parse.y: fix here-doc identifier with newline

* parse.y (heredoc_identifier): quoted here-document identifier
  must end within the same line.

  the only corner case that here-document identifier can contain a
  newline is that the closing qoute is placed at the beginning of
  the next line, and has been warned since 2.4.

  ```ruby
  <<"EOS
  " # warning: here document identifier ends with a newline
  EOS
  ```

NEWS

@@ -42,6 +42,12 @@ sufficient information, see the ChangeLog file or Redmine
 * Setting <code>$,</code> to non-nil value is warned now.  Use of it in
   Array#join is warned too.
 
+* Quoted here-document identifier must end within the same line.
+
+     <<"EOS
+     " # This has been warned since 2.4
+     EOS
+
 === Core classes updates (outstanding ones only)
 
 Enumerable::

doc/syntax/literals.rdoc

@@ -255,6 +255,9 @@ behaves like Kernel#`:
   cat #{__FILE__}
   HEREDOC
 
+When surrounding with quotes, any characters but that quote and newline
+can be used as the identifier.
+
 To call a method on a heredoc place it after the opening identifier:
 
   expected_result = <<-EXPECTED.chomp

parse.y

@@ -6788,7 +6788,6 @@ heredoc_identifier(struct parser_params *p)
     int c = nextc(p), term, func = 0, term_len = 2;
     enum yytokentype token = tSTRING_BEG;
     long len;
-    int newline = 0;
     int indent = 0;
 
     if (c == '-') {
@@ -6821,23 +6820,14 @@ heredoc_identifier(struct parser_params *p)
 	tokadd(p, func);
 	term = c;
 	while ((c = nextc(p)) != -1 && c != term) {
+	    if (c == '\n') goto unterminated;
 	    if (tokadd_mbchar(p, c) == -1) return 0;
-	    if (!newline && c == '\n') newline = 1;
-	    else if (newline) newline = 2;
 	}
 	if (c == -1) {
+	  unterminated:
 	    yyerror(NULL, p, "unterminated here document identifier");
 	    return -1;
 	}
-	switch (newline) {
-	  case 1:
-	    rb_warn0("here document identifier ends with a newline");
-	    if (--p->tokidx > 0 && p->tokenbuf[p->tokidx] == '\r') --p->tokidx;
-	    break;
-	  case 2:
-	    compile_error(p, "here document identifier across newlines, never match");
-	    return -1;
-	}
 	break;
 
       default:

test/ruby/test_syntax.rb

@@ -852,10 +852,9 @@ eom
     assert_syntax_error("puts <<""EOS\n""ng\n""EOS\r""NO\n", /can't find string "EOS" anywhere before EOF/)
   end
 
-  def test_heredoc_newline
-    assert_warn(/ends with a newline/) do
-      eval("<<\"EOS\n\"\nEOS\n")
-    end
+  def test_unterminated_heredoc
+    assert_syntax_error("<<\"EOS\n\nEOS\n", /unterminated/)
+    assert_syntax_error("<<\"EOS\n\"\nEOS\n", /unterminated/)
   end
 
   def test__END___cr