Added a test that joining main is UB.

Author: Vytautas Astrauskas

tests/compile-fail/concurrency/libc_pthread_join_main.rs

@@ -0,0 +1,20 @@
+// ignore-windows: Concurrency on Windows is not supported yet.
+
+// Joining the main thread is undefined behavior.
+
+#![feature(rustc_private)]
+
+extern crate libc;
+
+use std::{ptr, thread};
+
+fn main() {
+    let thread_id: libc::pthread_t = unsafe { libc::pthread_self() };
+    let handle = thread::spawn(move || {
+        unsafe {
+            assert_eq!(libc::pthread_join(thread_id, ptr::null_mut()), 0); //~ ERROR: Undefined Behavior: trying to join a detached or already joined thread
+        }
+    });
+    thread::yield_now();
+    handle.join().unwrap();
+}