Report duplicate rule definitions as errors

Based on a pull request by Futago-za Ryuu (@futagoza):

  https://github.com/pegjs/pegjs/pull/329

Resolves #318.

lib/compiler/index.js

@@ -20,6 +20,7 @@ var compiler = {
   passes: {
     check: {
       reportMissingRules:   require("./passes/report-missing-rules"),
+      reportDuplicateRules: require("./passes/report-duplicate-rules"),
       reportLeftRecursion:  require("./passes/report-left-recursion"),
       reportInfiniteLoops:  require("./passes/report-infinite-loops")
     },

lib/compiler/passes/report-duplicate-rules.js

@@ -0,0 +1,26 @@
+"use strict";
+
+var GrammarError = require("../../grammar-error"),
+    visitor      = require("../visitor");
+
+/* Checks that each rule is defined only once. */
+function reportDuplicateRules(ast) {
+  var rules = {};
+
+  var check = visitor.build({
+    rule: function(node) {
+      if (rules.hasOwnProperty(node.name)) {
+        throw new GrammarError(
+          "Rule \"" + node.name + "\" is already defined.",
+          node.location
+        );
+      }
+
+      rules[node.name] = true;
+    }
+  });
+
+  check(ast);
+}
+
+module.exports = reportDuplicateRules;

package.json

@@ -31,6 +31,7 @@
     "lib/compiler/passes/report-left-recursion.js",
     "lib/compiler/passes/report-infinite-loops.js",
     "lib/compiler/passes/report-missing-rules.js",
+    "lib/compiler/passes/report-duplicate-rules.js",
     "lib/grammar-error.js",
     "lib/parser.js",
     "lib/peg.js",

spec/index.html

@@ -10,6 +10,7 @@
     <script src="unit/parser.spec.js"></script>
     <script src="unit/compiler/passes/helpers.js"></script>
     <script src="unit/compiler/passes/report-missing-rules.spec.js"></script>
+    <script src="unit/compiler/passes/report-duplicate-rules.spec.js"></script>
     <script src="unit/compiler/passes/report-left-recursion.spec.js"></script>
     <script src="unit/compiler/passes/report-infinite-loops.spec.js"></script>
     <script src="unit/compiler/passes/remove-proxy-rules.spec.js"></script>

spec/unit/compiler/passes/report-duplicate-rules.spec.js

@@ -0,0 +1,20 @@
+/* global peg */
+
+"use strict";
+
+describe("compiler pass |reportDuplicateRules|", function() {
+  var pass = peg.compiler.passes.check.reportDuplicateRules;
+
+  it("reports duplicate rules", function() {
+    expect(pass).toReportError([
+      'start = "a"',
+      'start = "b"'
+    ].join('\n'), {
+      message:  'Rule "start" is already defined.',
+      location: {
+        start: { offset: 12, line: 2, column:  1 },
+        end:   { offset: 23, line: 2, column: 12 }
+      }
+    });
+  });
+});