Disallow $($v:vis)*. Fix #42755.

src/libsyntax/ext/tt/macro_rules.rs

@@ -299,6 +299,7 @@ fn check_lhs_no_empty_seq(sess: &ParseSess, tts: &[quoted::TokenTree]) -> bool {
             TokenTree::Sequence(span, ref seq) => {
                 if seq.separator.is_none() && seq.tts.iter().all(|seq_tt| {
                     match *seq_tt {
+                        TokenTree::MetaVarDecl(_, _, id) => id.name == "vis",
                         TokenTree::Sequence(_, ref sub_seq) =>
                             sub_seq.op == quoted::KleeneOp::ZeroOrMore,
                         _ => false,

src/test/compile-fail/issue-42755.rs

@@ -0,0 +1,17 @@
+// Copyright 2017 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.
+
+#![feature(macro_vis_matcher)]
+
+macro_rules! foo {
+    ($($p:vis)*) => {} //~ ERROR repetition matches empty token tree
+}
+
+foo!(a);