Add a linter rule for determining when single quotes should be used over double quotes.

Summary: Personally, I am a strong fan of this rule. There is currently a similar rule provided by PHP_CodeSniffer.

Test Plan: Wrote and executed unit tests.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9117

src/lint/linter/ArcanistXHPASTLinter.php

@@ -43,6 +43,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
   const LINT_CLOSING_DECL_PAREN        = 38;
   const LINT_REUSED_ITERATOR_REFERENCE = 39;
   const LINT_KEYWORD_CASING            = 40;
+  const LINT_DOUBLE_QUOTE              = 41;
 
   private $naminghook;
   private $switchhook;
@@ -99,6 +100,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
       self::LINT_CLOSING_DECL_PAREN        => 'Declaration Formatting',
       self::LINT_REUSED_ITERATOR_REFERENCE => 'Reuse of Iterator References',
       self::LINT_KEYWORD_CASING            => 'Keyword Conventions',
+      self::LINT_DOUBLE_QUOTE              => 'Unnecessary Double Quotes',
     );
   }
 
@@ -132,6 +134,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
       self::LINT_CLOSING_DECL_PAREN        => $warning,
       self::LINT_REUSED_ITERATOR_REFERENCE => $warning,
       self::LINT_KEYWORD_CASING            => $warning,
+      self::LINT_DOUBLE_QUOTE              => $advice,
 
       // This is disabled by default because it implies a very strict policy
       // which isn't necessary in the general case.
@@ -245,6 +248,7 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
       'lintClosingCallParen' => self::LINT_CLOSING_CALL_PAREN,
       'lintClosingDeclarationParen' => self::LINT_CLOSING_DECL_PAREN,
       'lintKeywordCasing' => self::LINT_KEYWORD_CASING,
+      'lintStrings' => self::LINT_DOUBLE_QUOTE,
     );
 
     foreach ($method_codes as $method => $codes) {
@@ -2355,6 +2359,78 @@ final class ArcanistXHPASTLinter extends ArcanistBaseXHPASTLinter {
     }
   }
 
+  private function lintStrings(XHPASTNode $root) {
+    $nodes = $root->selectDescendantsOfTypes(array(
+      'n_CONCATENATION_LIST',
+      'n_STRING_SCALAR'));
+
+    foreach ($nodes as $node) {
+      $strings = array();
+
+      if ($node->getTypeName() === 'n_CONCATENATION_LIST') {
+        $strings = $node->selectDescendantsOfType('n_STRING_SCALAR');
+      } else if ($node->getTypeName() === 'n_STRING_SCALAR') {
+        $strings = array($node);
+
+        if ($node->getParentNode()->getTypeName() === 'n_CONCATENATION_LIST') {
+          continue;
+        }
+      }
+
+      $valid = false;
+      $invalid_nodes = array();
+      $fixes = array();
+
+      foreach ($strings as $string) {
+        $concrete_string = $string->getConcreteString();
+        $single_quoted = ($concrete_string[0] === "'");
+        $contents = substr($concrete_string, 1, -1);
+
+        // Double quoted strings are allowed when the string contains the
+        // following characters.
+        static $allowed_chars = array(
+          '\0',
+          '\n',
+          '\r',
+          '\f',
+          '\t',
+          '\v',
+          '\x',
+          '\b',
+          '\'',
+        );
+
+        $contains_special_chars = false;
+        foreach ($allowed_chars as $allowed_char) {
+          if (strpos($contents, $allowed_char) !== false) {
+            $contains_special_chars = true;
+          }
+        }
+
+        if (!$string->isConstantString()) {
+          $valid = true;
+        } else if ($contains_special_chars && !$single_quoted) {
+          $valid = true;
+        } else if (!$contains_special_chars && !$single_quoted) {
+          $invalid_nodes[] = $string;
+          $fixes[$string->getID()] = "'".$contents."'";
+        }
+      }
+
+      if (!$valid) {
+        foreach ($invalid_nodes as $invalid_node) {
+          $this->raiseLintAtNode(
+            $invalid_node,
+            self::LINT_DOUBLE_QUOTE,
+            pht(
+              'String does not require double quotes. For consistency, '.
+              'prefer single quotes.'),
+            $fixes[$invalid_node->getID()]);
+        }
+      }
+    }
+  }
+
   public function getSuperGlobalNames() {
     return array(
       '$GLOBALS',

src/lint/linter/__tests__/xhpast/double-quote.lint-test

@@ -0,0 +1,23 @@
+<?php
+'foobar';
+"foobar";
+"foobar\n";
+"'foobar'";
+"foo{$bar}";
+'foo"bar"';
+pht(
+  "This string requires \x12\x34 double quotes, but ".
+  "this string does not. Here, they are used for consistency.");
+~~~~~~~~~~
+advice:3:1
+~~~~~~~~~~
+<?php
+'foobar';
+'foobar';
+"foobar\n";
+"'foobar'";
+"foo{$bar}";
+'foo"bar"';
+pht(
+  "This string requires \x12\x34 double quotes, but ".
+  "this string does not. Here, they are used for consistency.");

src/lint/linter/__tests__/xhpast/switches.lint-test

@@ -15,7 +15,7 @@ switch ($x) {
     return;
   case 4:
     $x++;
-    throw new Exception("!");
+    throw new Exception('!');
   case 5:
     break 2;
   case 6:

src/lint/linter/__tests__/xhpast/wrong-concat-operator.lint-test

@@ -1,8 +1,8 @@
 <?php
-"a"."b";
+'a'.'b';
-"a" + "b";
+'a' + 'b';
-"a" + $x;
+'a' + $x;
-$x + $y + $z + "q" + 0;
+$x + $y + $z + 'q' + 0;
 ~~~~~~~~~~
 error:3:1
 error:4:1