Fix ESLint errors in lib/parser.js

Fix the following errors:

    31:9   error  "parser" is defined but never used    no-unused-vars
   406:14  error  "expected" is defined but never used  no-unused-vars
  1304:15  error  "s1" is defined but never used        no-unused-vars
  1386:15  error  "s1" is defined but never used        no-unused-vars
  1442:15  error  "s1" is defined but never used        no-unused-vars

Makefile

@@ -17,8 +17,9 @@ NODE_MODULES_BIN_DIR = $(NODE_MODULES_DIR)/.bin
 
 MAIN_FILE = $(LIB_DIR)/peg.js
 
-PARSER_SRC_FILE = $(SRC_DIR)/parser.pegjs
-PARSER_OUT_FILE = $(LIB_DIR)/parser.js
+PARSER_SRC_FILE     = $(SRC_DIR)/parser.pegjs
+PARSER_OUT_FILE     = $(LIB_DIR)/parser.js
+PARSER_OUT_FILE_NEW = $(LIB_DIR)/parser.js.new
 
 BROWSER_FILE_DEV = $(BROWSER_DIR)/peg-$(PEGJS_VERSION).js
 BROWSER_FILE_MIN = $(BROWSER_DIR)/peg-$(PEGJS_VERSION).min.js
@@ -41,7 +42,20 @@ all: browser
 
 # Generate the grammar parser
 parser:
-	$(PEGJS) $(PARSER_SRC_FILE) $(PARSER_OUT_FILE)
+	# We need to prepend ESLint header to the generated parser file because we
+	# don't want the various unused variables there to get reported. This is a bit
+	# tricky because the file is used when generating its own new version, which
+	# means we can't start writing the header there until we call $(PEGJS).
+
+	$(PEGJS) $(PARSER_SRC_FILE) $(PARSER_OUT_FILE_NEW)
+
+	rm -f $(PARSER_OUT_FILE)
+
+	echo '/* eslint no-unused-vars: 0 */' >> $(PARSER_OUT_FILE)
+	echo                                  >> $(PARSER_OUT_FILE)
+	cat $(PARSER_OUT_FILE_NEW)            >> $(PARSER_OUT_FILE)
+
+	rm $(PARSER_OUT_FILE_NEW)
 
 # Build the browser version of the library
 browser:

lib/parser.js

@@ -1,3 +1,5 @@
+/* eslint no-unused-vars: 0 */
+
 module.exports = (function() {
   "use strict";