rollup merge of #19918: pnkfelix/ast-refactor-make-place-in-exprbox-an-option

This is to allow us to migrate away from UnUniq in a followup commit,
and thus unify the code paths related to all forms of `box`.

src/libsyntax/print/pprust.rs

@@ -1501,7 +1501,7 @@ impl<'a> State<'a> {
             ast::ExprBox(ref p, ref e) => {
                 try!(word(&mut self.s, "box"));
                 try!(word(&mut self.s, "("));
-                try!(self.print_expr(&**p));
+                try!(p.as_ref().map_or(Ok(()), |e|self.print_expr(&**e)));
                 try!(self.word_space(")"));
                 try!(self.print_expr(&**e));
             }