Disallow underscore suffix for string-like literals.

Author: qnighy

src/libsyntax/parse/lexer/mod.rs

@@ -479,11 +479,7 @@ impl<'a> StringReader<'a> {
         }
 
         self.with_str_from(start, |string| {
-            if string == "_" {
-                None
-            } else {
-                Some(Symbol::intern(string))
-            }
+            Some(Symbol::intern(string))
         })
     }
 

src/test/parse-fail/underscore-suffix-for-string.rs

@@ -0,0 +1,13 @@
+// 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.
+
+fn main() {
+    let a = "Foo"_; //~ ERROR string literal with a suffix is invalid
+}