Add a linter rule to detect unnecessary semicolons

Summary:
Ref T7409. Adds a rule to detect unnecessary semicolons. The most common scenario I've seen in the wild is the use of semicolons after a class definition:

```lang=php
class MyClass {
  // ...
};
```

Test Plan: Added unit tests.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7409

Differential Revision: https://secure.phabricator.com/D12139

src/lint/linter/ArcanistXHPASTLinter.php

@@ -55,6 +55,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
   const LINT_CALL_TIME_PASS_BY_REF      = 53;
   const LINT_FORMATTED_STRING           = 54;
   const LINT_UNNECESSARY_FINAL_MODIFIER = 55;
+  const LINT_UNNECESSARY_SEMICOLON      = 56;
 
   private $blacklistedFunctions = array();
   private $naminghook;
@@ -123,6 +124,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
       self::LINT_CALL_TIME_PASS_BY_REF      => 'Call-Time Pass-By-Reference',
       self::LINT_FORMATTED_STRING           => 'Formatted String',
       self::LINT_UNNECESSARY_FINAL_MODIFIER => 'Unnecessary Final Modifier',
+      self::LINT_UNNECESSARY_SEMICOLON      => 'Unnecessary Semicolon',
     );
   }
 
@@ -166,6 +168,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
       self::LINT_CONSTRUCTOR_PARENTHESES    => $advice,
       self::LINT_IMPLICIT_VISIBILITY        => $advice,
       self::LINT_UNNECESSARY_FINAL_MODIFIER => $advice,
+      self::LINT_UNNECESSARY_SEMICOLON      => $advice,
     );
   }
 
@@ -233,7 +236,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
 
   public function getVersion() {
     // The version number should be incremented whenever a new rule is added.
-    return '17';
+    return '18';
   }
 
   protected function resolveFuture($path, Future $future) {
@@ -312,6 +315,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
       'lintCallTimePassByReference' => self::LINT_CALL_TIME_PASS_BY_REF,
       'lintFormattedString' => self::LINT_FORMATTED_STRING,
       'lintUnnecessaryFinalModifier' => self::LINT_UNNECESSARY_FINAL_MODIFIER,
+      'lintUnnecessarySemicolons' => self::LINT_UNNECESSARY_SEMICOLON,
     );
 
     foreach ($method_codes as $method => $codes) {
@@ -3229,6 +3233,24 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
     }
   }
 
+  private function lintUnnecessarySemicolons(XHPASTNode $root) {
+    $statements = $root->selectDescendantsOfType('n_STATEMENT');
+
+    foreach ($statements as $statement) {
+      if ($statement->getParentNode()->getTypeName() == 'n_DECLARE') {
+        continue;
+      }
+
+      if ($statement->getSemanticString() == ';') {
+        $this->raiseLintAtNode(
+          $statement,
+          self::LINT_UNNECESSARY_SEMICOLON,
+          pht('Unnecessary semicolons after statement.'),
+          '');
+      }
+    }
+  }
+
   public function getSuperGlobalNames() {
     return array(
       '$GLOBALS',

src/lint/linter/__tests__/xhpast/empty-statement.lint-test

@@ -0,0 +1,11 @@
+<?php
+final class Foo {};
+$x = null;;
+~~~~~~~~~~
+error:2:13 XHP19
+advice:2:19
+advice:3:11
+~~~~~~~~~~
+<?php
+final class Foo {}
+$x = null;