libsyntax: fix pretty printing of macro with braces

Pretty printing of macro with braces but without terminated semicolon
removed more boxes from stack than it put there, resulting in panic.
This fixes the issue #30731.

src/libsyntax/print/pprust.rs

@@ -1781,11 +1781,8 @@ pub fn print_mac(&mut self, m: &ast::Mac, delim: token::DelimToken)
             token::Paren => try!(self.popen()),
             token::Bracket => try!(word(&mut self.s, "[")),
             token::Brace => {
-                // head-ibox, will be closed by bopen()
-                try!(self.ibox(0));
-                // Don't ask me why the regular bopen() does
-                // more then just opening a brace...
-                try!(self.bopen())
+                try!(self.head(""));
+                try!(self.bopen());
             }
         }
         try!(self.print_tts(&m.node.tts));

src/test/pretty/issue-30731.rs

@@ -0,0 +1,17 @@
+// Copyright 2016 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.
+//
+// Test pretty printing of macro with braces but without terminating semicolon,
+// this used to panic before fix.
+
+// pretty-compare-only
+// pp-exact
+
+fn main() { b!{ } c }