Auto merge of #31254 - tmiasko:macro-pretty-print-fix, r=sfackler

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.

Author: bors

src/libsyntax/print/pprust.rs

@@ -1781,11 +1781,8 @@ impl<'a> State<'a> {
             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 }