Add a linter rule for use statement namespace prefixes

Summary:
When importing or aliases a symbol with a `use` statement, the leading namespace separator is optional and does not modify the behavior. That is, `use \X` is equivalent to `use X`. As such, the latter syntax should be preferred because it is more concise.

According to the [[http://php.net/manual/en/language.namespaces.importing.php | PHP documentation]]:

> Note that for namespaced names (fully qualified namespace names containing namespace separator, such as `Foo\Bar` as opposed to global names that do not, such as `FooBar`), the leading backslash is unnecessary and not recommended, as import names must be fully qualified, and are not processed relative to the current namespace.

Test Plan: Added test cases.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D14517

src/__phutil_library_map__.php

@@ -352,6 +352,8 @@ phutil_register_library_map(array(
     'ArcanistUpgradeWorkflow' => 'workflow/ArcanistUpgradeWorkflow.php',
     'ArcanistUploadWorkflow' => 'workflow/ArcanistUploadWorkflow.php',
     'ArcanistUsageException' => 'exception/ArcanistUsageException.php',
+    'ArcanistUseStatementNamespacePrefixXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistUseStatementNamespacePrefixXHPASTLinterRule.php',
+    'ArcanistUseStatementNamespacePrefixXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistUseStatementNamespacePrefixXHPASTLinterRuleTestCase.php',
     'ArcanistUselessOverridingMethodXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistUselessOverridingMethodXHPASTLinterRule.php',
     'ArcanistUselessOverridingMethodXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistUselessOverridingMethodXHPASTLinterRuleTestCase.php',
     'ArcanistUserAbortException' => 'exception/usage/ArcanistUserAbortException.php',
@@ -730,6 +732,8 @@ phutil_register_library_map(array(
     'ArcanistUpgradeWorkflow' => 'ArcanistWorkflow',
     'ArcanistUploadWorkflow' => 'ArcanistWorkflow',
     'ArcanistUsageException' => 'Exception',
+    'ArcanistUseStatementNamespacePrefixXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
+    'ArcanistUseStatementNamespacePrefixXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistUselessOverridingMethodXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistUselessOverridingMethodXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistUserAbortException' => 'ArcanistUsageException',

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

@@ -0,0 +1,38 @@
+<?php
+
+final class ArcanistUseStatementNamespacePrefixXHPASTLinterRule
+  extends ArcanistXHPASTLinterRule {
+
+  const ID = 97;
+
+  public function getLintName() {
+    return pht('`%s` Statement Namespace Prefix', 'use');
+  }
+
+  public function getLintSeverity() {
+    return ArcanistLintSeverity::SEVERITY_WARNING;
+  }
+
+  public function process(XHPASTNode $root) {
+    $use_lists = $root->selectDescendantsOfType('n_USE_LIST');
+
+    foreach ($use_lists as $use_list) {
+      $uses = $use_list->selectDescendantsOfType('n_USE');
+
+      foreach ($uses as $use) {
+        $symbol = $use->getChildOfType(0, 'n_SYMBOL_NAME');
+        $symbol_name = $symbol->getConcreteString();
+
+        if ($symbol_name[0] == '\\') {
+          $this->raiseLintAtNode(
+            $symbol,
+            pht(
+              'Imported symbols should not be prefixed with `%s`.',
+              '\\'),
+            substr($symbol_name, 1));
+        }
+      }
+    }
+  }
+
+}

src/lint/linter/xhpast/rules/__tests__/ArcanistUseStatementNamespacePrefixXHPASTLinterRuleTestCase.php

@@ -0,0 +1,11 @@
+<?php
+
+final class ArcanistUseStatementNamespacePrefixXHPASTLinterRuleTestCase
+  extends ArcanistXHPASTLinterRuleTestCase {
+
+  public function testLinter() {
+    $this->executeTestsInDirectory(
+      dirname(__FILE__).'/use-statement-namespace-prefix/');
+  }
+
+}

src/lint/linter/xhpast/rules/__tests__/use-statement-namespace-prefix/use.lint-test

@@ -0,0 +1,11 @@
+<?php
+namespace SomeNamespace;
+use X\Y\Z;
+use \X\Y\Z;
+~~~~~~~~~~
+warning:4:5
+~~~~~~~~~~
+<?php
+namespace SomeNamespace;
+use X\Y\Z;
+use X\Y\Z;