Add a linter rule for list assignments

Summary: Add a linter rule to prevent trailing commas in a list assignment. `list($x, $y,)` is equivalent to `list($x, $y)`, but the latter is cleaner and should be preferred.

Test Plan: Added test cases.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13870

src/__phutil_library_map__.php

@@ -156,6 +156,7 @@ phutil_register_library_map(array(
     'ArcanistLinter' => 'lint/linter/ArcanistLinter.php',
     'ArcanistLinterTestCase' => 'lint/linter/__tests__/ArcanistLinterTestCase.php',
     'ArcanistLintersWorkflow' => 'workflow/ArcanistLintersWorkflow.php',
+    'ArcanistListAssignmentXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistListAssignmentXHPASTLinterRule.php',
     'ArcanistListWorkflow' => 'workflow/ArcanistListWorkflow.php',
     'ArcanistLogicalOperatorsXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistLogicalOperatorsXHPASTLinterRule.php',
     'ArcanistLowercaseFunctionsXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistLowercaseFunctionsXHPASTLinterRule.php',
@@ -435,6 +436,7 @@ phutil_register_library_map(array(
     'ArcanistLinter' => 'Phobject',
     'ArcanistLinterTestCase' => 'PhutilTestCase',
     'ArcanistLintersWorkflow' => 'ArcanistWorkflow',
+    'ArcanistListAssignmentXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistListWorkflow' => 'ArcanistWorkflow',
     'ArcanistLogicalOperatorsXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistLowercaseFunctionsXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',

src/lint/linter/__tests__/xhpast/list-assignment.lint-test

@@ -0,0 +1,11 @@
+<?php
+list($x, $y, , ,) = array();
+list($x, $y, , , $z) = array();
+~~~~~~~~~~
+warning:2:12
+warning:2:14
+warning:2:16
+~~~~~~~~~~
+<?php
+list($x, $y  ) = array();
+list($x, $y, , , $z) = array();

src/lint/linter/xhpast/rules/ArcanistListAssignmentXHPASTLinterRule.php

@@ -0,0 +1,38 @@
+<?php
+
+final class ArcanistListAssignmentXHPASTLinterRule
+  extends ArcanistXHPASTLinterRule {
+
+  const ID = 77;
+
+  public function getLintName() {
+    return pht('List Assignment');
+  }
+
+  public function getLintSeverity() {
+    return ArcanistLintSeverity::SEVERITY_WARNING;
+  }
+
+  public function process(XHPASTNode $root) {
+    $assignment_lists = $root->selectDescendantsOfType('n_ASSIGNMENT_LIST');
+
+    foreach ($assignment_lists as $assignment_list) {
+      $tokens = array_slice($assignment_list->getTokens(), 1, -1);
+
+      foreach (array_reverse($tokens) as $token) {
+        if ($token->getTypeName() == ',') {
+          $this->raiseLintAtToken(
+            $token,
+            pht('Unnecessary comma in list assignment.'),
+            '');
+          continue;
+        }
+
+        if ($token->isSemantic()) {
+          break;
+        }
+      }
+    }
+  }
+
+}