Add a mean to get the current threadpool

It is useful when you want to share the same threadpool across
threads outside the threadpool.

rayon-core/src/lib.rs

@@ -109,6 +109,15 @@ pub fn current_num_threads() -> usize {
     crate::registry::Registry::current_num_threads()
 }
 
+/// Returns the current threadpool.
+///
+/// It is a shorthand for [`Threadpool::current()`][tc].
+///
+/// [tc]: struct.ThreadPool.html#method.current
+pub fn current() -> ThreadPool {
+    ThreadPool::current()
+}
+
 /// Error when initializing a thread pool.
 #[derive(Debug)]
 pub struct ThreadPoolBuildError {

rayon-core/src/thread_pool/mod.rs

@@ -252,6 +252,12 @@ impl ThreadPool {
         // We assert that `self.registry` has not terminated.
         unsafe { spawn::spawn_fifo_in(op, &self.registry) }
     }
+
+    /// Returns the current threadpool.
+    pub fn current() -> ThreadPool {
+        let registry = crate::registry::Registry::current();
+        ThreadPool { registry }
+    }
 }
 
 impl Drop for ThreadPool {