introduce DynSend and DynSync auto trait

src/librustdoc/lib.rs

@@ -730,6 +730,9 @@ fn main_args(at_args: &[String]) -> MainResult {
         }
     };
 
+    // Set parallel mode early as the error handler will already create locks.
+    interface::set_parallel_mode(&options.unstable_opts);
+
     let diag = core::new_handler(
         options.error_format,
         None,