Unchecked thread spawning

src/libstd/thread/mod.rs

@@ -386,6 +386,74 @@ impl Builder {
     #[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
+    #[unstable(feature = "thread_spawn_unchecked", issue = "55132")]
+    pub unsafe fn spawn_unchecked<F, T>(self, f: F) -> io::Result<JoinHandle<T>> where
+        F: FnOnce() -> T, F: Send, T: Send
     {
         let Builder { name, stack_size } = self;
 
@@ -402,22 +470,19 @@ impl Builder {
             if let Some(name) = their_thread.cname() {
                 imp::Thread::set_name(name);
             }
-            unsafe {
-                thread_info::set(imp::guard::current(), their_thread);
-                #[cfg(feature = "backtrace")]
-                let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
-                    ::sys_common::backtrace::__rust_begin_short_backtrace(f)
-                }));
-                #[cfg(not(feature = "backtrace"))]
-                let try_result = panic::catch_unwind(panic::AssertUnwindSafe(f));
-                *their_packet.get() = Some(try_result);
-            }
+
+            thread_info::set(imp::guard::current(), their_thread);
+            #[cfg(feature = "backtrace")]
+            let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+                ::sys_common::backtrace::__rust_begin_short_backtrace(f)
+            }));
+            #[cfg(not(feature = "backtrace"))]
+            let try_result = panic::catch_unwind(panic::AssertUnwindSafe(f));
+            *their_packet.get() = Some(try_result);
         };
 
         Ok(JoinHandle(JoinInner {
-            native: unsafe {
-                Some(imp::Thread::new(stack_size, Box::new(main))?)
-            },
+            native: Some(imp::Thread::new(stack_size, Box::new(main))?),
             thread: my_thread,
             packet: Packet(my_packet),
         }))