diff --git a/src/libproc_macro/lib.rs b/src/libproc_macro/lib.rs
index cf1d4024373..1163c1a98d5 100644
--- a/src/libproc_macro/lib.rs
+++ b/src/libproc_macro/lib.rs
@@ -38,6 +38,7 @@
 #![feature(rustc_private)]
 #![feature(staged_api)]
 #![feature(lang_items)]
+#![feature(optin_builtin_traits)]
 
 #[macro_use]
 extern crate syntax;
@@ -310,6 +311,11 @@ pub struct SourceFile {
     filemap: Lrc<FileMap>,
 }
 
+#[unstable(feature = "proc_macro", issue = "38356")]
+impl !Send for SourceFile {}
+#[unstable(feature = "proc_macro", issue = "38356")]
+impl !Sync for SourceFile {}
+
 impl SourceFile {
     /// Get the path to this source file.
     ///