [`Err`] of the value given to
+//! a call to [`panic!`] if the child panicked.
+//!
+//! ## Configuring threads
+//!
+//! A new thread can be configured before it is spawned via the [`Builder`] type,
+//! which currently allows you to set the name and stack size for the child thread:
+//!
+//! ```rust
+//! # #![allow(unused_must_use)]
+//! use std::thread;
+//!
+//! thread::Builder::new().name("child1".to_string()).spawn(move || {
+//!     println!("Hello, world!");
+//! });
+//! ```
+//!
+//! ## The `Thread` type
+//!
+//! Threads are represented via the [`Thread`] type, which you can get in one of
+//! two ways:
+//!
+//! * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
+//!   function, and calling [`thread`][`JoinHandle::thread`] on the [`JoinHandle`].
+//! * By requesting the current thread, using the [`thread::current`] function.
+//!
+//! The [`thread::current`] function is available even for threads not spawned
+//! by the APIs of this module.
+//!
+//! ## Thread-local storage
+//!
+//! This module also provides an implementation of thread-local storage for Rust
+//! programs. Thread-local storage is a method of storing data into a global
+//! variable that each thread in the program will have its own copy of.
+//! Threads do not share this data, so accesses do not need to be synchronized.
+//!
+//! A thread-local key owns the value it contains and will destroy the value when the
+//! thread exits. It is created with the [`thread_local!`] macro and can contain any
+//! value that is `'static` (no borrowed pointers). It provides an accessor function,
+//! [`with`], that yields a shared reference to the value to the specified
+//! closure. Thread-local keys allow only shared access to values, as there would be no
+//! way to guarantee uniqueness if mutable borrows were allowed. Most values
+//! will want to make use of some form of **interior mutability** through the
+//! [`Cell`] or [`RefCell`] types.
+//!
+//! ## Naming threads
+//!
+//! Threads are able to have associated names for identification purposes. By default, spawned
+//! threads are unnamed. To specify a name for a thread, build the thread with [`Builder`] and pass
+//! the desired thread name to [`Builder::name`]. To retrieve the thread name from within the
+//! thread, use [`Thread::name`]. A couple examples of where the name of a thread gets used:
+//!
+//! * If a panic occurs in a named thread, the thread name will be printed in the panic message.
+//! * The thread name is provided to the OS where applicable (e.g., `pthread_setname_np` in
+//!   unix-like platforms).
+//!
+//! ## Stack size
+//!
+//! The default stack size for spawned threads is 2 MiB, though this particular stack size is
+//! subject to change in the future. There are two ways to manually specify the stack size for
+//! spawned threads:
+//!
+//! * Build the thread with [`Builder`] and pass the desired stack size to [`Builder::stack_size`].
+//! * Set the `RUST_MIN_STACK` environment variable to an integer representing the desired stack
+//!   size (in bytes). Note that setting [`Builder::stack_size`] will override this.
+//!
+//! Note that the stack size of the main thread is *not* determined by Rust.
+//!
+//! [channels]: ../../std/sync/mpsc/index.html
+//! [`Arc`]: ../../std/sync/struct.Arc.html
+//! [`spawn`]: ../../std/thread/fn.spawn.html
+//! [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
+//! [`JoinHandle::thread`]: ../../std/thread/struct.JoinHandle.html#method.thread
+//! [`join`]: ../../std/thread/struct.JoinHandle.html#method.join
+//! [`Result`]: ../../std/result/enum.Result.html
+//! [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
+//! [`Err`]: ../../std/result/enum.Result.html#variant.Err
+//! [`panic!`]: ../../std/macro.panic.html
+//! [`Builder`]: ../../std/thread/struct.Builder.html
+//! [`Builder::stack_size`]: ../../std/thread/struct.Builder.html#method.stack_size
+//! [`Builder::name`]: ../../std/thread/struct.Builder.html#method.name
+//! [`thread::current`]: ../../std/thread/fn.current.html
+//! [`thread::Result`]: ../../std/thread/type.Result.html
+//! [`Thread`]: ../../std/thread/struct.Thread.html
+//! [`park`]: ../../std/thread/fn.park.html
+//! [`unpark`]: ../../std/thread/struct.Thread.html#method.unpark
+//! [`Thread::name`]: ../../std/thread/struct.Thread.html#method.name
+//! [`thread::park_timeout`]: ../../std/thread/fn.park_timeout.html
+//! [`Cell`]: ../cell/struct.Cell.html
+//! [`RefCell`]: ../cell/struct.RefCell.html
+//! [`thread_local!`]: ../macro.thread_local.html
+//! [`with`]: struct.LocalKey.html#method.with
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+use crate::any::Any;
+use crate::cell::UnsafeCell;
+use crate::ffi::{CStr, CString};
+use crate::fmt;
+use crate::io;
+use crate::mem;
+use crate::num::NonZeroU64;
+use crate::panic;
+use crate::panicking;
+use crate::str;
+use crate::sync::atomic::AtomicUsize;
+use crate::sync::atomic::Ordering::SeqCst;
+use crate::sync::{Arc, Condvar, Mutex};
+use crate::sys::thread as imp;
+use crate::sys_common::mutex;
+use crate::sys_common::thread;
+use crate::sys_common::thread_info;
+use crate::sys_common::{AsInner, IntoInner};
+use crate::time::Duration;
+
+////////////////////////////////////////////////////////////////////////////////
+// Thread-local storage
+////////////////////////////////////////////////////////////////////////////////
+
+#[macro_use]
+mod local;
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use self::local::{AccessError, LocalKey};
+
+// The types used by the thread_local! macro to access TLS keys. Note that there
+// are two types, the "OS" type and the "fast" type. The OS thread local key
+// type is accessed via platform-specific API calls and is slow, while the fast
+// key type is accessed via code generated via LLVM, where TLS keys are set up
+// by the elf linker. Note that the OS TLS type is always available: on macOS
+// the standard library is compiled with support for older platform versions
+// where fast TLS was not available; end-user code is compiled with fast TLS
+// where available, but both are needed.
+
+#[unstable(feature = "libstd_thread_internals", issue = "none")]
+#[cfg(target_thread_local)]
+#[doc(hidden)]
+pub use self::local::fast::Key as __FastLocalKeyInner;
+#[unstable(feature = "libstd_thread_internals", issue = "none")]
+#[doc(hidden)]
+pub use self::local::os::Key as __OsLocalKeyInner;
+#[unstable(feature = "libstd_thread_internals", issue = "none")]
+#[cfg(all(target_arch = "wasm32", not(target_feature = "atomics")))]
+#[doc(hidden)]
+pub use self::local::statik::Key as __StaticLocalKeyInner;
+
+////////////////////////////////////////////////////////////////////////////////
+// Builder
+////////////////////////////////////////////////////////////////////////////////
+
+/// Thread factory, which can be used in order to configure the properties of
+/// a new thread.
+///
+/// Methods can be chained on it in order to configure it.
+///
+/// The two configurations available are:
+///
+/// - [`name`]: specifies an [associated name for the thread][naming-threads]
+/// - [`stack_size`]: specifies the [desired stack size for the thread][stack-size]
+///
+/// The [`spawn`] method will take ownership of the builder and create an
+/// [`io::Result`] to the thread handle with the given configuration.
+///
+/// The [`thread::spawn`] free function uses a `Builder` with default
+/// configuration and [`unwrap`]s its return value.
+///
+/// You may want to use [`spawn`] instead of [`thread::spawn`], when you want
+/// to recover from a failure to launch a thread, indeed the free function will
+/// panic where the `Builder` method will return a [`io::Result`].
+///
+/// # Examples
+///
+/// ```
+/// use std::thread;
+///
+/// let builder = thread::Builder::new();
+///
+/// let handler = builder.spawn(|| {
+///     // thread code
+/// }).unwrap();
+///
+/// handler.join().unwrap();
+/// ```
+///
+/// [`thread::spawn`]: ../../std/thread/fn.spawn.html
+/// [`stack_size`]: ../../std/thread/struct.Builder.html#method.stack_size
+/// [`name`]: ../../std/thread/struct.Builder.html#method.name
+/// [`spawn`]: ../../std/thread/struct.Builder.html#method.spawn
+/// [`io::Result`]: ../../std/io/type.Result.html
+/// [`unwrap`]: ../../std/result/enum.Result.html#method.unwrap
+/// [naming-threads]: ./index.html#naming-threads
+/// [stack-size]: ./index.html#stack-size
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
+pub struct Builder {
+    // A name for the thread-to-be, for identification in panic messages
+    name: Option<String>,
+    // The size of the stack for the spawned thread in bytes
+    stack_size: Option<usize>,
+}
+
+impl Builder {
+    /// Generates the base configuration for spawning a thread, from which
+    /// configuration methods can be chained.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new()
+    ///                               .name("foo".into())
+    ///                               .stack_size(32 * 1024);
+    ///
+    /// let handler = builder.spawn(|| {
+    ///     // thread code
+    /// }).unwrap();
+    ///
+    /// handler.join().unwrap();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new() -> Builder {
+        Builder { name: None, stack_size: None }
+    }
+
+    /// Names the thread-to-be. Currently the name is used for identification
+    /// only in panic messages.
+    ///
+    /// The name must not contain null bytes (`\0`).
+    ///
+    /// For more information about named threads, see
+    /// [this module-level documentation][naming-threads].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new()
+    ///     .name("foo".into());
+    ///
+    /// let handler = builder.spawn(|| {
+    ///     assert_eq!(thread::current().name(), Some("foo"))
+    /// }).unwrap();
+    ///
+    /// handler.join().unwrap();
+    /// ```
+    ///
+    /// [naming-threads]: ./index.html#naming-threads
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn name(mut self, name: String) -> Builder {
+        self.name = Some(name);
+        self
+    }
+
+    /// Sets the size of the stack (in bytes) for the new thread.
+    ///
+    /// The actual stack size may be greater than this value if
+    /// the platform specifies a minimal stack size.
+    ///
+    /// For more information about the stack size for threads, see
+    /// [this module-level documentation][stack-size].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new().stack_size(32 * 1024);
+    /// ```
+    ///
+    /// [stack-size]: ./index.html#stack-size
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn stack_size(mut self, size: usize) -> Builder {
+        self.stack_size = Some(size);
+        self
+    }
+
+    /// Spawns a new thread by taking ownership of the `Builder`, and returns an
+    /// [`io::Result`] to its [`JoinHandle`].
+    ///
+    /// The spawned thread may outlive the caller (unless the caller thread
+    /// is the main thread; the whole process is terminated when the main
+    /// thread finishes). The join handle can be used to block on
+    /// termination of the child thread, including recovering its panics.
+    ///
+    /// For a more complete documentation see [`thread::spawn`][`spawn`].
+    ///
+    /// # Errors
+    ///
+    /// Unlike the [`spawn`] free function, this method yields an
+    /// [`io::Result`] to capture any failure to create the thread at
+    /// the OS level.
+    ///
+    /// [`spawn`]: ../../std/thread/fn.spawn.html
+    /// [`io::Result`]: ../../std/io/type.Result.html
+    /// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
+    ///
+    /// # Panics
+    ///
+    /// Panics if a thread name was set and it contained null bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new();
+    ///
+    /// let handler = builder.spawn(|| {
+    ///     // thread code
+    /// }).unwrap();
+    ///
+    /// handler.join().unwrap();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn spawn<F, T>(self, f: F) -> io::Result<JoinHandle<T>>
+    where
+        F: FnOnce() -> T,
+        F: Send + 'static,
+        T: Send + 'static,
+    {
+        unsafe { self.spawn_unchecked(f) }
+    }
+
+    /// Spawns a new thread without any lifetime restrictions by taking ownership
+    /// of the `Builder`, and returns an [`io::Result`] to its [`JoinHandle`].
+    ///
+    /// The spawned thread may outlive the caller (unless the caller thread
+    /// is the main thread; the whole process is terminated when the main
+    /// thread finishes). The join handle can be used to block on
+    /// termination of the child thread, including recovering its panics.
+    ///
+    /// This method is identical to [`thread::Builder::spawn`][`Builder::spawn`],
+    /// except for the relaxed lifetime bounds, which render it unsafe.
+    /// For a more complete documentation see [`thread::spawn`][`spawn`].
+    ///
+    /// # Errors
+    ///
+    /// Unlike the [`spawn`] free function, this method yields an
+    /// [`io::Result`] to capture any failure to create the thread at
+    /// the OS level.
+    ///
+    /// # Panics
+    ///
+    /// Panics if a thread name was set and it contained null bytes.
+    ///
+    /// # Safety
+    ///
+    /// The caller has to ensure that no references in the supplied thread closure
+    /// or its return type can outlive the spawned thread's lifetime. This can be
+    /// guaranteed in two ways:
+    ///
+    /// - ensure that [`join`][`JoinHandle::join`] is called before any referenced
+    /// data is dropped
+    /// - use only types with `'static` lifetime bounds, i.e., those with no or only
+    /// `'static` references (both [`thread::Builder::spawn`][`Builder::spawn`]
+    /// and [`thread::spawn`][`spawn`] enforce this property statically)
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(thread_spawn_unchecked)]
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new();
+    ///
+    /// let x = 1;
+    /// let thread_x = &x;
+    ///
+    /// let handler = unsafe {
+    ///     builder.spawn_unchecked(move || {
+    ///         println!("x = {}", *thread_x);
+    ///     }).unwrap()
+    /// };
+    ///
+    /// // caller has to ensure `join()` is called, otherwise
+    /// // it is possible to access freed memory if `x` gets
+    /// // dropped before the thread closure is executed!
+    /// handler.join().unwrap();
+    /// ```
+    ///
+    /// [`spawn`]: ../../std/thread/fn.spawn.html
+    /// [`Builder::spawn`]: ../../std/thread/struct.Builder.html#method.spawn
+    /// [`io::Result`]: ../../std/io/type.Result.html
+    /// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
+    /// [`JoinHandle::join`]: ../../std/thread/struct.JoinHandle.html#method.join
+    #[unstable(feature = "thread_spawn_unchecked", issue = "55132")]
+    pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>>
+    where
+        F: FnOnce() -> T,
+        F: Send + 'a,
+        T: Send + 'a,
+    {
+        let Builder { name, stack_size } = self;
+
+        let stack_size = stack_size.unwrap_or_else(thread::min_stack);
+
+        let my_thread = Thread::new(name);
+        let their_thread = my_thread.clone();
+
+        let my_packet: Arc<UnsafeCell<Option<Result<T>>>> = Arc::new(UnsafeCell::new(None));
+        let their_packet = my_packet.clone();
+
+        let main = move || {
+            if let Some(name) = their_thread.cname() {
+                imp::Thread::set_name(name);
+            }
+
+            thread_info::set(imp::guard::current(), their_thread);
+            let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+                crate::sys_common::backtrace::__rust_begin_short_backtrace(f)
+            }));
+            *their_packet.get() = Some(try_result);
+        };
+
+        Ok(JoinHandle(JoinInner {
+            // `imp::Thread::new` takes a closure with a `'static` lifetime, since it's passed
+            // through FFI or otherwise used with low-level threading primitives that have no
+            // notion of or way to enforce lifetimes.
+            //
+            // As mentioned in the `Safety` section of this function's documentation, the caller of
+            // this function needs to guarantee that the passed-in lifetime is sufficiently long
+            // for the lifetime of the thread.
+            //
+            // Similarly, the `sys` implementation must guarantee that no references to the closure
+            // exist after the thread has terminated, which is signaled by `Thread::join`
+            // returning.
+            native: Some(imp::Thread::new(
+                stack_size,
+                mem::transmute::<Box<dyn FnOnce() + 'a>, Box<dyn FnOnce() + 'static>>(Box::new(
+                    main,
+                )),
+            )?),
+            thread: my_thread,
+            packet: Packet(my_packet),
+        }))
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Free functions
+////////////////////////////////////////////////////////////////////////////////
+
+/// Spawns a new thread, returning a [`JoinHandle`] for it.
+///
+/// The join handle will implicitly *detach* the child thread upon being
+/// dropped. In this case, the child thread may outlive the parent (unless
+/// the parent thread is the main thread; the whole process is terminated when
+/// the main thread finishes). Additionally, the join handle provides a [`join`]
+/// method that can be used to join the child thread. If the child thread
+/// panics, [`join`] will return an [`Err`] containing the argument given to
+/// [`panic`].
+///
+/// This will create a thread using default parameters of [`Builder`], if you
+/// want to specify the stack size or the name of the thread, use this API
+/// instead.
+///
+/// As you can see in the signature of `spawn` there are two constraints on
+/// both the closure given to `spawn` and its return value, let's explain them:
+///
+/// - The `'static` constraint means that the closure and its return value
+///   must have a lifetime of the whole program execution. The reason for this
+///   is that threads can `detach` and outlive the lifetime they have been
+///   created in.
+///   Indeed if the thread, and by extension its return value, can outlive their
+///   caller, we need to make sure that they will be valid afterwards, and since
+///   we *can't* know when it will return we need to have them valid as long as
+///   possible, that is until the end of the program, hence the `'static`
+///   lifetime.
+/// - The [`Send`] constraint is because the closure will need to be passed
+///   *by value* from the thread where it is spawned to the new thread. Its
+///   return value will need to be passed from the new thread to the thread
+///   where it is `join`ed.
+///   As a reminder, the [`Send`] marker trait expresses that it is safe to be
+///   passed from thread to thread. [`Sync`] expresses that it is safe to have a
+///   reference be passed from thread to thread.
+///
+/// # Panics
+///
+/// Panics if the OS fails to create a thread; use [`Builder::spawn`]
+/// to recover from such errors.
+///
+/// # Examples
+///
+/// Creating a thread.
+///
+/// ```
+/// use std::thread;
+///
+/// let handler = thread::spawn(|| {
+///     // thread code
+/// });
+///
+/// handler.join().unwrap();
+/// ```
+///
+/// As mentioned in the module documentation, threads are usually made to
+/// communicate using [`channels`], here is how it usually looks.
+///
+/// This example also shows how to use `move`, in order to give ownership
+/// of values to a thread.
+///
+/// ```
+/// use std::thread;
+/// use std::sync::mpsc::channel;
+///
+/// let (tx, rx) = channel();
+///
+/// let sender = thread::spawn(move || {
+///     tx.send("Hello, thread".to_owned())
+///         .expect("Unable to send on channel");
+/// });
+///
+/// let receiver = thread::spawn(move || {
+///     let value = rx.recv().expect("Unable to receive from channel");
+///     println!("{}", value);
+/// });
+///
+/// sender.join().expect("The sender thread has panicked");
+/// receiver.join().expect("The receiver thread has panicked");
+/// ```
+///
+/// A thread can also return a value through its [`JoinHandle`], you can use
+/// this to make asynchronous computations (futures might be more appropriate
+/// though).
+///
+/// ```
+/// use std::thread;
+///
+/// let computation = thread::spawn(|| {
+///     // Some expensive computation.
+///     42
+/// });
+///
+/// let result = computation.join().unwrap();
+/// println!("{}", result);
+/// ```
+///
+/// [`channels`]: ../../std/sync/mpsc/index.html
+/// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
+/// [`join`]: ../../std/thread/struct.JoinHandle.html#method.join
+/// [`Err`]: ../../std/result/enum.Result.html#variant.Err
+/// [`panic`]: ../../std/macro.panic.html
+/// [`Builder::spawn`]: ../../std/thread/struct.Builder.html#method.spawn
+/// [`Builder`]: ../../std/thread/struct.Builder.html
+/// [`Send`]: ../../std/marker/trait.Send.html
+/// [`Sync`]: ../../std/marker/trait.Sync.html
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn spawn<F, T>(f: F) -> JoinHandle<T>
+where
+    F: FnOnce() -> T,
+    F: Send + 'static,
+    T: Send + 'static,
+{
+    Builder::new().spawn(f).expect("failed to spawn thread")
+}
+
+/// Gets a handle to the thread that invokes it.
+///
+/// # Examples
+///
+/// Getting a handle to the current thread with `thread::current()`:
+///
+/// ```
+/// use std::thread;
+///
+/// let handler = thread::Builder::new()
+///     .name("named thread".into())
+///     .spawn(|| {
+///         let handle = thread::current();
+///         assert_eq!(handle.name(), Some("named thread"));
+///     })
+///     .unwrap();
+///
+/// handler.join().unwrap();
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn current() -> Thread {
+    thread_info::current_thread().expect(
+        "use of std::thread::current() is not possible \
+         after the thread's local data has been destroyed",
+    )
+}
+
+/// Cooperatively gives up a timeslice to the OS scheduler.
+///
+/// This is used when the programmer knows that the thread will have nothing
+/// to do for some time, and thus avoid wasting computing time.
+///
+/// For example when polling on a resource, it is common to check that it is
+/// available, and if not to yield in order to avoid busy waiting.
+///
+/// Thus the pattern of `yield`ing after a failed poll is rather common when
+/// implementing low-level shared resources or synchronization primitives.
+///
+/// However programmers will usually prefer to use [`channel`]s, [`Condvar`]s,
+/// [`Mutex`]es or [`join`] for their synchronization routines, as they avoid
+/// thinking about thread scheduling.
+///
+/// Note that [`channel`]s for example are implemented using this primitive.
+/// Indeed when you call `send` or `recv`, which are blocking, they will yield
+/// if the channel is not available.
+///
+/// # Examples
+///
+/// ```
+/// use std::thread;
+///
+/// thread::yield_now();
+/// ```
+///
+/// [`channel`]: ../../std/sync/mpsc/index.html
+/// [`spawn`]: ../../std/thread/fn.spawn.html
+/// [`join`]: ../../std/thread/struct.JoinHandle.html#method.join
+/// [`Mutex`]: ../../std/sync/struct.Mutex.html
+/// [`Condvar`]: ../../std/sync/struct.Condvar.html
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn yield_now() {
+    imp::Thread::yield_now()
+}
+
+/// Determines whether the current thread is unwinding because of panic.
+///
+/// A common use of this feature is to poison shared resources when writing
+/// unsafe code, by checking `panicking` when the `drop` is called.
+///
+/// This is usually not needed when writing safe code, as [`Mutex`es][Mutex]
+/// already poison themselves when a thread panics while holding the lock.
+///
+/// This can also be used in multithreaded applications, in order to send a
+/// message to other threads warning that a thread has panicked (e.g., for
+/// monitoring purposes).
+///
+/// # Examples
+///
+/// ```should_panic
+/// use std::thread;
+///
+/// struct SomeStruct;
+///
+/// impl Drop for SomeStruct {
+///     fn drop(&mut self) {
+///         if thread::panicking() {
+///             println!("dropped while unwinding");
+///         } else {
+///             println!("dropped while not unwinding");
+///         }
+///     }
+/// }
+///
+/// {
+///     print!("a: ");
+///     let a = SomeStruct;
+/// }
+///
+/// {
+///     print!("b: ");
+///     let b = SomeStruct;
+///     panic!()
+/// }
+/// ```
+///
+/// [Mutex]: ../../std/sync/struct.Mutex.html
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn panicking() -> bool {
+    panicking::panicking()
+}
+
+/// Puts the current thread to sleep for at least the specified amount of time.
+///
+/// The thread may sleep longer than the duration specified due to scheduling
+/// specifics or platform-dependent functionality. It will never sleep less.
+///
+/// This function is blocking, and should not be used in `async` functions.
+///
+/// # Platform-specific behavior
+///
+/// On Unix platforms, the underlying syscall may be interrupted by a
+/// spurious wakeup or signal handler. To ensure the sleep occurs for at least
+/// the specified duration, this function may invoke that system call multiple
+/// times.
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::thread;
+///
+/// // Let's sleep for 2 seconds:
+/// thread::sleep_ms(2000);
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_deprecated(since = "1.6.0", reason = "replaced by `std::thread::sleep`")]
+pub fn sleep_ms(ms: u32) {
+    sleep(Duration::from_millis(ms as u64))
+}
+
+/// Puts the current thread to sleep for at least the specified amount of time.
+///
+/// The thread may sleep longer than the duration specified due to scheduling
+/// specifics or platform-dependent functionality. It will never sleep less.
+///
+/// This function is blocking, and should not be used in `async` functions.
+///
+/// # Platform-specific behavior
+///
+/// On Unix platforms, the underlying syscall may be interrupted by a
+/// spurious wakeup or signal handler. To ensure the sleep occurs for at least
+/// the specified duration, this function may invoke that system call multiple
+/// times.
+/// Platforms which do not support nanosecond precision for sleeping will
+/// have `dur` rounded up to the nearest granularity of time they can sleep for.
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::{thread, time};
+///
+/// let ten_millis = time::Duration::from_millis(10);
+/// let now = time::Instant::now();
+///
+/// thread::sleep(ten_millis);
+///
+/// assert!(now.elapsed() >= ten_millis);
+/// ```
+#[stable(feature = "thread_sleep", since = "1.4.0")]
+pub fn sleep(dur: Duration) {
+    imp::Thread::sleep(dur)
+}
+
+// constants for park/unpark
+const EMPTY: usize = 0;
+const PARKED: usize = 1;
+const NOTIFIED: usize = 2;
+
+/// Blocks unless or until the current thread's token is made available.
+///
+/// A call to `park` does not guarantee that the thread will remain parked
+/// forever, and callers should be prepared for this possibility.
+///
+/// # park and unpark
+///
+/// Every thread is equipped with some basic low-level blocking support, via the
+/// [`thread::park`][`park`] function and [`thread::Thread::unpark`][`unpark`]
+/// method. [`park`] blocks the current thread, which can then be resumed from
+/// another thread by calling the [`unpark`] method on the blocked thread's
+/// handle.
+///
+/// Conceptually, each [`Thread`] handle has an associated token, which is
+/// initially not present:
+///
+/// * The [`thread::park`][`park`] function blocks the current thread unless or
+///   until the token is available for its thread handle, at which point it
+///   atomically consumes the token. It may also return *spuriously*, without
+///   consuming the token. [`thread::park_timeout`] does the same, but allows
+///   specifying a maximum time to block the thread for.
+///
+/// * The [`unpark`] method on a [`Thread`] atomically makes the token available
+///   if it wasn't already. Because the token is initially absent, [`unpark`]
+///   followed by [`park`] will result in the second call returning immediately.
+///
+/// In other words, each [`Thread`] acts a bit like a spinlock that can be
+/// locked and unlocked using `park` and `unpark`.
+///
+/// Notice that being unblocked does not imply any synchronization with someone
+/// that unparked this thread, it could also be spurious.
+/// For example, it would be a valid, but inefficient, implementation to make both [`park`] and
+/// [`unpark`] return immediately without doing anything.
+///
+/// The API is typically used by acquiring a handle to the current thread,
+/// placing that handle in a shared data structure so that other threads can
+/// find it, and then `park`ing in a loop. When some desired condition is met, another
+/// thread calls [`unpark`] on the handle.
+///
+/// The motivation for this design is twofold:
+///
+/// * It avoids the need to allocate mutexes and condvars when building new
+///   synchronization primitives; the threads already provide basic
+///   blocking/signaling.
+///
+/// * It can be implemented very efficiently on many platforms.
+///
+/// # Examples
+///
+/// ```
+/// use std::thread;
+/// use std::sync::{Arc, atomic::{Ordering, AtomicBool}};
+/// use std::time::Duration;
+///
+/// let flag = Arc::new(AtomicBool::new(false));
+/// let flag2 = Arc::clone(&flag);
+///
+/// let parked_thread = thread::spawn(move || {
+///     // We want to wait until the flag is set. We *could* just spin, but using
+///     // park/unpark is more efficient.
+///     while !flag2.load(Ordering::Acquire) {
+///         println!("Parking thread");
+///         thread::park();
+///         // We *could* get here spuriously, i.e., way before the 10ms below are over!
+///         // But that is no problem, we are in a loop until the flag is set anyway.
+///         println!("Thread unparked");
+///     }
+///     println!("Flag received");
+/// });
+///
+/// // Let some time pass for the thread to be spawned.
+/// thread::sleep(Duration::from_millis(10));
+///
+/// // Set the flag, and let the thread wake up.
+/// // There is no race condition here, if `unpark`
+/// // happens first, `park` will return immediately.
+/// // Hence there is no risk of a deadlock.
+/// flag.store(true, Ordering::Release);
+/// println!("Unpark the thread");
+/// parked_thread.thread().unpark();
+///
+/// parked_thread.join().unwrap();
+/// ```
+///
+/// [`Thread`]: ../../std/thread/struct.Thread.html
+/// [`park`]: ../../std/thread/fn.park.html
+/// [`unpark`]: ../../std/thread/struct.Thread.html#method.unpark
+/// [`thread::park_timeout`]: ../../std/thread/fn.park_timeout.html
+//
+// The implementation currently uses the trivial strategy of a Mutex+Condvar
+// with wakeup flag, which does not actually allow spurious wakeups. In the
+// future, this will be implemented in a more efficient way, perhaps along the lines of
+//   http://cr.openjdk.java.net/~stefank/6989984.1/raw_files/new/src/os/linux/vm/os_linux.cpp
+// or futuxes, and in either case may allow spurious wakeups.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn park() {
+    let thread = current();
+
+    // If we were previously notified then we consume this notification and
+    // return quickly.
+    if thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
+        return;
+    }
+
+    // Otherwise we need to coordinate going to sleep
+    let mut m = thread.inner.lock.lock().unwrap();
+    match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
+        Ok(_) => {}
+        Err(NOTIFIED) => {
+            // We must read here, even though we know it will be `NOTIFIED`.
+            // This is because `unpark` may have been called again since we read
+            // `NOTIFIED` in the `compare_exchange` above. We must perform an
+            // acquire operation that synchronizes with that `unpark` to observe
+            // any writes it made before the call to unpark. To do that we must
+            // read from the write it made to `state`.
+            let old = thread.inner.state.swap(EMPTY, SeqCst);
+            assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
+            return;
+        } // should consume this notification, so prohibit spurious wakeups in next park.
+        Err(_) => panic!("inconsistent park state"),
+    }
+    loop {
+        m = thread.inner.cvar.wait(m).unwrap();
+        match thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
+            Ok(_) => return, // got a notification
+            Err(_) => {}     // spurious wakeup, go back to sleep
+        }
+    }
+}
+
+/// Use [`park_timeout`].
+///
+/// Blocks unless or until the current thread's token is made available or
+/// the specified duration has been reached (may wake spuriously).
+///
+/// The semantics of this function are equivalent to [`park`] except
+/// that the thread will be blocked for roughly no longer than `dur`. This
+/// method should not be used for precise timing due to anomalies such as
+/// preemption or platform differences that may not cause the maximum
+/// amount of time waited to be precisely `ms` long.
+///
+/// See the [park documentation][`park`] for more detail.
+///
+/// [`park_timeout`]: fn.park_timeout.html
+/// [`park`]: ../../std/thread/fn.park.html
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_deprecated(since = "1.6.0", reason = "replaced by `std::thread::park_timeout`")]
+pub fn park_timeout_ms(ms: u32) {
+    park_timeout(Duration::from_millis(ms as u64))
+}
+
+/// Blocks unless or until the current thread's token is made available or
+/// the specified duration has been reached (may wake spuriously).
+///
+/// The semantics of this function are equivalent to [`park`][park] except
+/// that the thread will be blocked for roughly no longer than `dur`. This
+/// method should not be used for precise timing due to anomalies such as
+/// preemption or platform differences that may not cause the maximum
+/// amount of time waited to be precisely `dur` long.
+///
+/// See the [park documentation][park] for more details.
+///
+/// # Platform-specific behavior
+///
+/// Platforms which do not support nanosecond precision for sleeping will have
+/// `dur` rounded up to the nearest granularity of time they can sleep for.
+///
+/// # Examples
+///
+/// Waiting for the complete expiration of the timeout:
+///
+/// ```rust,no_run
+/// use std::thread::park_timeout;
+/// use std::time::{Instant, Duration};
+///
+/// let timeout = Duration::from_secs(2);
+/// let beginning_park = Instant::now();
+///
+/// let mut timeout_remaining = timeout;
+/// loop {
+///     park_timeout(timeout_remaining);
+///     let elapsed = beginning_park.elapsed();
+///     if elapsed >= timeout {
+///         break;
+///     }
+///     println!("restarting park_timeout after {:?}", elapsed);
+///     timeout_remaining = timeout - elapsed;
+/// }
+/// ```
+///
+/// [park]: fn.park.html
+#[stable(feature = "park_timeout", since = "1.4.0")]
+pub fn park_timeout(dur: Duration) {
+    let thread = current();
+
+    // Like `park` above we have a fast path for an already-notified thread, and
+    // afterwards we start coordinating for a sleep.
+    // return quickly.
+    if thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
+        return;
+    }
+    let m = thread.inner.lock.lock().unwrap();
+    match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
+        Ok(_) => {}
+        Err(NOTIFIED) => {
+            // We must read again here, see `park`.
+            let old = thread.inner.state.swap(EMPTY, SeqCst);
+            assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
+            return;
+        } // should consume this notification, so prohibit spurious wakeups in next park.
+        Err(_) => panic!("inconsistent park_timeout state"),
+    }
+
+    // Wait with a timeout, and if we spuriously wake up or otherwise wake up
+    // from a notification we just want to unconditionally set the state back to
+    // empty, either consuming a notification or un-flagging ourselves as
+    // parked.
+    let (_m, _result) = thread.inner.cvar.wait_timeout(m, dur).unwrap();
+    match thread.inner.state.swap(EMPTY, SeqCst) {
+        NOTIFIED => {} // got a notification, hurray!
+        PARKED => {}   // no notification, alas
+        n => panic!("inconsistent park_timeout state: {}", n),
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// ThreadId
+////////////////////////////////////////////////////////////////////////////////
+
+/// A unique identifier for a running thread.
+///
+/// A `ThreadId` is an opaque object that has a unique value for each thread
+/// that creates one. `ThreadId`s are not guaranteed to correspond to a thread's
+/// system-designated identifier. A `ThreadId` can be retrieved from the [`id`]
+/// method on a [`Thread`].
+///
+/// # Examples
+///
+/// ```
+/// use std::thread;
+///
+/// let other_thread = thread::spawn(|| {
+///     thread::current().id()
+/// });
+///
+/// let other_thread_id = other_thread.join().unwrap();
+/// assert!(thread::current().id() != other_thread_id);
+/// ```
+///
+/// [`id`]: ../../std/thread/struct.Thread.html#method.id
+/// [`Thread`]: ../../std/thread/struct.Thread.html
+#[stable(feature = "thread_id", since = "1.19.0")]
+#[derive(Eq, PartialEq, Clone, Copy, Hash, Debug)]
+pub struct ThreadId(NonZeroU64);
+
+impl ThreadId {
+    // Generate a new unique thread ID.
+    fn new() -> ThreadId {
+        // We never call `GUARD.init()`, so it is UB to attempt to
+        // acquire this mutex reentrantly!
+        static GUARD: mutex::Mutex = mutex::Mutex::new();
+        static mut COUNTER: u64 = 1;
+
+        unsafe {
+            let _guard = GUARD.lock();
+
+            // If we somehow use up all our bits, panic so that we're not
+            // covering up subtle bugs of IDs being reused.
+            if COUNTER == u64::MAX {
+                panic!("failed to generate unique thread ID: bitspace exhausted");
+            }
+
+            let id = COUNTER;
+            COUNTER += 1;
+
+            ThreadId(NonZeroU64::new(id).unwrap())
+        }
+    }
+
+    /// This returns a numeric identifier for the thread identified by this
+    /// `ThreadId`.
+    ///
+    /// As noted in the documentation for the type itself, it is essentially an
+    /// opaque ID, but is guaranteed to be unique for each thread. The returned
+    /// value is entirely opaque -- only equality testing is stable. Note that
+    /// it is not guaranteed which values new threads will return, and this may
+    /// change across Rust versions.
+    #[unstable(feature = "thread_id_value", issue = "67939")]
+    pub fn as_u64(&self) -> NonZeroU64 {
+        self.0
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Thread
+////////////////////////////////////////////////////////////////////////////////
+
+/// The internal representation of a `Thread` handle
+struct Inner {
+    name: Option<CString>, // Guaranteed to be UTF-8
+    id: ThreadId,
+
+    // state for thread park/unpark
+    state: AtomicUsize,
+    lock: Mutex<()>,
+    cvar: Condvar,
+}
+
+#[derive(Clone)]
+#[stable(feature = "rust1", since = "1.0.0")]
+/// A handle to a thread.
+///
+/// Threads are represented via the `Thread` type, which you can get in one of
+/// two ways:
+///
+/// * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
+///   function, and calling [`thread`][`JoinHandle::thread`] on the
+///   [`JoinHandle`].
+/// * By requesting the current thread, using the [`thread::current`] function.
+///
+/// The [`thread::current`] function is available even for threads not spawned
+/// by the APIs of this module.
+///
+/// There is usually no need to create a `Thread` struct yourself, one
+/// should instead use a function like `spawn` to create new threads, see the
+/// docs of [`Builder`] and [`spawn`] for more details.
+///
+/// [`Builder`]: ../../std/thread/struct.Builder.html
+/// [`JoinHandle::thread`]: ../../std/thread/struct.JoinHandle.html#method.thread
+/// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
+/// [`thread::current`]: ../../std/thread/fn.current.html
+/// [`spawn`]: ../../std/thread/fn.spawn.html
+
+pub struct Thread {
+    inner: Arc<Inner>,
+}
+
+impl Thread {
+    // Used only internally to construct a thread object without spawning
+    // Panics if the name contains nuls.
+    pub(crate) fn new(name: Option<String>) -> Thread {
+        let cname =
+            name.map(|n| CString::new(n).expect("thread name may not contain interior null bytes"));
+        Thread {
+            inner: Arc::new(Inner {
+                name: cname,
+                id: ThreadId::new(),
+                state: AtomicUsize::new(EMPTY),
+                lock: Mutex::new(()),
+                cvar: Condvar::new(),
+            }),
+        }
+    }
+
+    /// Atomically makes the handle's token available if it is not already.
+    ///
+    /// Every thread is equipped with some basic low-level blocking support, via
+    /// the [`park`][park] function and the `unpark()` method. These can be
+    /// used as a more CPU-efficient implementation of a spinlock.
+    ///
+    /// See the [park documentation][park] for more details.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    /// use std::time::Duration;
+    ///
+    /// let parked_thread = thread::Builder::new()
+    ///     .spawn(|| {
+    ///         println!("Parking thread");
+    ///         thread::park();
+    ///         println!("Thread unparked");
+    ///     })
+    ///     .unwrap();
+    ///
+    /// // Let some time pass for the thread to be spawned.
+    /// thread::sleep(Duration::from_millis(10));
+    ///
+    /// println!("Unpark the thread");
+    /// parked_thread.thread().unpark();
+    ///
+    /// parked_thread.join().unwrap();
+    /// ```
+    ///
+    /// [park]: fn.park.html
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn unpark(&self) {
+        // To ensure the unparked thread will observe any writes we made
+        // before this call, we must perform a release operation that `park`
+        // can synchronize with. To do that we must write `NOTIFIED` even if
+        // `state` is already `NOTIFIED`. That is why this must be a swap
+        // rather than a compare-and-swap that returns if it reads `NOTIFIED`
+        // on failure.
+        match self.inner.state.swap(NOTIFIED, SeqCst) {
+            EMPTY => return,    // no one was waiting
+            NOTIFIED => return, // already unparked
+            PARKED => {}        // gotta go wake someone up
+            _ => panic!("inconsistent state in unpark"),
+        }
+
+        // There is a period between when the parked thread sets `state` to
+        // `PARKED` (or last checked `state` in the case of a spurious wake
+        // up) and when it actually waits on `cvar`. If we were to notify
+        // during this period it would be ignored and then when the parked
+        // thread went to sleep it would never wake up. Fortunately, it has
+        // `lock` locked at this stage so we can acquire `lock` to wait until
+        // it is ready to receive the notification.
+        //
+        // Releasing `lock` before the call to `notify_one` means that when the
+        // parked thread wakes it doesn't get woken only to have to wait for us
+        // to release `lock`.
+        drop(self.inner.lock.lock().unwrap());
+        self.inner.cvar.notify_one()
+    }
+
+    /// Gets the thread's unique identifier.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let other_thread = thread::spawn(|| {
+    ///     thread::current().id()
+    /// });
+    ///
+    /// let other_thread_id = other_thread.join().unwrap();
+    /// assert!(thread::current().id() != other_thread_id);
+    /// ```
+    #[stable(feature = "thread_id", since = "1.19.0")]
+    pub fn id(&self) -> ThreadId {
+        self.inner.id
+    }
+
+    /// Gets the thread's name.
+    ///
+    /// For more information about named threads, see
+    /// [this module-level documentation][naming-threads].
+    ///
+    /// # Examples
+    ///
+    /// Threads by default have no name specified:
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new();
+    ///
+    /// let handler = builder.spawn(|| {
+    ///     assert!(thread::current().name().is_none());
+    /// }).unwrap();
+    ///
+    /// handler.join().unwrap();
+    /// ```
+    ///
+    /// Thread with a specified name:
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new()
+    ///     .name("foo".into());
+    ///
+    /// let handler = builder.spawn(|| {
+    ///     assert_eq!(thread::current().name(), Some("foo"))
+    /// }).unwrap();
+    ///
+    /// handler.join().unwrap();
+    /// ```
+    ///
+    /// [naming-threads]: ./index.html#naming-threads
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn name(&self) -> Option<&str> {
+        self.cname().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
+    }
+
+    fn cname(&self) -> Option<&CStr> {
+        self.inner.name.as_deref()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl fmt::Debug for Thread {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Thread").field("id", &self.id()).field("name", &self.name()).finish()
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// JoinHandle
+////////////////////////////////////////////////////////////////////////////////
+
+/// A specialized [`Result`] type for threads.
+///
+/// Indicates the manner in which a thread exited.
+///
+/// The value contained in the `Result::Err` variant
+/// is the value the thread panicked with;
+/// that is, the argument the `panic!` macro was called with.
+/// Unlike with normal errors, this value doesn't implement
+/// the [`Error`](crate::error::Error) trait.
+///
+/// Thus, a sensible way to handle a thread panic is to either:
+/// 1. `unwrap` the `Result<T>`, propagating the panic
+/// 2. or in case the thread is intended to be a subsystem boundary
+/// that is supposed to isolate system-level failures,
+/// match on the `Err` variant and handle the panic in an appropriate way.
+///
+/// A thread that completes without panicking is considered to exit successfully.
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::thread;
+/// use std::fs;
+///
+/// fn copy_in_thread() -> thread::Result<()> {
+///     thread::spawn(move || { fs::copy("foo.txt", "bar.txt").unwrap(); }).join()
+/// }
+///
+/// fn main() {
+///     match copy_in_thread() {
+///         Ok(_) => println!("this is fine"),
+///         Err(_) => println!("thread panicked"),
+///     }
+/// }
+/// ```
+///
+/// [`Result`]: ../../std/result/enum.Result.html
+#[stable(feature = "rust1", since = "1.0.0")]
+pub type Result<T> = crate::result::Result<T, Box<dyn Any + Send + 'static>>;
+
+// This packet is used to communicate the return value between the child thread
+// and the parent thread. Memory is shared through the `Arc` within and there's
+// no need for a mutex here because synchronization happens with `join()` (the
+// parent thread never reads this packet until the child has exited).
+//
+// This packet itself is then stored into a `JoinInner` which in turns is placed
+// in `JoinHandle` and `JoinGuard`. Due to the usage of `UnsafeCell` we need to
+// manually worry about impls like Send and Sync. The type `T` should
+// already always be Send (otherwise the thread could not have been created) and
+// this type is inherently Sync because no methods take &self. Regardless,
+// however, we add inheriting impls for Send/Sync to this type to ensure it's
+// Send/Sync and that future modifications will still appropriately classify it.
+struct Packet<T>(Arc<UnsafeCell<Option<Result<T>>>>);
+
+unsafe impl<T: Send> Send for Packet<T> {}
+unsafe impl<T: Sync> Sync for Packet<T> {}
+
+/// Inner representation for JoinHandle
+struct JoinInner<T> {
+    native: Option<imp::Thread>,
+    thread: Thread,
+    packet: Packet<T>,
+}
+
+impl<T> JoinInner<T> {
+    fn join(&mut self) -> Result<T> {
+        self.native.take().unwrap().join();
+        unsafe { (*self.packet.0.get()).take().unwrap() }
+    }
+}
+
+/// An owned permission to join on a thread (block on its termination).
+///
+/// A `JoinHandle` *detaches* the associated thread when it is dropped, which
+/// means that there is no longer any handle to thread and no way to `join`
+/// on it.
+///
+/// Due to platform restrictions, it is not possible to [`Clone`] this
+/// handle: the ability to join a thread is a uniquely-owned permission.
+///
+/// This `struct` is created by the [`thread::spawn`] function and the
+/// [`thread::Builder::spawn`] method.
+///
+/// # Examples
+///
+/// Creation from [`thread::spawn`]:
+///
+/// ```
+/// use std::thread;
+///
+/// let join_handle: thread::JoinHandle<_> = thread::spawn(|| {
+///     // some work here
+/// });
+/// ```
+///
+/// Creation from [`thread::Builder::spawn`]:
+///
+/// ```
+/// use std::thread;
+///
+/// let builder = thread::Builder::new();
+///
+/// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
+///     // some work here
+/// }).unwrap();
+/// ```
+///
+/// Child being detached and outliving its parent:
+///
+/// ```no_run
+/// use std::thread;
+/// use std::time::Duration;
+///
+/// let original_thread = thread::spawn(|| {
+///     let _detached_thread = thread::spawn(|| {
+///         // Here we sleep to make sure that the first thread returns before.
+///         thread::sleep(Duration::from_millis(10));
+///         // This will be called, even though the JoinHandle is dropped.
+///         println!("♫ Still alive ♫");
+///     });
+/// });
+///
+/// original_thread.join().expect("The thread being joined has panicked");
+/// println!("Original thread is joined.");
+///
+/// // We make sure that the new thread has time to run, before the main
+/// // thread returns.
+///
+/// thread::sleep(Duration::from_millis(1000));
+/// ```
+///
+/// [`Clone`]: ../../std/clone/trait.Clone.html
+/// [`thread::spawn`]: fn.spawn.html
+/// [`thread::Builder::spawn`]: struct.Builder.html#method.spawn
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct JoinHandle<T>(JoinInner<T>);
+
+#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
+unsafe impl<T> Send for JoinHandle<T> {}
+#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
+unsafe impl<T> Sync for JoinHandle<T> {}
+
+impl<T> JoinHandle<T> {
+    /// Extracts a handle to the underlying thread.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new();
+    ///
+    /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
+    ///     // some work here
+    /// }).unwrap();
+    ///
+    /// let thread = join_handle.thread();
+    /// println!("thread id: {:?}", thread.id());
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn thread(&self) -> &Thread {
+        &self.0.thread
+    }
+
+    /// Waits for the associated thread to finish.
+    ///
+    /// In terms of [atomic memory orderings],  the completion of the associated
+    /// thread synchronizes with this function returning. In other words, all
+    /// operations performed by that thread are ordered before all
+    /// operations that happen after `join` returns.
+    ///
+    /// If the child thread panics, [`Err`] is returned with the parameter given
+    /// to [`panic`].
+    ///
+    /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
+    /// [`panic`]: ../../std/macro.panic.html
+    /// [atomic memory orderings]: ../../std/sync/atomic/index.html
+    ///
+    /// # Panics
+    ///
+    /// This function may panic on some platforms if a thread attempts to join
+    /// itself or otherwise may create a deadlock with joining threads.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    ///
+    /// let builder = thread::Builder::new();
+    ///
+    /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
+    ///     // some work here
+    /// }).unwrap();
+    /// join_handle.join().expect("Couldn't join on the associated thread");
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn join(mut self) -> Result<T> {
+        self.0.join()
+    }
+}
+
+impl<T> AsInner<imp::Thread> for JoinHandle<T> {
+    fn as_inner(&self) -> &imp::Thread {
+        self.0.native.as_ref().unwrap()
+    }
+}
+
+impl<T> IntoInner<imp::Thread> for JoinHandle<T> {
+    fn into_inner(self) -> imp::Thread {
+        self.0.native.unwrap()
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl<T> fmt::Debug for JoinHandle<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("JoinHandle { .. }")
+    }
+}
+
+fn _assert_sync_and_send() {
+    fn _assert_both<T: Send + Sync>() {}
+    _assert_both::<JoinHandle<()>>();
+    _assert_both::<Thread>();
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Tests
+////////////////////////////////////////////////////////////////////////////////
+
+#[cfg(all(test, not(target_os = "emscripten")))]
+mod tests {
+    use super::Builder;
+    use crate::any::Any;
+    use crate::mem;
+    use crate::result;
+    use crate::sync::mpsc::{channel, Sender};
+    use crate::thread::{self, ThreadId};
+    use crate::time::Duration;
+
+    // !!! These tests are dangerous. If something is buggy, they will hang, !!!
+    // !!! instead of exiting cleanly. This might wedge the buildbots.       !!!
+
+    #[test]
+    fn test_unnamed_thread() {
+        thread::spawn(move || {
+            assert!(thread::current().name().is_none());
+        })
+        .join()
+        .ok()
+        .expect("thread panicked");
+    }
+
+    #[test]
+    fn test_named_thread() {
+        Builder::new()
+            .name("ada lovelace".to_string())
+            .spawn(move || {
+                assert!(thread::current().name().unwrap() == "ada lovelace".to_string());
+            })
+            .unwrap()
+            .join()
+            .unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_invalid_named_thread() {
+        let _ = Builder::new().name("ada l\0velace".to_string()).spawn(|| {});
+    }
+
+    #[test]
+    fn test_run_basic() {
+        let (tx, rx) = channel();
+        thread::spawn(move || {
+            tx.send(()).unwrap();
+        });
+        rx.recv().unwrap();
+    }
+
+    #[test]
+    fn test_join_panic() {
+        match thread::spawn(move || panic!()).join() {
+            result::Result::Err(_) => (),
+            result::Result::Ok(()) => panic!(),
+        }
+    }
+
+    #[test]
+    fn test_spawn_sched() {
+        let (tx, rx) = channel();
+
+        fn f(i: i32, tx: Sender<()>) {
+            let tx = tx.clone();
+            thread::spawn(move || {
+                if i == 0 {
+                    tx.send(()).unwrap();
+                } else {
+                    f(i - 1, tx);
+                }
+            });
+        }
+        f(10, tx);
+        rx.recv().unwrap();
+    }
+
+    #[test]
+    fn test_spawn_sched_childs_on_default_sched() {
+        let (tx, rx) = channel();
+
+        thread::spawn(move || {
+            thread::spawn(move || {
+                tx.send(()).unwrap();
+            });
+        });
+
+        rx.recv().unwrap();
+    }
+
+    fn avoid_copying_the_body<F>(spawnfn: F)
+    where
+        F: FnOnce(Box<dyn Fn() + Send>),
+    {
+        let (tx, rx) = channel();
+
+        let x: Box<_> = box 1;
+        let x_in_parent = (&*x) as *const i32 as usize;
+
+        spawnfn(Box::new(move || {
+            let x_in_child = (&*x) as *const i32 as usize;
+            tx.send(x_in_child).unwrap();
+        }));
+
+        let x_in_child = rx.recv().unwrap();
+        assert_eq!(x_in_parent, x_in_child);
+    }
+
+    #[test]
+    fn test_avoid_copying_the_body_spawn() {
+        avoid_copying_the_body(|v| {
+            thread::spawn(move || v());
+        });
+    }
+
+    #[test]
+    fn test_avoid_copying_the_body_thread_spawn() {
+        avoid_copying_the_body(|f| {
+            thread::spawn(move || {
+                f();
+            });
+        })
+    }
+
+    #[test]
+    fn test_avoid_copying_the_body_join() {
+        avoid_copying_the_body(|f| {
+            let _ = thread::spawn(move || f()).join();
+        })
+    }
+
+    #[test]
+    fn test_child_doesnt_ref_parent() {
+        // If the child refcounts the parent thread, this will stack overflow when
+        // climbing the thread tree to dereference each ancestor. (See #1789)
+        // (well, it would if the constant were 8000+ - I lowered it to be more
+        // valgrind-friendly. try this at home, instead..!)
+        const GENERATIONS: u32 = 16;
+        fn child_no(x: u32) -> Box<dyn Fn() + Send> {
+            return Box::new(move || {
+                if x < GENERATIONS {
+                    thread::spawn(move || child_no(x + 1)());
+                }
+            });
+        }
+        thread::spawn(|| child_no(0)());
+    }
+
+    #[test]
+    fn test_simple_newsched_spawn() {
+        thread::spawn(move || {});
+    }
+
+    #[test]
+    fn test_try_panic_message_static_str() {
+        match thread::spawn(move || {
+            panic!("static string");
+        })
+        .join()
+        {
+            Err(e) => {
+                type T = &'static str;
+                assert!(e.is::<T>());
+                assert_eq!(*e.downcast::<T>().unwrap(), "static string");
+            }
+            Ok(()) => panic!(),
+        }
+    }
+
+    #[test]
+    fn test_try_panic_message_owned_str() {
+        match thread::spawn(move || {
+            panic!("owned string".to_string());
+        })
+        .join()
+        {
+            Err(e) => {
+                type T = String;
+                assert!(e.is::<T>());
+                assert_eq!(*e.downcast::<T>().unwrap(), "owned string".to_string());
+            }
+            Ok(()) => panic!(),
+        }
+    }
+
+    #[test]
+    fn test_try_panic_message_any() {
+        match thread::spawn(move || {
+            panic!(box 413u16 as Box<dyn Any + Send>);
+        })
+        .join()
+        {
+            Err(e) => {
+                type T = Box<dyn Any + Send>;
+                assert!(e.is::<T>());
+                let any = e.downcast::<T>().unwrap();
+                assert!(any.is::<u16>());
+                assert_eq!(*any.downcast::<u16>().unwrap(), 413);
+            }
+            Ok(()) => panic!(),
+        }
+    }
+
+    #[test]
+    fn test_try_panic_message_unit_struct() {
+        struct Juju;
+
+        match thread::spawn(move || panic!(Juju)).join() {
+            Err(ref e) if e.is::<Juju>() => {}
+            Err(_) | Ok(()) => panic!(),
+        }
+    }
+
+    #[test]
+    fn test_park_timeout_unpark_before() {
+        for _ in 0..10 {
+            thread::current().unpark();
+            thread::park_timeout(Duration::from_millis(u32::MAX as u64));
+        }
+    }
+
+    #[test]
+    fn test_park_timeout_unpark_not_called() {
+        for _ in 0..10 {
+            thread::park_timeout(Duration::from_millis(10));
+        }
+    }
+
+    #[test]
+    fn test_park_timeout_unpark_called_other_thread() {
+        for _ in 0..10 {
+            let th = thread::current();
+
+            let _guard = thread::spawn(move || {
+                super::sleep(Duration::from_millis(50));
+                th.unpark();
+            });
+
+            thread::park_timeout(Duration::from_millis(u32::MAX as u64));
+        }
+    }
+
+    #[test]
+    fn sleep_ms_smoke() {
+        thread::sleep(Duration::from_millis(2));
+    }
+
+    #[test]
+    fn test_size_of_option_thread_id() {
+        assert_eq!(mem::size_of::<Option<ThreadId>>(), mem::size_of::<ThreadId>());
+    }
+
+    #[test]
+    fn test_thread_id_equal() {
+        assert!(thread::current().id() == thread::current().id());
+    }
+
+    #[test]
+    fn test_thread_id_not_equal() {
+        let spawned_id = thread::spawn(|| thread::current().id()).join().unwrap();
+        assert!(thread::current().id() != spawned_id);
+    }
+
+    // NOTE: the corresponding test for stderr is in ui/thread-stderr, due
+    // to the test harness apparently interfering with stderr configuration.
+}