Add error for using null characters in export_name

src/test/ui/error-codes/E0648.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[export_name="\0foo"] //~ ERROR E0648
+pub fn bar() {}
+
+fn main() {}

src/test/ui/error-codes/E0648.stderr

@@ -0,0 +1,9 @@
+error[E0648]: `export_name` may not contain null characters
+  --> $DIR/E0648.rs:11:1
+   |
+LL | #[export_name="/0foo"] //~ ERROR E0648
+   | ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0648`.